U.S. patent application number 10/175084 was filed with the patent office on 2003-01-09 for method and apparatus for radiation detection.
Invention is credited to Tumer, Tumay O..
Application Number | 20030006376 10/175084 |
Document ID | / |
Family ID | 46256048 |
Filed Date | 2003-01-09 |
United States Patent
Application |
20030006376 |
Kind Code |
A1 |
Tumer, Tumay O. |
January 9, 2003 |
Method and apparatus for radiation detection
Abstract
A detection system is provided. In one embodiment a silicon
Compton recoil electron detector uses the Compton double scatter
technique with recoil electron tracking to detect medium energy
gamma rays from 0.05 to 10 MeV. Two detector layers are required, a
silicon microstrip hodoscope and a calorimeter. The incoming photon
Compton scatters in the hodoscope. The second scatter layer is the
calorimeter where the scattered gamma ray is totally absorbed. The
recoil electron in the hodoscope is tracked through several
detector planes until it stops. The x and y position signals from
the first two planes of the electron track determine the direction
of the recoil electron while the energy loss from all planes
determines the energy of the recoil electron. In another embodiment
of the invention, the Compton double scatter technique with recoil
electron tracking is used to detect x-rays from 300 to 5,000 keV.
This embodiment is useful for nondestructive, real time imaging,
for example for use in medical imaging and the inspection of
munition items. In another embodiment, a high sensitivity, high
spatial resolution and electronically collimated single photon
emission computed tomography system which is sensitive from 81 keV
to 511 keV gamma ray photons is provided. In another embodiment, a
Compton scatter positron emission tomography system is
provided.
Inventors: |
Tumer, Tumay O.; (Riverside,
CA) |
Correspondence
Address: |
Ronald R. Snider
Snider & Associates
P.O. Box 27613
Washington
DC
20038-7613
US
|
Family ID: |
46256048 |
Appl. No.: |
10/175084 |
Filed: |
June 20, 2002 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
10175084 |
Jun 20, 2002 |
|
|
|
09822177 |
Apr 2, 2001 |
|
|
|
6420711 |
|
|
|
|
09822177 |
Apr 2, 2001 |
|
|
|
09135184 |
Aug 17, 1998 |
|
|
|
6236050 |
|
|
|
|
09135184 |
Aug 17, 1998 |
|
|
|
08784176 |
Jan 15, 1997 |
|
|
|
5821541 |
|
|
|
|
60011135 |
Feb 2, 1996 |
|
|
|
Current U.S.
Class: |
250/370.09 ;
257/E31.086 |
Current CPC
Class: |
A61B 6/037 20130101;
H01L 27/14659 20130101; G01T 1/2928 20130101; H01L 31/115 20130101;
G01T 1/006 20130101; G01T 1/2985 20130101 |
Class at
Publication: |
250/370.09 |
International
Class: |
G01T 001/24 |
Goverment Interests
[0002] This invention was made with U.S. Government support under
Contract Numbers DASG60-92-C-0200 and DAAA21-93-C-1014, both
awarded by the Department of Defense. The U.S. Government has
certain rights in the invention.
Claims
What is claimed is:
1. A imaging system for imaging an object, comprising: an x-ray
source emitting x-rays; a detection system comprised of a
hodoscope, wherein said object is located between said x-ray source
and said hodoscope, said hodoscope comprised of a plurality of
position sensitive detector planes, wherein a portion of said
x-rays passing through said object pass into said hodoscope and are
scattered within said hodoscope; a multi-channel readout system
coupled to said plurality of position sensitive detector planes; a
shielding member, said shielding member substantially preventing
exposure of said multi-channel readout system to said emitted
x-rays; a processor coupled to said multi-channel readout system;
and a monitor coupled to said processor, said monitor displaying an
image of said object.
2. The imaging system of claim 1, wherein said shield collimates
said emitted x-rays.
3. The imaging system of claim 1, wherein said plurality of
position sensitive detector planes is comprised of a plurality of
silicon detection planes.
4. The imaging system of claim 3, wherein each of said plurality of
silicon detection planes has an area between 4 square centimeters
and 1,000 square centimeters.
5. The imaging system of claim 3, wherein each of said plurality of
silicon detection planes has an area between 16 square centimeters
and 144 square centimeters.
6. The imaging system of claim 3, wherein each of said plurality of
silicon detection planes has a thickness between about 0.1 and
about 10 millimeter.
7. The imaging system of claim 3, wherein each of said plurality of
silicon detection planes has a thickness between about 0.5 and
about 1 millimeter.
8. The imaging system of claim 3, wherein a portion of said emitted
x-rays undergo at least one Compton scatter within said plurality
of silicon detection planes to yield a track direction
corresponding to each of said portion of emitted x-rays, wherein a
total energy corresponding to each of said portion of said emitted
x-rays is absorbed within said plurality of silicon detection
planes, and wherein said track direction and said total energy
corresponding to each of said portion of emitted x-rays is combined
by said processor to generate said image.
9. The imaging system of claim 3, wherein a recoil electron is
formed by a portion of said emitted x-rays undergoing Compton
scatter within said plurality of silicon detection planes, said
recoil electron passing through a portion of said plurality of
silicon detection planes, wherein a position of said recoil
electron is recorded for each of said portion of said plurality of
silicon detection planes.
10. The imaging system of claim 3, wherein said plurality of
silicon detection planes have a predetermined orientation with
respect to said emitted x-rays, said predetermined orientation
selected from the group consisting of parallel, perpendicular, or
an angle.
11. The imaging system of claim 3, wherein said plurality of
silicon detection planes is selected from the group consisting of
silicon microstrip detectors, silicon strip detectors, silicon pad
detectors, silicon pixel detectors, double-sided silicon microstrip
detectors, and double-sided silicon strip detectors.
12. The imaging system of claim 1, wherein said plurality of
position sensitive detector planes is selected from the group
consisting of CdTe microstrip detectors, CdTe strip detectors, CdTe
pad detectors, CdTe pixel detectors, double-sided CdTe microstrip
detectors, double-sided CdTe strip detectors, CdZnTe microstrip
detectors, CdZnTe strip detectors, CdZnTe pad detectors, CdZnTe
pixel detectors, double-sided CdZnTe microstrip detectors, and
double-sided CdZnTe strip detectors
13. The imaging system of claim 1, wherein said emitted x-rays have
an energy in the range of 50 to 600 keV.
14. The imaging system of claim 1, wherein said emitted x-rays have
an energy in the range of 600 to 5000 keV.
15. The imaging system of claim 1, wherein said emitted x-rays are
monoenergetic.
16. The imaging system of claim 1, wherein said emitted x-rays are
from a radioactive source with multiple emission lines.
17. The imaging system of claim 1, wherein said x-ray source is a
continuous energy x-ray source.
18. The imaging system of claim 1, said detection system further
comprising a calorimeter at least partially enclosing said
hodoscope.
19. The imaging system of claim 18, wherein said calorimeter is
shielded from said emitted x-rays not passing through said
object.
20. The imaging system of claim 18, wherein said calorimeter is
comprised of CsI(Tl) crystals.
21. The imaging system of claim 20, wherein said CsI(Tl) crystals
are coupled to PIN photodiodes.
22. The imaging system of claim 18, wherein said calorimeter is
selected from the group of calorimeter detector materials
consisting of HPGe, BGO, LSO, GSO, CdWO.sub.4, CsF, NaI(Tl),
CsI(Na), CsI(Tl), CdTe, CdZnTe, HgI.sub.2, GaAs, and PbI.sub.2.
23. The imaging system of claim 1, further comprising a rotation
stage coupled to said object, wherein said rotation stage rotates
said object relative to said detection system, wherein said image
is a three-dimensional tomographic image.
24. The imaging system of claim 17, wherein said image includes
energy spectrum information.
25. The imaging system of claim 24, wherein different energy
spectra of said energy spectrum information are represented in said
image by different colors.
26. The imaging system of claim 25, wherein an intensity
corresponding to each of said different energy spectra is
represented by a color intensity corresponding to said different
colors.
27. The imaging system of claim 1, wherein said image is a
two-dimensional image.
28. The imaging system of claim 1, wherein said image is a
three-dimensional image.
29. The imaging system of claim 18, wherein said calorimeter is
coupled to said multi-channel readout system.
30. The imaging system of claim 18, wherein said scattered x-rays
passing through said hodoscope form recoil electrons during passage
through said plurality of position sensitive detector planes,
wherein said hodoscope determines a track direction by a first
scatter vertex and an energy associated with said recoil electrons,
wherein said scattered x-rays are totally absorbed within said
calorimeter, and wherein an energy of said absorbed x-rays is
determined by said calorimeter.
31. The imaging system of claim 1, comprising a second shielding
member proximate to an entrance aperture of said hodoscope.
32. The imaging system of claim 3, wherein said multi-channel
readout system is comprised of ASIC chips, wherein said plurality
of silicon detection planes is comprised of silicon strip
detectors, and wherein said strips are fanned in to match a chip
bonding pitch corresponding to said ASIC chips.
33. A positron emission tomography system for imaging a portion of
a living organism, said portion treated with a radionuclides, said
radionuclide emitting positrons, said emitted positrons creating
photon pairs within said portion of said living organism, said
system comprising: a first and a second detection system, said
portion of said living organism interposed between said first and
second detection systems, wherein said first and second detection
systems are diametrically opposed, wherein said first and second
detection systems are comprised of a plurality of position
sensitive strip detectors, wherein a portion of said photons
undergo multiple Compton scatters within said detection systems;
means for rotating the relative positions of said first and second
detection systems to said portion of said living organism; a
multi-channel readout system coupled to said plurality of position
sensitive strip detectors; a processor coupled to said
multi-channel readout system, said processor determining track
directions and total energies for said portion of said photons; and
a monitor coupled to said processor, said monitor displaying an
image of said portion of said living organism.
34. The positron emission tomography system of claim 33, said
rotation means comprising a rotation stage coupled to said first
and second detection systems for rotating said detection systems
relative to said portion of said living organism.
35. The positron emission tomography system of claim 33, said
rotation means comprising a rotation stage coupled to said living
organism for rotating said portion of said living organism relative
to said first and second detection systems.
36. A positron emission tomography system for imaging a portion of
a living organism, said portion treated with a radionuclide, said
radionuclide emitting positrons, said emitted positrons creating
photon pairs within said portion of said living organism, said
system comprising: a plurality of detection systems at least
partially surrounding said portion of said living organism, wherein
said plurality of detection systems are comprised of diametrically
opposed detection system pairs, wherein each of said plurality of
detection systems is comprised of a hodoscope, each of said
hodoscopes comprised of a plurality of position sensitive
detectors, wherein a portion of said photons undergo Compton
scatters within said detection systems; a multi-channel readout
system coupled to said plurality of position sensitive strip
detectors; a processor coupled to said multi-channel readout
system, said processor determining track directions and total
energies for said portion of said photons; and a monitor coupled to
said processor, said monitor displaying an image of said portion of
said living organism.
37. The positron emission tomography system of claim 36, said
radionuclide selected from the group consisting of carbon-11,
nitrogen-13, oxygen-15, fluorine-18, gallium-68, bromine-75, and
strontium-82.
38. The positron emission tomography system of claim 36, wherein
said position sensitive detectors are CdZnTe strip detectors.
39. The positron emission tomography system of claim 36, said
plurality of detection systems further comprising a plurality of
calorimeters, said calorimeters positioned proximate an end portion
of each of said hodoscopes.
40. The positron emission tomography system of claim 39, wherein
said plurality of calorimeters are CdZnTe calorimeters.
41. The positron emission tomography system of claim 36, further
comprising passive septa interposed between said detection
systems.
42. The positron emission tomography system of claim 36, further
comprising active septa interposed between said detection systems,
wherein said active septa are calorimeters.
43. The positron emission tomography system of claim 36, wherein
said position sensitive detectors are selected from the group
consisting of pad detectors and strip detectors.
44. A radiation detector for detecting gamma rays, comprising: a
detector aperture, said aperture limiting said detector to a
predetermined field-of-view; a detection system comprised of a
hodoscope, said hodoscope comprised of a plurality of position
sensitive detector planes, wherein a portion of said gamma rays
pass through said aperture into said hodoscope and are scattered
within said hodoscope; a multi-channel readout system coupled to
said detection system; a processor coupled to said multi-channel
readout system; and an output device coupled to said multi-channel
readout system, said output device outputting information
corresponding to said detected gamma rays.
45. The radiation detector of claim 44, wherein said plurality of
position sensitive detector planes is comprised of a plurality of
silicon detection planes.
46. The radiation detector of claim 44, wherein said gamma rays
have an energy between about 0.05 and 10 MeV.
47. The radiation detector of claim 44, wherein said output device
is a monitor, said monitor displaying an image corresponding to
said detected gamma rays.
48. The radiation detector of claim 45, wherein a portion of said
gamma rays undergo at least one Compton scatter within said
plurality of silicon detection planes to yield a track direction
corresponding to each of said portion of gamma rays, wherein a
total energy corresponding to each of said portion of said gamma
rays is absorbed within said plurality of silicon detection planes,
and wherein said track direction and said total energy
corresponding to each of said portion of gamma rays is combined by
said processor to generate said outputted information.
49. The radiation detector of claim 45, wherein a recoil electron
is formed by a portion of said gamma rays undergoing Compton
scatter within said plurality of silicon detection planes, said
recoil electron passing through a portion of said plurality of
silicon detection planes, wherein a position of said recoil
electron is recorded for each of said portion of said plurality of
silicon detection planes.
50. The radiation detector of claim 45, wherein said plurality of
silicon detection planes have a predetermined orientation with
respect to said gamma rays, said predetermined orientation selected
from the group consisting of parallel, perpendicular, or an
angle.
51. The radiation detector of claim 45, wherein said plurality of
silicon detection planes is selected from the group consisting of
silicon microstrip detectors, silicon strip detectors, silicon pad
detectors, silicon pixel detectors, double-sided silicon microstrip
detectors, and double-sided silicon strip detectors.
52. The radiation detector of claim 44, wherein said plurality of
position sensitive detector planes is selected from the group
consisting of CdTe microstrip detectors, CdTe strip detectors, CdTe
pad detectors, CdTe pixel detectors, double-sided CdTe microstrip
detectors, double-sided CdTe strip detectors, CdZnTe microstrip
detectors, CdZnTe strip detectors, CdZnTe pad detectors, CdZnTe
pixel detectors, double-sided CdZnTe microstrip detectors, and
double-sided CdZnTe strip detectors.
53. The radiation detector of claim 44, said detection system
further comprising a calorimeter at least partially enclosing said
hodoscope.
54. The radiation detector of claim 53, wherein said calorimeter is
coupled to said multi-channel readout system.
55. The radiation detector of claim 53, wherein said calorimeter is
selected from the group of calorimeter detector materials
consisting of HPGe, BGO, LSO, GSO, CdWO.sub.4, CsF, NaI(Tl),
CsI(Na), CsI(Tl), CdTe, CdZnTe, HgI.sub.2, GaAs, and PbI.sub.2.
56. The radiation detector of claim 53, wherein said scattered
gamma rays passing through said hodoscope form recoil electrons
during passage through said plurality of position sensitive
detector planes, wherein said hodoscope determines a track
direction by a first scatter vertex and an energy associated with
said recoil electrons, wherein said scattered gamma rays are
totally absorbed within said calorimeter, and wherein an energy of
said absorbed gamma rays is determined by said calorimeter.
57. The radiation detector of claim 45, wherein said multi-channel
readout system is comprised of ASIC chips, wherein said plurality
of silicon detection planes is comprised of silicon strip
detectors, and wherein said strips are fanned in to match a chip
bonding pitch corresponding to said ASIC chips.
58. The radiation detector of claim 53, further comprising an
anti-coincidence shield substantially surrounding said hodoscope,
said anti-coincidence shield not covering said detector
aperture.
59. The radiation detector of claim 53, further comprising an
anti-coincidence shield substantially surrounding said
calorimeter.
60. The radiation detector of claim 53, said detection system
further comprising a position sensitive detection layer interposed
between said hodoscope and said calorimeter.
61. The radiation detector of claim 53, said position sensitive
detection layer having a thickness between about 0.1 millimeter and
1 centimeter.
62. The radiation detector of claim 44, further comprising a scout
interceptor missile, wherein said detection system is located
within said scout interceptor missile.
63. The radiation detector of claim 62, wherein a portion of said
gamma rays is emitted by warheads.
64. The radiation detector of claim 63, wherein said warheads are
launched from an ICBM bus.
65. The radiation detector of claim 63, wherein said warheads are
irradiated by an external source.
66. The radiation detector of claim 65, wherein said external
source is selected from the group consisting of directed energy
sources, particle beams, and neutron bombs.
67. The radiation detector of claim 62, wherein a portion of said
gamma rays is emitted by a nuclear powered satellite.
Description
CROSS-REFERENCES TO RELATED APPLICATIONS
[0001] This application is a continuation-in-part of U.S.
application Ser. No. 08/784,176, filed Jan. 15, 1997 which is a
continuation of U.S. Provisional Application Serial No. 60/011,135,
filed Feb. 2, 1996 (now abandoned). This application is related to
co-pending U.S. application Ser. No. 09/119,144, filed Jul. 20,
1998, entitled Method and Apparatus for Gamma Ray Detection
(Attorney Docket No. 16219-5-1).
FIELD OF THE INVENTION
[0003] The present invention relates generally to detection
systems, and more particularly, to a method and apparatus for
imaging gamma rays, x-rays, and positrons.
BACKGROUND OF THE INVENTION
[0004] The dominant absorption process for gamma rays of about 0.05
to 30 MeV is the Compton interaction. At present the best method to
detect these gamma rays is the Compton double scatter technique
since single Compton scattering alone gives neither the direction
nor the energy of the incident gamma ray. In contrast, the Compton
double scatter technique yields both the direction and the energy
of the incoming photon.
[0005] Present detectors which use the Compton double scatter
technique determine the direction of the incoming photon to a ring
in the sky since the direction of the recoil electron at the first
scatter cannot be measured. Time-of-flight measurements are
normally used to discriminate between gamma rays coming through the
field-of-view and those entering through the back of the system.
Typically this requires that the first scatterer (i.e., the
hodoscope) is separated from the second scatterer (i.e.,
calorimeter) by approximately 1.5 meters.
[0006] Emission computed tomography (ECT) and associated
technologies are mainly used for the detection and imaging of the
radiation produced by radiotracers and radiopharmaceuticals. The
primary application for ECT systems is in medical study and
diagnosis due to the potential for imaging organ functions in real
time. The two major ECT instruments presently used are Single
Photon Emission Computed Tomography (SPECT) and Positron Emission
Tomography (PET).
[0007] Positron scanners for use in locating brain tumors were
first developed in the early 1960s with the first PET system
completed in 1975. Pet systems have become an essential medical
diagnostic tool for a variety of reasons. For one, very high
efficiencies utilizing positron emitting labels can be achieved
through the coincidence collimation of the annihilation radiation.
Another advantage is that the common radiopharmaceuticals used with
PET systems typically have very short life times, thus allowing
large doses to be administered to a patient as well as the
performance of repetitive studies. Recently, the utilization of
photon time-of-flight information with fast scintillators has
improved the SNR that can be obtained in images of the distribution
of positron emitting radionuclides.
[0008] Present PET systems use bismuth germanate oxide (BGO)
crystals. BGO crystals have the highest effective atomic number and
stopping power of any scintillator crystal available today. This
translates into a higher photopeak fraction and a lower Compton
continuum than other crystals such as NaI and BaF.sub.2. Gadolinium
orthosilicate (GSO) crystal is an alternative which has a slower
decay time but larger pulse yield than BGO. PbCO.sub.3 crystals
nearly equal the stopping power of BGO but the light output is
about 10 times lower.
[0009] From the foregoing, it is apparent that an improved imaging
system for use with gamma rays, x-rays, and positrons is
desired.
SUMMARY OF THE INVENTION
[0010] The present invention provides a high sensitivity, low
background detector for use in a variety of applications. The
detector also has excellent angular resolution for accurate
direction measurement and imaging; good energy resolution for the
identification of the source material by its energy spectrum; and
low power consumption. Both the direction and energy of the
incident photons is measured using a Compton double scatter
technique with recoil electron tracking.
[0011] The Compton double scatter technique involves two detector
layers; a silicon microstrip hodoscope and a calorimeter. The
incoming photon Compton scatters in the hodoscope. The second
scatter layer is the calorimeter where the scattered gamma ray is
totally absorbed. The recoil electron in the hodoscope is tracked
through several detector planes until it stops. The x and y
position signals from the first two planes of the electron track
determine the direction of the recoil electron. The energy loss
from all planes is summed to determine the energy of the recoil
electron.
[0012] The hodoscope of the disclosed system is constructed of
position sensitive, double-sided silicon microstrip detectors,
preferably with a strip pitch of between 0.5 and 1 millimeter and a
thickness of between 200 and 1000 micrometers. The pixel size of
the microstrip detectors ranges from approximately 1 square
millimeter to approximately 1 square centimeter.
[0013] The measurement of the direction of the Compton recoil
electron track reduces the incident gamma ray event ring to an
event arc. The recoil electron direction calculation requires only
the x and y coordinates of the first two adjacent planes along the
track of the recoil electron. For the measurement of the direction
of motion of the recoil electron (i.e., moving forward or backward
in the hodoscope) a track which penetrates 2 or more adjacent
planes is required. If the energy of the recoil electron is low
enough, it may be absorbed in the same detector plane and not
produce a track. Thus for a 200 micrometer thick detector, recoil
electron tracking is effective for electrons with an energy of
greater than 0.25 MeV energy. Therefore gamma rays with energies
greater than 1 MeV will produce recoil electron tracks with high
probability. For low energy gamma rays which do not produce recoil
electron tracks, imaging is carried out using event rings instead
of arcs. This technique increases the background, resulting in a
decrease in sensitivity for low energy gamma rays. By utilizing
thinner detectors the recoil electron tracking threshold can be
reduced to even lower energies.
[0014] The selection of the detector for use as the second
scatterer depends primarily upon the desired detection energy as
well as the desired detection energy range. In one embodiment of
the invention, the second scatterer uses thallium activated cesium
iodide (i.e., CsI(Tl)) detectors viewed by photodiodes. In a second
embodiment used to obtain higher energy resolution, a germanium
array calorimeter is used. The germanium array requires
refrigeration to liquid nitrogen temperatures. Alternatively, a
detection system using only a hodoscope with a large number of
silicon detector planes can be used for high energy resolution.
[0015] The double Compton scatter measurement determines the
direction of the incident photon to a cone with a half angle equal
to the scatter angle. This type of measurement requires special
data analysis software. The data analysis can be carried out by
cone interaction, Maximum Likelihood or Maximum Entropy techniques
or using a direct data analysis and imaging technique such as
Direct Linear Algebraic Deconvolution (DLAD).
[0016] In another embodiment of the invention, a Compton scatter
PET system is provided. This system can be designed as a
cylindrical detector with a length of approximately 30 centimeters
or greater. Cylindrical geometry leads to the production of
accurate 3-dimensional images. Alternately, a ring detector with a
width of about 7 centimeters or less can be used to provide a
multi-slice ring type system. Since the PET system according to the
present invention does not utilize PMTs, it is substantially less
bulky than present systems. As a result, the structural
requirements placed on the gantry are less stringent.
[0017] Photon attenuation inside the patient can be corrected using
the techniques already developed. For example, the boundary method
has already been successfully applied to attenuation correction in
PET image reconstruction. In this method, the organ boundaries are
determined by transmission tomography and each region is enclosed
by a boundary and assigned an average attenuation coefficient.
Attenuation correction factors for all angular views can be
calculated from the quantized image.
[0018] The PET embodiment of the present invention preferably uses
thin film strip detectors with high stopping power such as position
sensitive, double-sided CdZnTe strip detectors with an approximate
strip size of at least 0.1 millimeters in both the x and y
dimensions. The CdZnTe detectors have a thickness in the range of
250 micrometers to 2 millimeters. This embodiment uses several
planes of detectors with minimal detection plane spacing. The
incident photons undergo Compton scatter in one of the detector
planes, which is the dominant process for photons above
approximately 200 keV in CdZnTe. The energy of the Compton recoil
electron in this detector wafer is measured. This process is
repeated until the scattered photon with reduced energy either gets
absorbed through the photoelectric effect in another detector plane
or escapes without further interaction. If the scattered photon is
fully absorbed within the detector planes, the sum of the measured
energies yields the total energy of the incident photon. The
straight line (i.e., cord) joining the first interaction points of
an annihilation photon pair at the CdZnTe strip detector in
coincidence will determine the geometry. Escaped photons will
produce a tail at lower energies in the energy spectrum. Such
events can be rejected because the measured total energy is smaller
than the expected energy of the known incident energy of 511
keV.
[0019] A further understanding of the nature and advantages of the
present invention may be realized by reference to the remaining
portions of the specification and the drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0020] FIG. 1 is an illustration of the gamma ray linear
attenuation coefficients for a silicon detector;
[0021] FIG. 2 is an illustration of the Compton double scatter
technique for detecting gamma rays;
[0022] FIG. 3 is an illustration of a typical double-sided silicon
microstrip or strip detector;
[0023] FIG. 4 is an illustration of the cross-section of the
detector shown in FIG. 3;
[0024] FIG. 5 is an illustration of one side of an individual
double-sided silicon strip detector design;
[0025] FIG. 6 is an illustration of a silicon microstrip detector
module that includes four silicon detector arrays, a PC board, four
front mounted FEE chips, and a ceramic chip carrier;
[0026] FIG. 7 is an illustration of a top view of a
CsI(Tl)/photodiode calorimeter module design according to one
embodiment of the invention;
[0027] FIG. 8 is across-sectional view of the design illustrated in
FIG. 7;
[0028] FIG. 9 is an illustration of the side view of an embodiment
of a single module of a two-dimensional detector module utilizing
CdZnTe pad detectors;
[0029] FIG. 10 is an illustration of the top view of the detector
module illustrated in FIG. 9;
[0030] FIG. 11 is a schematic diagram of a possible multi-channel
silicon microstrip detector readout chip with fast data readout and
trigger output capability;
[0031] FIG. 12 is a block diagram of a real time data acquisition
system for use with the present invention;
[0032] FIG. 13 is an illustration of the cross-section of the
detection system according to one embodiment of the invention;
[0033] FIG. 14 is an illustration of the top view of the detection
system illustrated in FIG. 13;
[0034] FIG. 15 is an illustration of the cross-section of a
detection system similar to that shown in FIG. 13 which includes an
additional detection level;
[0035] FIG. 16 is an illustration of the top view of the detection
system illustrated in FIG. 15 utilizing a square cross-section;
[0036] FIG. 17 is an illustration of the top view of the detection
system illustrated in FIG. 15 utilizing a cylindrical
cross-section;
[0037] FIG. 18 is an illustration of a hodoscope detection plane
utilizing four bridged double-sided silicon microstrip
detectors;
[0038] FIG. 19 is a graph of recoil electron track length
distributions in a silicon hodoscope for 1 MeV gamma rays;
[0039] FIG. 20 is a graph of recoil electron track length
distributions in a silicon hodoscope for 2 MeV gamma rays;
[0040] FIG. 21 is an illustration of a multiple scattering event in
which the multiple scatter angles along the electron track can be
used to determine the direction of motion of the electron;
[0041] FIG. 22 is an illustration of the bottom of a CsI(Tl)
calorimeter using an array of photodiodes to view the crystal;
[0042] FIG. 23 illustrates the use of the present invention for
exo-atmospheric mid-course discrimination of nuclear warheads from
decoys;
[0043] FIG. 24 illustrates the overlay of radar or infrared images
with the computer simulation provided by the present invention to
discriminate between nuclear warheads and decoys for the
application shown in FIG. 23;
[0044] FIG. 25 is a graph of the number of integrated gamma ray
photons versus time before impact for a nuclear warhead producing
5.times.10.sup.8 gamma rays per second;
[0045] FIG. 26 is a graph of the number of integrated gamma ray
photons versus distance before impact for a nuclear warhead
producing 5.times.10.sup.8 gamma rays per second;
[0046] FIG. 27 is an illustration of an embodiment of the invention
for use as a 2-dimensional inspection imaging system;
[0047] FIG. 28 is an illustration of the scattered photon
background discrimination technique using the incident photon
direction measurement;
[0048] FIG. 29 is an illustration of a configuration of the
invention in which a plurality of CsI(Tl)/photodiode modules are
placed on a hemisphere with most detection modules placed in a
forward direction with respect to the incident beam;
[0049] FIG. 30 is an illustration of an embodiment of a positron
imaging system according to the present invention;
[0050] FIG. 31 is an illustration of the cross-section of a portion
of a positron imager detection system in which the individual
hodoscopes are separated by calorimeter septa; and
[0051] FIG. 32 is an illustration of the cross-section of a
detector module with inner and outer calorimeter sections.
DESCRIPTION OF THE SPECIFIC EMBODIMENTS
[0052] Gamma Ray Detection
[0053] The most probable interaction mechanism for 0.05 to 10 MeV
gamma rays in silicon is the Compton scatter process. Therefore,
the detection of gamma rays in this energy range must use Compton
interaction to have maximum sensitivity. The detector must also
have excellent angular and energy resolution and a wide
field-of-view. The best detection technique that has all of these
features is the Compton double scatter method. This technique
incorporates Compton scattering, photoelectric absorption, and pair
production. The three gamma ray interaction mechanisms are briefly
discussed below.
[0054] Although a number of possible interaction mechanisms are
known for gamma rays in matter, only three major types play an
important role in radiation detection: photoelectric absorption,
Compton scattering, and pair production. Of these, only the first
two play a major roll in emission imaging. All of these processes
lead to the partial or complete transfer of the photon energy to
electron energy. They result in sudden and abrupt changes in the
photon history where the photon disappears entirely or is scattered
through a significant angle.
[0055] FIG. 1 is an illustration of the gamma ray linear
attenuation coefficients for silicon microstrip detectors through
these three processes. The photoelectric absorption dominates below
about 50 keV for silicon. Compton scattering becomes important at
80 keV and it stays the dominant process up to about 10 MeV, where
pair production takes over. Compton scattered gamma ray photons
with energies less than 50 keV are readily absorbed due to the
photoelectric effect.
[0056] In the photoelectric absorption process, a photon undergoes
an interaction with an absorber atom in which the photon completely
disappears. In its place, an energetic photoelectron is ejected by
the atom from one of its bound shells. The interaction is with the
atom as a whole and cannot take place with free electrons. The
photoelectron appears with an energy, E.sub.e, given by
E.sub.e=h.upsilon.-E.sub.b
[0057] where h.upsilon. is the incident photon energy and E.sub.b
represents the binding energy of the photoelectron in its original
shell. For gamma ray energies, h.upsilon., of more than a 100 keV,
the photoelectron carries off most of the original photon energy.
For silicon microstrip detectors, this process is only important
for low energy gamma rays in the range of 0. 5 to 50 keV. For
CsI(Tl) crystals the photoelectric effect is dominant up to about
0.3 MeV above which Compton scattering becomes important.
Photoelectric absorption falls nearly exponentially with an
increase in energy. Photoelectric absorption is excellent for the
determination of the scattered photon energy as the photon is
completely absorbed.
[0058] Compton scattering takes place between the incident gamma
ray and an electron in the absorbing material. In Compton
scattering, the incident gamma ray is deflected through an angle
.theta. with respect to its original direction as illustrated in
FIG. 2. The photon transfers a portion of its energy to the recoil
electron that was initially at rest. Because all angles of
scattering are possible, the energy transferred to the electron can
vary from zero to a large fraction of the gamma ray energy. This
has been a problem in the detection of gamma rays at energies
dominated by the Compton scatter process, since the detected recoil
electron alone does not give sufficient information to uniquely
determine the energy and direction of the incident photon. This has
been solved by the Compton double scatter technique described below
and illustrated in FIG. 2.
[0059] The total incident gamma ray energy, E.sub..gamma., and
Compton scatter angle, .theta., for the double scatter process are
given by:
E.sub..gamma.=E.sub.e1+E.sub..gamma.1
[0060] and
Cos .theta.=1-mc.sup.2(1/E.sub..gamma.1-1/E.sub..gamma.)
[0061] The incident gamma ray first scatters by the Compton process
in one of the silicon strip detectors 201, losing recoil energy
E.sub.e1. The scattered photon continues on until it interacts with
another silicon strip detector or is absorbed by a calorimeter 203.
If the second interaction is photoelectric absorption, the full
energy of the scatter photon is measured and the energy of the
incident photon and the scatter angle are determined. This is the
dominant process for the calorimeter as it is made of high Z
material and photoelectric absorption increases exponentially with
a decrease in the scattered photon energy. Another possibility is
that the second interaction can be another Compton scatter where
the photon escapes with a small amount of the energy. If the energy
of the escaping photon is sufficiently low, the energy
determination is not significantly effected. If there are enough
silicon planes, the escaped photon makes further interactions in
subsequent planes and gets fully absorbed by the photoelectric
effect. All of the energy measured after the second scatter is just
added to the energy of the second scatter, E.sub.e2, to correct for
the missing energy. If not enough silicon planes are used, for
example due to cost considerations, a calorimeter can be placed
such that it surrounds the sides and the bottom of the silicon
strip detector hodoscope. The surrounding calorimeter is used as a
second scatterer to measure the energy and direction of the
scattered photon or to catch the escaping photons and correct
E.sub.e2 for accurate incident photon scatter angle determination.
Since the calorimeter is a high Z and high density detector or
scintillator, there is a high probability that the escaped low
energy photon will be fully absorbed. The events that do not add up
to the full energy of the incident photon are rejected to reduce
scattered photon background.
[0062] The incident gamma ray direction lies on a cone segment in
the field-of-view with a half-angle .theta.. The cone axis is
determined by the interaction positions in the first and the second
scatters. This is because the direction of the scattered electron
in the top scintillator is not measured. The Compton scattered
electrons with energies in the range of 81 to 364 keV are fully
stopped within 0.03 and 0.3 millimeters of the silicon strip
detectors, respectively.
[0063] For a gamma ray with an energy larger than or equal to twice
the rest mass of an electron, pair production becomes energetically
possible. Since the probability of an occurrence remains very low
until the gamma ray approaches an energy of several MeV, pair
production is confined to high energy gamma rays. In the
interaction which takes place in the coulomb field of a nucleus, a
gamma ray photon disappears and is replaced by an electron-positron
pair. All excess energy carried by the photon above 1.022 MeV, the
energy required to create the pair, goes into kinetic energy shared
by the electron and the positron. The positron annihilates with an
electron after slowing down in the absorbing medium and produces
two annihilation photons each with an energy of 0.511 MeV in the
para state or 3 photons with different energies adding to 1.022 MeV
in the ortho state. The para state decay to 2 photons dominates
ortho state decay.
[0064] For silicon microstrip detectors, pair production becomes
significant for gamma rays above 10 MeV and dominates Compton
scattering above 15 MeV. Pair events are easily distinguishable as
they produce two tracks starting from a common vertex. Multiple
scattering in the silicon planes quickly separate the two tracks
which resemble an inverted V. Both the electron and the positron
loose their energy in the silicon planes and stop. The positron
quickly annihilates with an electron as it stops and creates
back-to-back 0.511 MeV gamma rays. One or more of the annihilation
gamma rays will likely be detected in the calorimeter in
coincidence with the electron-positron pair observed in the silicon
microstrip hodoscope.
[0065] Silicon Microstrip Detectors
[0066] In the preferred embodiment of the invention, silicon
microstrip detectors are used as the first scatterer (i.e.,
hodoscope). Silicon microstrip detectors have large active areas,
excellent energy and position resolution, and fast readout. Three
inch diameter wafers, typically 200 to 1500 micrometers thick, with
parallel readout strips of greater than 25 micrometers pitch on one
side have been available for several years. Pitch size can have any
value from 25 micrometers to several centimeters.
[0067] On the average, 1 electron-hole pair is produced per 3.6 eV
of deposited energy. The energy deposited by an 80 keV recoil
electron fully stopped in silicon is about 22,000 electrons (and
holes) which can be collected in less than 10 nanoseconds. This
leads to pulse rise times of less than 10 nanoseconds. Spatial
resolutions of less than 10 micrometers in one dimension are
obtainable by exploiting charge division between adjacent strips.
Superimposed on the signal is Gaussian-distributed noise related to
the detector strip and preamplifier input capacitances. This noise
or equivalent noise charge (e.g., ENC) is typically about 1,000
electrons at room temperature for detector capacitances of about 20
pF. Thus large signal-to-noise ratios, on the order of 22, are
obtainable for 80 keV electrons.
[0068] To date, silicon detectors have been primarily used in high
energy physics experiments to detect minimum ionizing high energy
charged particles. The Compton converter in the present invention
is different in that the recoil electron loses varying fractions of
its energy at each detector wafer that it traverses until it
completely stops. The energy and angular resolutions improve as the
number of electron-hole pairs created in the silicon increase. For
a 300 keV recoil electron stopping in silicon, about 83,000
electrons (i.e., 278 e/keV) are produced with an inherent energy
resolution of 0.8 percent (i.e., FWHM/E.sub.0=2.35/{squar- e root}N
where N is the number of electron-hole pairs). For 141 keV
electrons stopping inside the silicon wafer, the theoretical energy
resolution is calculated to be about 1.2 percent with a stopping
distance for the recoil electron of about 0.1 millimeters. The
theoretical resolution can be approached if the input capacitance
and the preamplifier noise can be kept low. The input capacitance
can be decreased substantially by mounting the chips next to the
strips or building them on the same silicon. In the present
invention preferably a low noise, 64 channel front end mixed signal
application specific integrated circuit (ASIC) readout chips is
used.
[0069] The individual detector thicknesses can be increased in
order to decrease the number of required planes. By increasing the
thickness of the individual detectors, the energy resolution is
improved while the accuracy of the recoil electron direction
determination is decreased. The optimum thickness is also driven by
the desired energy range of the detector.
[0070] Double-sided readout silicon microstrip detectors with
orthogonal strips on opposite sides have been developed. FIGS. 3
and 4 show the basic features of a double-sided silicon microstrip
or strip detector. The distinct advantage with this configuration
is that both x and y coordinates of a traversing particle are
determined in a single detector plane. For single-sided detectors,
the junction side of a standard p+n diode is segmented into many
strips. For double-sided detectors, the ohmic side of the n-type
silicon wafer is also segmented with orthogonal strips to provide
simultaneous readout of the particle impact point. Position
resolutions well below a square millimeter on both sides can be
achieved. The preferred detector in the present invention uses 200
to 1500 micrometer thick, and more preferably 200 to 300 micrometer
thick, double-sided, silicon microstrip detectors with about a
millimeter spaced strips orthogonal on the top and bottom surfaces.
Such detectors are now commercially available and fit well with the
present design. The x and y positions of the first two interaction
points on the recoil electron track determine the electron
direction. A combination of all interactions is used to determine
the energy of the recoil electron as well as the scatter angle.
[0071] In one embodiment of the invention, the detector is 6.4
centimeters by 6.4 centimeters, the detector being fabricated from
a 4 inch wafer. In another embodiment, 10 centimeter by 10
centimeter detectors are used. Bridged detectors with overall
lengths exceeding 25 centimeters can also be used with the present
invention. Bridging allows one preamplifier to be connected to a
series of strips on adjacent detectors with significant savings in
electronics.
[0072] A simple Monte Carlo calculation using Monte Carlo Neutron
Photon (MCNP) software from Los Alamos National Laboratory was
performed. The MCNP software gives the probability for a 141 keV
photon to Compton scatter in varying total silicon thicknesses. For
example, about 50 percent of the 141 keV photons will Compton
scatter in a silicon detector 2 centimeters thick. If 2 millimeter
thick silicon strip detectors are used, then 10 planes will be
required. For lower energy photons, a lower total thickness is
required.
[0073] The FWIIM angular resolution of the scatter angle in a
Compton double scatter detector depends on the geometry of the
detector as well as its energy resolution. The FWIIM uncertainty in
the cone half-angle, .DELTA..theta., due to a detector of finite
energy resolution at first and second scattering planes, can be
calculated using the Compton scatter formula: 1 = mc 2 E 2 Sin { E
e 1 2 + [ E 2 E 1 2 - 1 ] 2 E e 2 2 } 1 / 2
[0074] where mc.sup.2 is the electron rest energy (511 keV),
.theta. is the Compton scatter angle, and E.sub..gamma. and
E.sub..gamma.1 are the incident and scattered photon energies.
Applying the formula, the energy resolution due to the statistical
fluctuation for electrons stopped inside the silicon microstrip
detectors varies from 1.3 percent at 100 keV to 0.75 percent at 350
keV. The electronics noise of the detector is about 2 keV.
Therefore the total energy resolution is dominated by the
electronics noise which is the same for both the converter and the
calorimeter.
[0075] The angular resolution is calculated with an energy
resolution of 2 keV (FWHM) where .DELTA..theta. for forward
scattered gamma rays (i.e., .theta.<90.degree.) varies from
5.degree. at a .theta. of 30.degree. to about 3.2.degree. at a
.theta. of 70.degree. for 141 keV .sup.99mTc incident photons. The
same calculation carried out for 364 keV .sup.131I gamma rays gives
angular resolutions of approximately 1.degree. for a 0 of between
20.degree. and 90.degree.. Thus the angular resolution improves
significantly with an increase in the photon energy. Also the
effects of amplifier noise are reduced as more electron-hole pairs
are created by higher energy scattered electrons. At a distance of
20 centimeters these angular resolutions produce effectively 6 to
3.5 millimeter spatial resolutions for 141 keV gamma rays and 3.5
to 1.5 millimeter spatial resolutions for 364 keV gamma rays. At a
distance of 2.5 centimeters the same energy gamma rays produce 2.2
to 1.4 millimeter spatial resolutions and 0.4 millimeter spatial
resolutions, respectively.
[0076] The geometric angular resolution, .DELTA..theta..sub.Geom,
is the FHWM variation of the axis of the image cone and is
dependent upon the silicon microstrip detector pixel size and the
distance between the first two scatters. The FWHM value can be
calculated similar to that for a collimator. Normally the geometric
angular resolution is kept much smaller than the scatter angle
variation which depends strongly on the energy resolution as shown
above. It is easier to adjust the geometric angular resolution in a
silicon microstrip detector as the pixel dimensions can be as small
as 25 microns. The pixel size for the simulated model is 1 square
millimeter.
[0077] Another important advantage of silicon microstrip detectors
is that they do not need high voltages or cooling to low
temperatures. Room temperature functionality is important to
produce small size, low cost, and low power detectors. They also
have a strong potential for mass production. However, a significant
number of wafers are needed to achieve the conversion rates
required for high sensitivity. Their small thickness and ultrasonic
wire bonding capability render them good candidates for compact
printed circuit board mounting with data acquisition ICs placed
next to them. The readout ICs are preferably designed to give fast
trigger outputs when events occur and output the address and the
analog content of the channel that has the data.
[0078] The schematic details of one embodiment of the individual
double-sided silicon microstrip detectors of the present invention
are shown in FIG. 5. The design includes a 64 pad output on both
junction and ohmic sides, fanned in to 250.times.250 micrometer
linear pad array. Fan-in connection dimensions are minimized but
not to the extent that performance is comprised. As viewed from one
side, the opposite side is an exact copy, rotated once by
90.degree..
[0079] Preferably silicon array 500 is exactly square so that it
may be rotated and used in the three other locations on the
detector module board to form a detector plane 600 as shown in FIG.
6. The number of guard rings are preferably set to 3 with a pad
placement preferably located toward the outside so that they are
neighbors when the silicon is rotated.
[0080] Preferably a multichannel front end electronics (FEE) chip
with self trigger output is mounted onto a ceramic carrier along
with the buffer electronics. Although the FEE chip is described
briefly below, a detailed description of the FEE chip is provided
in U.S. Pat. No. 5,696,458, issued Dec. 9, 1997 and in co-pending
U.S. patent application Ser. No. 08/866,117, filed Jun. 27, 1997,
both disclosures of which are incorporated herein for all
purposes.
[0081] In at least one embodiment, the silicon detector output pads
are bonded to 64 input pads on the PC board that support both the
silicon strip detectors and the FEE chips. The ceramic FEE chip
carrier is bonded onto the PC board pads. The FEE chip is bonded to
the pads on the ceramic carrier. Additional detectors can be placed
onto a PC board and connected to readout electronics to achieve
large effective areas. Alternately, the strips can be daisy chained
to decrease the number of readout electronics chips needed.
[0082] The above-described mounting technique allows the
fabrication of the extremely fine fan-in with the approximately 50
micrometer pitch required at the detector input side on the ceramic
carrier. Although such high resolution traces cannot be made easily
on a PC board, they can be fabricated on a ceramic substrate.
[0083] FIGS. 7 and 8 illustrate an embodiment of the invention
utilizing surface mount technology (SMT) to mount the photodiode by
surface mount connectors on one side of the board. By mounting the
FEE chip and the diode array on the same board, the length of the
connections are minimized thereby lowering the electronic noise.
Due to the density of diode pins, the through-hole design prevents
the FEE chip to be mounted on the other side of the board, thus
making SMT useful.
[0084] As shown in FIGS. 7 and 8, the array comprised of diodes 701
is elevated from board 801 by SMT connectors 803 and standoffs 805.
Due to the elevation of diodes 701, the surface mounted bias
resistors 807 and coupling capacitors 809 can be mounted underneath
the diodes. The other side of the board is used to mount FEE chip
811 as well as various other analog components and the by-pass
filter network. A plastic housing 813 is placed on the calorimeter
printed circuit board to protect the CsI(Tl)/photodiode array from
mechanical and humidity damage. This housing also provides improved
heat dissipation from the electronics on the other side of the
printed circuit board. Preferably the whole silicon hodoscope and
CsI(Tl) calorimeter is sealed as a unit.
[0085] The above-described design can also be applied to other
kinds of position sensitive solid state detectors such as CdTe,
CdZnTe, HgI.sub.2, HPGe, GaAs, BGO, CdWO.sub.4, CsF, NaI(Tl),
CsI(Na), CsI(Tl), PbI.sub.2, etc. The position sensitive detector
modules can be made by mounting individual units on the detector
module as shown in FIGS. 7 and 8, or the detector itself can be
position sensitive (i.e., single or two-dimensional arrays of pads
or pixels) and one or more of these detectors can be mounted
together onto the calorimeter module board. Although a
two-dimensional 8 by 8 array is shown in FIGS. 7 and 8, the pixel
dimensions can be of any size or form. A possible two-dimensional
combination of CdZnTe pad detectors is shown in FIGS. 9 and 10.
These detectors can also be made with negligible dead perimeter
area so that they can be abutted to form uniform large area
two-dimensional arrays.
[0086] Calorimeter
[0087] In at least one embodiment of the invention, a calorimeter
is placed around and at the bottom of the silicon microstrip
detectors in order to absorb the escaping Compton scattered
photons. A variety of different high density radiation detectors
can be used. Many of these detectors are relatively high cost
(e.g., HPGe, BGO, CdWO.sub.4 and CsF) and several require cooling
to liquid nitrogen temperatures (e.g., HPGe).
[0088] Sodium Iodide is the most popular high density scintillator.
It has a large light yield and its response to electrons and gamma
rays is close to linear over most of the significant energy range.
The NaI(Tl) crystal is fragile and hygroscopic and therefore must
be handled carefully and hermetically sealed. It has long decay
time and is not suitable for fast timing applications.
[0089] Cesium Iodide is another alkali halide that has gained
substantial popularity as a scintillator material. It is
commercially available with either thallium or sodium as the
activator material and has significantly different scintillation
properties with thallium. CsI has a larger gamma ray absorption
coefficient per unit size and is less brittle than NaI. The two
forms of CsI scintillators, CsI(Na) and CsI(Tl), are discussed
separately below.
[0090] CsI(Na) has an emission spectrum similar to NaI(Tl). Its
light yield is also comparable. CsI(Na) is hygroscopic and must be
hermetically sealed. Therefore, CsI(Na) is similar to NaI(Tl) and
has the same draw backs.
[0091] CsI(Tl) is different than NaI(Tl) and has unique properties.
It is also only slightly hygroscopic. Energy resolution of 5
percent FWHM at 0.662 MeV has been obtained with 2.5 centimeter
diameter by 2.5 centimeter thick CsI(Tl) scintillation crystals
coupled to large area (e.g., 2.5 centimeter diameter) mercuric
iodide photodetectors. This is about 50 percent better than the
NaI(Tl) detectors. The mercuric iodide photodiodes are not yet
available as commercial devices. Resolution of 6 percent at 0.662
MeV has been obtained for considerably smaller CsI(Tl) crystals
using avalanche photodiodes. Large area PIN diodes coupled to 1
centimeter by 2 centimeter CsI(Tl) crystals give a 7 percent
resolution at 0.662 MeV. These crystals produce 35 percent more
photons per MeV than NaI(Tl) and their light spectrum is much
better matched to the sensitivities of the photodiodes. A key to
improved energy resolution is good light collection by matching the
areas of the crystals to those of the photodiodes.
[0092] An important property of CsI(Tl) is its variable decay time
for different particles. Therefore pulse shape discrimination
techniques can be used to differentiate among various types of
radiation such as electrons, protons and alpha particles. The
CsI(Tl) light output is quoted lower than NaI(Tl) for bialkali
photomultiplier tubes (PMTs). The scintillation yield is actually
found to be larger than that of any other scintillator because its
light emission peaks at longer wavelengths. It can be used with
photodiodes with extended response in the red region of the
spectrum. Its energy resolution is equal to or better than the
energy resolution of the NaI(Tl) crystals. For these reasons
CsI(Tl) crystals are used in at least one embodiment of the
invention.
[0093] CdTe, CdZnTe, HPGe and HgI.sub.2 are solid state detectors
and can be made in arrays for position sensitive applications. They
are high Z and high density crystals.
[0094] They are used to detect x-rays and gamma rays directly
without need for photomultiplier tubes or PIN and avalanche
photodiodes. They produce much better energy resolution than the
other detectors which require photomultiplier tubes or PIN and
avalanche photodiodes since they convert the energy deposited by
the x-ray and gamma ray photons into light, not electron-hole
pairs.
[0095] High purity germanium (HPGe) offers extremely high energy
resolution and exhibits excellent gamma ray absorption properties,
making it the detector of choice for high accuracy spectroscopy.
Unfortunately since it only works at liquid nitrogen temperatures,
bulky refrigeration systems are required which further increase the
cost of this detector. HPGe is available in single small crystals
and works by collecting the electron hole pairs produced inside the
crystal similar to the silicon detectors and does not require PMTs.
Because of the large cost this detector is mainly used for
applications which require ultra high energy resolution and small
size detectors.
[0096] BGO, CdWO.sub.4 and CsF are excellent high density and high
Z scintillators. They have lower energy resolution and light
output. Their maximum light emissions peak around 430 nanometers,
similar to NaI(Tl), and require PMTs for detection. CdWO.sub.4 and
especially CsF have shorter decay constants and faster rise times
than the others and can be used for timing. However, since the
preferred detector of the present invention does not use
time-of-flight to determine the direction of the scattered gamma
ray photon, good time resolution is not important.
[0097] The preferred room temperature detector for the calorimeter
of the present invention is CdTe or CdZnTe. These detectors are
described in more detail below.
[0098] Data Acquisition System
[0099] As noted above, an important technological requirement for
the present invention is the FEE chip with self trigger output to
readout the silicon strip and calorimeter detectors. In order to
provide a readout chip with a trigger output capability,
comparators can be incorporated into the readout chip to detect a
strong signal above the externally set threshold and the outputs of
the comparators can be fanned in using a fast OR circuit to produce
the single trigger output.
[0100] The preferred FEE chip is a 64 channel, charge sensitive,
mixed signal ASIC CMOS chip, a version of which is illustrated in
FIG. 11. If desired, similar chips with fewer or greater numbers of
inputs (e.g., 16 or 32 inputs) can be used based on the same
design. Each channel of the chip consists of an analog section and
a digital section. The input from the silicon strip detector is
directly coupled to a low noise, charge sensitive amplifier. The
outputs of the charge sensitive amplifier are connected to a shaper
amplifier. The time constant of the shaper amplifier is typically
on the order of 100 nanoseconds although other time constants
(e.g., 1 microsecond) may also be used depending on the embodiment
of the invention. The output of the shaper amplifier goes into the
track and hold (T/H) switch. The T/H switch can be controlled
externally or activated internally from the trigger output with a
delay set to turn on the hold at the peak of the shaped pulse. The
T/H switch is connected to the input of the buffer amplifier
through the voltage following capacitor. When the T/H switch is
open the voltage on the capacitor is held constant and the voltage
level is buffered on to the analog output switch. A shift register
connects each buffer output to the single analog output pin in
sequence, from input 1 to N, by an external clock input. The shift
register also has an external clear input to reset it and a clock
output to daisy chain it to other readout chips. Only one clock
input is sufficient if the clock outputs are connected in serial to
the clock inputs of the adjacent readout chips. The charge
sensitive amplifier outputs can be fanned out to comparators with a
common external level adjustment. The outputs of the comparators
can be fanned in through a fast OR circuit which will produce a
trigger signal if any comparator input exceeds the set threshold.
The trigger signal can also be used with a suitable delay to
control the T/H switches to apply hold signal at the peak of the
pulse from the shaper amplifier.
[0101] The data acquisition speed of the readout chip can be
increased using the extra versatility introduced by the
comparators. The design shown in FIG. 11 does not tell which strip
has the information so all strips are readout to find the strip
that has the signal. A logic circuit can be added to the design
which detects the channel with the largest signal from the
comparator outputs, applies a track and hold signal, and connects
the strip with the signal to the analog output pin. At the same
time it can encode the address of the strip that has the
information and output it as the address of the strip with the
signal. There could be an occasional signal on more than one strip.
Multi-hits can be detected and an output can be generated to warn
of a multi-hit signal. The trigger signals are generated for each
readout chip. They have to be externally processed for the
hodoscope in coincidence with the calorimeter to produce the single
trigger signal to activate the data acquisition system. For
extremely high signal rates this may not be possible. In such a
case each wafer or front end readout chip can be separately readout
in parallel using independent data acquisition electronics and
tagging each event time by using an accurate clock. The calorimeter
crystals are also individually readout and event times tagged by
the same clock. Since the calorimeter is running at much slower
speeds, individual readout modules are not necessary and can be
readout in groups.
[0102] The data readout can be carried out in parallel and can be
stored on-board using individual module memory. This is the key to
achieve fast data throughput rates. The data can be asynchronously
accessed by the host computer, analyzed and displayed on screen in
real time. Data acquisition rates of 1 to 10 MHz per readout chip
(or silicon wafer) are achievable.
[0103] A block diagram for a possible readout electronics system is
shown in FIG. 12. The electronics has two similar sections for the
hodoscope and the calorimeter readout. A true event is a
coincidence between the hodoscope and the calorimeter. Since most
of the time both sides observe more than a single interaction, a
fan-in system is used to convert the several trigger signals into
one master trigger. The fan-in can be designed to recognize a track
with adjacent planes producing the signal and to reject random
coincidence events.
[0104] The two master trigger signals from the hodoscope and the
calorimeter are sent to a coincidence unit to create the Compton
double scatter event trigger. The Compton double scatter trigger
signal is only generated if there is a master trigger signal employ
the time tagged data readout method.
[0105] The Compton double scatter event trigger activates data
acquisition for both the hodoscope and the calorimeter
simultaneously. CAMAC, Fastbus, VME, or VXI bus modules can carry
out the data acquisition. The CAMAC system is the most cost
effective. The custom designed data acquisition modules for the
hodoscope produce the necessary microstrip readout chip control
electronics such as the T/H (if not generated internally in the
readout chip), a clear signal to reset the shift registers, and the
clock pulse to multiplex each strip to the analog output.
[0106] The analog input channels from different hodoscope planes
are read out synchronously with the clock pulse output. The module
converts the pulse height information received from the analog
output pin to a digital number. In parallel with reading the
hodoscope data, it also digitizes the signal(s) from the
calorimeter. Immediately after reading out the last signal it
clears the hodoscope to reset the readout chip so that it can
receive the next event. It is assumed that the analog output of
each readout chip in each detector plane is fanned-in to allow a
single signal to be sent to the readout module. It is also possible
to design a microstrip readout chip that can internally connect the
strip which has the maximum signal to the analog output and also
produce the encoded address of the strip. In such a case the clock
output will not be necessary and the silicon microstrip detectors
can be readout asynchronously at a much faster rate.
[0107] The custom made CAMAC, Fastbus, VME, or VXI modules are
connected to the bus or crate controllers which are standard
devices and available off the shelf. The controllers connect the
modules to the data acquisition computer. Depending on the data
rate and readout overhead, single or separate computers can be used
to read the hodoscope and the calorimeter. The computer stores data
on a hard disk, optical drive, or nonvolatile RAM depending on the
application. If the data acquisition overhead is not high then one
of the computers can analyze the data in real time or a separate
computer can access the storage media asynchronously. The results
of the data analysis are imaged onto the field-of-view through a
display system in real time.
[0108] Data Analysis
[0109] For each gamma ray detected by the first and second
generation Compton double scatter detectors, an "event ring" on the
sky containing the source direction can be measured. The half-angle
of the cone, as illustrated in FIG. 2, is the Compton scatter
angle, .theta.. The overlap of the rings gives the source
direction. The angular resolution is the angular width of the ring.
This angular resolution depends on the energy resolution in the
hodoscope and the calorimeter as well as the geometric uncertainty
in the scattered photon direction. Measuring the recoil electron
direction in the Compton double scatter detector reduces the event
ring to an arc centered on the source direction as illustrated in
FIG. 2. It significantly reduces background and provides a true
imaging capability for Compton detectors. The event arcs produces
dramatic improvement on the signal to noise ratio compared to event
rings. The data analysis methods discussed below can be applied to
both types of data.
[0110] True imaging is achieved by determining the x and y
coordinates of the interactions in the first two planes of the
Compton recoil electron track in the silicon microstrip hodoscope.
With both the electron and scattered gamma ray directions known and
their energies measured, a unique direction is found for each
event.
[0111] Many different data analysis methods can be applied to the
data of the detection system. Depending upon the application, the
applied data analysis techniques may closely resemble those used in
medical Computer Assisted Tomography (CAT) imaging. This type of
imaging is based on the Radon transform and back projection
techniques and is standard in the industry. New iterative
techniques such as Maximum Likelihood and Maximum Entropy methods
can also be applied to enhance the image quality.
[0112] Another data analysis technique that can be applied to the
Compton double scatter data is the Direct Linear Algebraic
Deconvolution (DLAD) technique. One advantage of this technique
resulting from its non-iterative approach is that it can produce
fast images from the data.
[0113] A concise explanation of the DLAD technique is provided
below. The reconstruction of the source image from the Compton
double scatter data can be represented by the following general
formula:
D(.chi.,.PSI.,.phi.,E)=.intg..sub..chi.,.PSI.,EI(.chi..sub.0,.PSI..sub.0,E-
')R(.chi.,.PSI.,.chi..sub.0,
.PSI..sub.0,.phi.,E',E)d.chi..sub.0d.PSI..sub-
.0dE'+B(.chi.,.PSI.,.phi.,E)
[0114] In the above formula, D(.chi., .PSI., .phi., E) is the
actual Compton scatter data observed by the detector in appropriate
coordinates; .chi. and .PSI. are the coordinates of the rectangular
image plane; .phi. is the Compton scatter angle; E is the energy of
the incident photon; I (.chi..sub.0, .PSI..sub.0, E.sub.0) is the
true image of the source and is not a function of the Compton
scatter angle; R (.chi., .PSI., .chi..sub.0, .PSI..sub.0, .phi.,
E', E) is the response function of the detector; and B(.chi.,
.PSI., .phi., E) is the gamma ray background. Normally the
calculation is carried out for all energies within the detector
sensitivity to determine the total gamma ray flux and for certain
energy bands to obtain an energy spectrum. For application to the
present invention, the energy spectrum is used to discriminate the
scattered photon background. The calculation can also be done for
different scatter angle bands. D and I are normally referred to as
the data and the image spaces, respectively.
[0115] The response function in the DLAD technique is the
concentric rings obtained by mapping the scattered photon direction
vector in the image plane. This can be used as an ideal detector
response function. The true detector response function, R, can be
represented by
R.sub.ij,.phi..sub..sub.s=.epsilon.(E,.theta..sub.j,.phi..sub.s).multidot.-
.DELTA..phi..sub.s.multidot.PSF.multidot.G(.theta..sub.i)
[0116] where i and j define the bins in the data and image spaces,
respectively; .phi..sub.s is the calculated Compton scatter angle
as given by Compton scatter formula; .epsilon. is the detector
efficiency; .theta..sub.i and .theta..sub.j are the incident zenith
angles in data and image spaces, respectively; .DELTA..phi..sub.s
is the scatter angle interval; PSF is the point spread function;
and G(.theta..sub.i) is the geometric factor. The PSF is the
distribution of the scattered photon vectors in the image plane.
The PSF can be represented by the two dimensional normal
distribution
PSF=C(.theta..sub.j,.phi..sub.s)e.sup.-{[(.phi..sub.i
.sup.-.phi..sub.s.sup.).sup..sup.2.sup.]/[2.sigma..sup..sup.2.sup.(E)]}
[0117] where C is the normalization constant determined by the
requirement that PSF.times.G(.theta..sub.i) is equal to 1. The PSF
and G(.theta..sub.i) are symmetric in the azimuth, thus giving a
two-dimensional image. The present invention can produce
three-dimensional images due to the Compton scatter process.
Therefore, either two-dimensional image slices parallel to the
converter planes are produced or a direct three-dimensional image
can be constructed.
[0118] The DLAD technique can produce fluctuations on the image
space that are due to the geometric factor forcing data space to
zero at the comers and edges of the field-of-view where the data
may be scarce and the Poisson fluctuations are large. This effect
can be improved by applying the positivity requirement. The
positivity requirement is based on the fact that in image space one
cannot get negative fluxes. The positivity constraint has been
introduced into DLAD. The new technique is called Constrained
Linear Algebraic Deconvolution (CLAD).
[0119] System
[0120] FIGS. 13 and 14 illustrate the cross-section and a top view,
respectively, of one embodiment of the invention. In this
embodiment, a calorimeter 1301 surrounds a hodoscope comprised of
between approximately 1 and 100 silicon strip detector planes 1303,
and preferably between approximately 10 and 25 planes 1303.
Calorimeter 1301 is shaped to detect the large angle Compton
scattered photons. FIGS. 15-17 illustrate a variation on this
embodiment which includes a middle detector layer 1501 between the
hodoscope and calorimeter 1301. Detector layer 1501 is made of
position sensitive x-ray and gamma-ray detectors.
[0121] Preferably detector layer 1501 has a thickness of
approximately 0.1 millimeters to 1 centimeter. This layer performs
two functions. First, it increases the energy and angular
resolutions for the lower energy x-rays and gamma rays which will
primarily be stopped within this layer. Second, it allows for three
level scatter instead of two, thus providing more information on
the incident photon. For example, the total energy and direction of
the incident photon can be determined even if the photon makes a
Compton scatter in the calorimeter and escapes.
[0122] The silicon microstrip detectors 1303 preferably have a
thickness of between 200 and 300 micrometers, the selected
thickness being dependent upon the desired performance as well as
the availability of the detectors. Preferably detectors 1303 are
double-sided with approximately 1 millimeter pitch strips
orthogonal to each other on the two sides. In order to achieve the
desired detector plane area, each detector plane 1303 is preferably
comprised of four individual detectors 1801 bridged together to
form a square plane of approximately 100 square centimeters as
illustrated in FIG. 18.
[0123] Bridging detectors as illustrated in FIG. 18 decreases the
readout channel number and related electronics significantly.
Preferably ultrasonic bonding is used to form the bridges and to
connect them to readout chips 1803. Readout chips 1803 are mounted
as near as possible to detectors 1801 in order to minimize the size
of front end PC readout board 1805. The fan in from the detector
strips to readout chip pins are preferably gold plated for good
quality ultrasonic bonding.
[0124] FIG. 18 shows the junction side of board 1805. The strip
detectors on the ohmic side (i.e., the back side) of board 1805 run
orthogonal to the junction side so that both the x and y dimensions
of an interaction in the silicon are measured simultaneously. The
bridging on the ohmic side is similar to the junction side with the
position of ohmic side readout chips 1807 preferably being mounted
on the reverse side of board 1805. The output and control signals
for the readout chips are not shown as they depend on the chip
design.
[0125] A simple Monte Carlo calculation using MCNP software was
performed. FIGS. 19 and 20 provide the track lengths for incident
photon energies of 1 and 2 MeV, respectively, assuming 200
micrometer thick detector planes. For 1 MeV gamma rays the recoil
electron traverses an average of 3 detector planes. This increases
significantly as the energy of the incident photon increases. For 2
and 6 MeV gamma rays an average of 8 and 25 detector planes are
traversed, respectively. Depending upon the number of silicon
detector planes, typically the recoil electrons with long tracks
will escape the hodoscope and enter the calorimeter. This effect
does not present a problem since the missing energy of the recoil
electron in the hodoscope is recovered by adding the energy
deposited in the calorimeter, thus allowing the event to be fully
determined.
[0126] FIG. 21 illustrates the multiple scatter along a recoil
electron track produced through Compton scattering of a 2 MeV gamma
ray 2101. As electron 2105 loses energy traversing silicon planes
2101, the deflection in its trajectory increases significantly.
Thus the start and end of the recoil electron track can be
identified by measuring the deflections in the track at each
detector plane.
[0127] In a multiple scatter event, the multiple scatter angle,
.theta..sub.0, is inversely proportional to the momentum and
velocity of the particle. The multiple scatter angle increases
strongly with the decrease in the momentum and velocity as
demonstrated in the simplified formula:
.theta..sub.0={[14.1
MeV/c]/p.beta.}Z.sub.inc{[L/L.sub.R][1+({fraction
(1/9)})log.sub.10(L/L.sub.R)]}1/2
[0128] where p is the momentum in MeV/c, .beta. is the velocity,
Z.sub.inc is the charge number of the incident particle, and
L/L.sub.R is the thickness of the scattering medium in radiation
lengths. This formula is accurate to about 5 percent for a value of
L/L.sub.R between 10.sup.-3 and 10, excluding the cases of very
light elements or low velocity particles where the error is about
10 to 20 percent.
[0129] The multiple scatter angles can be calculated using the MCNP
program which utilizes the more rigorous Moliere theory. There is a
large increase in the multiple scatter angles towards the end of
the track due to the decrease in the momentum and the velocity of
the particle before it stops. At least 4 interactions are required
to get the minimum 2 multiple scatter angles per track that are
required to determine the direction of motion of recoil electron
2105.
[0130] The energy loss suffered by recoil electron 2105 at each
silicon detector plane 2103 is not uniform. The energy loss in each
detector plane 2103 increases as the kinetic energy decreases, thus
the energy deposited in the first interaction plane is normally
much lower than the energy deposited in the last plane where it
stops. This gives another signature to measure the direction of
motion for a recoil electron. The energy deposition of the recoil
electron is also calculated by the Monte Carlo program. For high
energy relativistic particles, the energy loss in a medium is
constant and minimum. Such high energy particles are referred to as
minimum ionizing particles. As the particle velocity decreases the
energy loss by ionization increases. A minimum of 2 interaction
points may be sufficient to apply this method since the energy
deposition is a scalar quantity. This allows the determination of
direction of motion of recoil electrons even with 2 interaction
points, also the minimum required to calculate the direction of the
recoil electron to reduce the event ring into an event arc. The
formula to calculate the direction of motion factor, F, if there
are only 2 or 3 interaction points in a track is given by:
F.sub.2,3=[E.sub.Last-E.sub.First]/[E.sub.Last+E.sub.First]
[0131] where E.sub.First and E.sub.Last are the energies deposited
in the first and last interaction points, respectively, assuming
that the recoil electron is moving into the silicon hodoscope from
front to back. The factor F varies from -1 to 1. Positive values
indicate that the recoil electron track is progressing from the
front towards the back of the detector while negative values mean
the opposite is true. The differential scatter angle and energy
loss can also be used to determine the direction of motion for the
recoil electron.
[0132] The factor, F, based on track lengths of at least 4
interactions, is revised to make use of the large differences in
the first and last energy deposition values as well as the strong
difference between the sum of the first and second halves of the
multiple scatter angle distribution. The revised formula is given
as: 2 F 4 = E Last i SecondHalf i - E first i FirstHalf i E Last i
SecondHalf i + E First i FirstHalf i
[0133] where .theta. is the multiple scattering angle of the recoil
electron at each detector plane and E.sub.First and E.sub.Last are
the energy deposited in the first and last interaction points,
respectively. Other techniques can also be used to determine the
direction of the recoil electron track.
[0134] The interactions of the incident photon in the hodoscope and
the calorimeter produce an ambiguity since it is not known whether
the photon made a Compton scatter in the hodoscope or calorimeter
first. The situation is completely symmetric. If this ambiguity is
not resolved then the direction of the background photons incident
on the back of the detector can be mistaken for true events
incident on the detector aperture. Application of the direction of
motion determination described above can resolve this ambiguity
since the direction of the incident photon can be derived from the
direction of the recoil electron.
[0135] The summation is carried out for the first and second halves
of the recoil electron track, assuming the track is moving from the
front to the back of the hodoscope. Therefore if F is positive, the
track is progressing from the front to the back of the hodoscope
while if F is negative, the opposite is true. If F is zero, the
direction is indeterminate.
[0136] The effectiveness of this technique was tested for many
tracks of gamma ray of predetermined energy. The results of the
calculation for 2 MeV gamma rays showed that about 4 percent of the
calculated events are negative and resemble upward moving recoil
electron tracks progressing from the back of the hodoscope to the
front. This is a small effect and decreases at higher incident
photon energies. For example, the tracks mistakenly calculated to
be moving backward increases to 11 percent at gamma ray energies of
1 MeV and decreases to 2 percent at 6 MeV.
[0137] In the embodiment of the invention illustrated in FIGS.
13-17, calorimeter 1301 is preferably made from CsI(Tl) crystals
viewed by photodiodes. The calorimeter can also be built from
NaI(Tl) crystals or any other high density scintillator with good
characteristics. The calorimeter is formed in the shape of a well
surrounding all sides of the hodoscope except the front aperture.
The selected calorimeter material as well as the amount of area
covered is primarily dependent upon the cost of the scintillator
for a given thickness. The high density scintillator also acts as
an excellent active shield for background gamma rays.
[0138] The position sensitive CsI(Tl) crystal calorimeter is
preferably constructed from rectangular bars with a cross-section
between approximately 1 square centimeter and over 6 square
centimeters and a length varying from 1.5 to 2.5 centimeters. The
wide variation of length is due to the energy range of detection.
The bottom section of the calorimeter needs longer crystals since
the forward scattered gamma rays carry most of the primary photon
energy. The CsI(Tl) crystals on the side walls of the calorimeter
can be short, as the photons with large Compton scatter angles
carry a much smaller fraction of the primary photon energy. Since
the Compton scatter angle increases with the height of the
calorimeter at the side walls, a tapered crystal length can be
used; longer CsI(Tl) crystals can be placed on the walls near the
bottom of the calorimeter and the crystal length can be gradually
reduced upwards toward the rim.
[0139] If desired, for example to reduce the costs associated with
the calorimeter, the height of the calorimeter side walls can be
reduced to below the top of the silicon hodoscope. This will cause
backscattered gamma rays at sufficiently large angles to miss the
calorimeter and not be detected. Given that the probability of
backscattered gamma rays is lower than that for forward Compton
scatters, especially at higher incident photon energies, this loss
is negligible for many cases.
[0140] In this embodiment, the calorimeter is comprised of a mosaic
of individual CsI(Tl) crystals where each crystal is individually
viewed by a photodiode. As a result of this configuration, the
number of photodiodes required is the same as the number of
crystals. An alternative method illustrated in FIG. 22 uses flat
CsI(Tl) crystals 2201 with sufficient size to cover the bottom and
the sides of the calorimeter. Each flat crystal 2201 is viewed by a
plurality of photodiodes 2203 placed at equal distance to each
other on the back surface of crystal 2201. The position of the
interaction point is determined from the centroid of the pulse
heights observed by adjacent photodiodes surrounding the
interaction point.
[0141] Preferably an anti-coincidence shield 1305 surrounds all
sides of the detection system as shown in FIGS. 13-17. Shield 1305
is mainly used to veto charge particles such as electrons,
positrons and protons incident on the detector from all directions.
Charged particles deflect in the Earth's magnetic field and produce
a nearly isotropic background. The best low cost material for an
anti-coincidence shield is a fast plastic scintillator such as
NE-102A with a thickness of between 0.5 and 1.5 centimeters. If an
anti-coincidence shield is used, typically the aperture of the
detection system uses a thinner scintillator in order to reduce the
Compton scatter of gamma rays in the plastic which change the
direction of the incident gamma rays. Since the plastic
scintillators have low density and low average Z, the Compton
scatter probability is very low and a thickness of up to 1.5
centimeters can be used in front of the detector aperture without
significant effect. For applications where charge particle
background is not significant, shield 805 can be omitted.
Furthermore the top hodoscope layer can serve as an
anti-coincidence shield by requiring that there is no track in this
layer in all events that are accepted as x-ray and gamma-ray
events.
[0142] i) Energy and Angular Resolution
[0143] Position resolutions are assumed at the hodoscope to be
approximately 1 millimeter by 1 millimeter (i.e., double-sided
silicon microstrip detectors with a 1 millimeter pitch) and at the
calorimeter of approximately 1 centimeter by 1 centimeter. The
calculations were carried out for 0.5, 1, 2, 6, 10, and 25 MeV.
Compton scatter angles were not restricted and threshold energies
of 0.05 MeV were applied to the hodoscope and the calorimeter.
[0144] The energy resolution of a gamma ray photopeak depends upon
the electrical noise, the light collection efficiency, and the
intensity spread of the collected light. The electrical noise,
however, contributes only a small amount to the full width at half
maximum (FWHM) resolution of the photopeak.
[0145] When a scintillator is mounted onto a photodiode, the
photopeak width increases. The effect of the light collection
efficiency from the scintillator results in a much broader peak
than with a bare photodiode. For example, the efficiency of light
collection from a pulse close to the photodiode surface, near the
center of the scintillator, may be significantly different from the
collection efficiency when the pulse is from a skewed location. A
measured photopeak will be a convolution of all of these results of
varying pulse locations. The result is a wide peak. Typical FWHM
values even with low electrical noise are about 45 channels. Since
both sources of "noise" are independent of each other, the half
width at half maximum (HWHM) C can be written as
.sigma..sub.Total.sup.2=.sigma..sub.Opt.sup.2+.sigma..sub.Elec.sup.2
[0146] where .sigma..sub.Total is the total measured half-width,
.sigma..sub.Elec is the half-width contribution due to electrical
noise, and .sigma..sub.Opt is the spread due to the
scintillator/photodiode geometry. Since the electrical part and the
total part are known from measurements, the optical component can
be estimated to be
.sigma..sub.Opt.sup.2=.sigma..sub.Total.sup.2-.sigma.Elec.sup.2.
Thus a typical value for a 1.0 by 1.0 by 1.75 cubic centimeter
CsI(Tl) crystal is 2.sigma..sub.Opt=40. This means that the spread
in the photopeak has very little to do with the electrical noise
performance of the photodiode. That is, with these numbers, it
makes little difference if the photodiode noise FWHM is 20 or 22
channels (10% noisier); the total photopeak width would only
increase from 45 to 46 channels, causing the resolution to go from
5 percent to 5.1 percent. It also points out that any effort to
reduce noise by lowering the temperature will only be beneficial if
the noise is reduced by a large fraction.
[0147] ii) Effective Area Efficiency Factor and Sensitivity
[0148] Untracked events are detected when the Compton scattered
recoil electron stops in the same silicon detector in which it is
created. In this case the recoil electron direction cannot be
measured and the direction of the incident electron is only known
as a ring in the field-of-view. If the recoil electron traverses
two or more silicon detectors then the incident gamma ray direction
is restricted to a short arc in the field-of-view. For incident
gamma rays with energies above 1 MeV most of the events are tracked
while for incident gamma rays with energies below 1 MeV most event
are untracked, assuming silicon detector thicknesses of between 200
and 300 micrometers.
[0149] To find the effect of the angle of incidence on the detector
efficiency, the response of the detector to gamma rays generated
with uniform isotropic distribution must be studied. The results
show that the detector efficiency is high for gamma rays entering
the detector aperture with zenith angles from 0.degree. to
60.degree.. The gamma rays incident from the side and back of the
detector at zenith angles from 90.degree. to 180.degree. have a low
efficiency as expected. The efficiency for gamma rays incident from
the front of the detector is about a factor of 30 higher than for
gamma rays incident from the back.
[0150] The sensitivity of this detector is best for incident gamma
rays between 1 and 5 MeV. At higher energies the sensitivity is
reasonably constant, but there is some loss of sensitivity as the
stopping power of the detector decreases with an increase in the
energy due to the smaller size of this embodiment of the detector.
At lower energies the sensitivity is also reduced as the large
angle Compton scattered gamma rays that miss the calorimeter are
lost.
[0151] iii) Detection of Pair Produced Events
[0152] Pair produced events are different than Compton scattered
events. The signature of a pair produced event in the present
invention is two simultaneous tracks in the form of an inverted V
with a single common vertex point in the silicon hodoscope. The
dual track is due to the electron-positron pair created in the
hodoscope. The inverted V track is accompanied by one or two 0.511
MeV interactions in the calorimeter resulting from the absorption
of the 0.511 MeV photon pair created by the annihilation of the
positron. One or both of the electron-positron pair can escape the
hodoscope and enter the calorimeter. These will be legitimate
events and the missing particle energies can be obtained from the
calorimeter since the tracks of these particles are already
measured and their position of interaction at the calorimeter can
be determined. The pair production starts to become important for
incident photon energies above 5 MeV.
[0153] Examples of System Applications
[0154] As illustrated in FIG. 23, a detection system 2301 according
to the above-described embodiment can be mounted within the cone of
a scout interceptor missile 2303, i.e., an interceptor missile
launched prior to the launch of other interceptors. The scout
interceptor is timed to intercept an ICBM bus 2305 at the initial
stages of RV deployment. The detector of the present invention is
used to measure the direction and energy of the emitted gamma rays
for each of the RVs in real time. The images of the RVs obtained by
the onboard radar or infrared sensors can be superimposed onto the
computer simulation provided by the present invention as
illustrated in FIG. 24, thus allowing decoys 2307 to be rapidly
distinguished from warheads 2309. In the scenario shown in FIG. 24,
target RV 2401 is a decoy.
[0155] In the present application, the flight time prior to impact
is quite short, resulting in negligible diffuse cosmic background
gamma ray flux on the order of 0.6 photons in a 10 millisecond
period. As scout interceptor 2303 approaches target 2401, the
images of the RVs move radially outward from the center of the
field-of-view. The direction of motion of the RVs can be determined
in real time from the information obtained by the detection system
of the present invention. This information as well as the
information on which RVs contain warheads can either be transmitted
to the ground control or to the interceptors following the
scout.
[0156] FIGS. 25 and 26 show the results of the calculation of the
total number of photons detected against time or distance before
impact, assuming that the nuclear warhead encapsulated within the
carrier missile produces 5.times.10.sup.8 gamma rays per
second.
[0157] The area-efficiency factor for the present embodiment of the
invention is taken as 500 square centimeters. The RVs and the scout
missile are assumed to approach each other at a rate of 7
kilometers per second. The formula for the accumulated counts, C,
is derived by integrating the count rate formula:
i
dC/dt=(LA.epsilon.)/(4.pi.R.sup.2)={(LA.epsilon.)/(4.pi.[vt].sup.2)}
[0158] to obtain the formula for counts:
C={(LA.epsilon.)/(4.pi.v.sup.2)}{(1/t.sub.1)-(1/t.sub.0)}
[0159] where L is the source luminosity (5.times.10.sup.8
.gamma./s), A.epsilon. is the area-efficiency factor (i.e., 500
cm.sup.2), R is the distance to the source, v is the approach speed
(14 km/s), t.sub.0 and t.sub.1 are the time to impact at the start
and at the end of integration. If t.sub.0 is large compared to
t.sub.1, C is proportional to 1/t.sub.1.
[0160] As the distance between the scout interceptor and the target
decreases, the number of detected photons increases by a factor of
1/.sub.to impact. The detection of a significant number of photons
starts at approximately 10 milliseconds before impact. The 10
millisecond time before impact is equal to a distance of about 140
meters. In the 10 millisecond time interval, the number of diffuse
galactic background gamma ray photons
(F.sub.Diff=1.1.times.10.sup.-2 E.sup.-2.3 photons cm.sup.-2
s.sup.-1 sr.sup.-1 MeV.sup.-1) detected is negligible (i.e.,
approximately 0.6 photons). Since there is practically no
background counts during the observation interval, even a few gamma
rays detected from the source are significant especially if the
detected gamma rays coincide with the position of one or more of
the monitored RVs.
[0161] Therefore the proposed technique requires 10 milliseconds to
detect, analyze, and transmit the information to the ground control
or to the interceptors following the scout. The information to be
transmitted contains the number of RVs, their position and
direction of motion, and the identity of the RVs containing
warheads. If additional discrimination time is required, either a
slower scout interceptor missile can be used or the area-efficiency
factor of the detection system can be increased.
[0162] The application of the invention illustrated in FIGS. 23 and
24 assume that the RVs are all deployed within a relatively short
time. An alternative approach is for the RVs to be deployed one by
one. In such a case a different technique can be applied in which
the scout approaches the first RV in a collision course. If the
scout determines that the first RV is a decoy, it alters its
direction to bypass the first RV and initiates a collision course
with the second RV. This process continues until a RV containing a
warhead is detected. Once such a RV is detected, the interceptor is
allowed to collide with the RV, destroying it in mid-course. One
method of improving this technique is to use a slow scout
interceptor with high maneuverability.
[0163] In an alternate configuration, the scout interceptor is
aimed to meet and follow the ICBM bus or the RVs. This technique
allows much longer observation times for the scout, on the order of
minutes compared to milliseconds for the head-on interception.
Assuming an observation time of approximately 5 minutes and a
requirement of 20 counts per RV to detect the warheads with high
significance and without ambiguity, then the maximum range of
warhead detection is about 5.5 kilometers. In 5 minutes, the
diffuse galactic gamma ray background radiation produces
approximately 1 gamma ray event per 1.degree. by 1.degree. sky bin
which is the FWHM angular resolution. Therefore, the minimum signal
to noise ratio is about 20. This technique can be effective for
exoatmospheric midcourse warhead discrimination if fast deployment
of scout interceptors can be achieved.
[0164] Alternatively, the present invention can be used to observe
covert satellites carrying nuclear warheads, assuming that the
warheads have similar gamma ray emissions as the missile based
warheads described above. In this case the present detection system
could be tens of kilometers away, assuming a detection time on the
order of hours in order to provide a statistically significant
number of photons. Assuming a distance of 1 kilometer, an energy
spectrum of 10,000 photons can be obtained within about one hour.
The higher energy photons above 1 MeV with the characteristic
nuclear lines can be used as the signal for final
identification.
[0165] In another configuration the RVs can be irradiated, for
example using a directed energy weapon, particle beam, or small
scale neutron bomb, and the produced secondary emission can be used
for discrimination purposes. In this configuration much higher
numbers of gamma rays are expected, leading to warhead
discrimination at distances on the order of 1,000 kilometers. The
estimated gamma ray rate that may be observed from a neutral
particle beam irradiated warhead at a range of 1,000 kilometers is
0.1 to 1 photon cm.sup.-2 s.sup.-1. Therefore in 1 second 50 to 500
photons with energies reaching up to 10 MeV will be observed from
such an interaction. This allows the detection system of the
present invention to be mounted on a stationary space based
platform. Its insensitivity to neutrons, wide field-of-view and
imaging capability will enable discrimination and/or kill
determination within a few seconds of the interaction.
[0166] An intact warhead contains significant amounts of shielding
inside thus significantly reducing the amount of gamma rays
observed from the outside as long as the casing is intact. During a
kill the warhead structure is disrupted, leading to a much larger
release of gamma rays that can last for seconds. Assuming a factor
of 1,000 increase in the gamma ray flux, a detection system
according to the present invention as described above is expected
to detect 100 gamma rays from the killed warhead for a duration of
1 second and at a distance of about 5 kilometers.
[0167] Precursor nuclear blasts can be employed in space before a
nuclear attack in order to disrupt communications and defense
systems. During such a blast the number of gamma rays released is
extensive. For example, at 1,000 kilometers the gamma ray flux from
a one megaton fission yield is approximately 10.sup.7 photons
cm.sup.-2 s.sup.-1 with energies reaching up to 10 MeV. The present
invention can be used to monitor such explosions from a distance
and determine their position, extent, and duration. At the above
photon fluxes, a small detection system in accordance with the
present invention can be mounted on a fixed space platform to
monitor precursor blasts from distances in excess of 10,000
kilometers. The same detection system could also be used to monitor
nuclear powered satellites.
[0168] Inspection System
[0169] In another embodiment of the invention illustrated in FIG.
27, the detection system is used with 80 to 2,000 keV x-rays to
provide a 2-dimensional imaging system, for example for use as a
real-time munitions inspection system. FIG. 27 is an illustration
of an embodiment of the invention for use as a 2-dimensional
inspection imaging system.
[0170] A strong x-ray source is required for real-time inspection
of munition items with 2-dimensional imaging. As this configuration
requires that at least some portions of the detector be in the
direct path of the x-ray beam, the system must be highly resistance
to radiation damage. In the illustrated embodiment of the
invention, only the active area of silicon microstrip hodoscope
2701 is in the direct path of the x-ray beam, a portion of the
detection system which is expected to exhibit negligible effects
under the desired x-ray source flux. The radiation damage to the
FEE chips is also insignificant as these chips, in the preferred
embodiment of the invention, are placed at the perimeter of the
hodoscope and are therefore effectively shielded from the incident
x-ray beam. Alternatively, the FEE chips may be manufactured using
radiation hardened electronic circuits.
[0171] In one embodiment of the invention, a source 2703 provides
x-rays of relatively low energy, on the order of 80 to 300 keV. In
this embodiment the detection system only uses silicon microstrip
hodoscope 2701 as opposed to a combination of a hodoscope and a
calorimeter. For imaging a small object 2704, the area of each
detection plane 2705 of hodoscope 2701 is typically between
approximately 16 square centimeters and 144 square centimeters with
a plane thickness of between 0.5 and 1 millimeter. Assuming a total
silicon thickness of between about 2 and 10 centimeters, the
Compton scatter probability ranges from 45 to 95 percent for 300
keV x-rays. Pixel sizes can be as low as approximately 625 square
micrometers for super high resolution imaging. The object size must
be at most equal to the detector area, otherwise only a section of
the object is imaged.
[0172] The detector of this embodiment can be used with both energy
and angular discrimination of the scattered photon background. In
the single scatter mode it will accumulate all events without any
cut. In the double scatter mode it will measure the total energy
deposited in the hodoscope from multiple Compton scatters ending
most likely with photo-absorption. The direction of the incident
photon can also be determined from the energies deposited at each
interaction point and the pixel coordinates of the first and the
last scatters. As this embodiment does not include a calorimeter
section, it exhibits excellent energy and angular resolution and
can be used with both monoenergetic and continuous energy x-ray
sources.
[0173] Although the present hodoscope only embodiment can be used
with a higher energy x-ray source 2703 in order to accommodate
larger objects 2704, this approach would require that detection
planes 2705 have a much larger area and that hodoscope 2701 be
comprised of many more planes 2705. A more practical solution is to
combine hodoscope 2701 with a calorimeter 2707 placed behind a
collimating shield 2709.
[0174] As illustrated in FIG. 27, the collimated x-ray beam from
source 2703 is incident on object 2704 under inspection. Although
collimating shield 2709 may not be necessary if the x-ray beam is
already collimated to the size of hodoscope 2701, preferably shield
2709 is used to shield calorimeter 2707 and the hodoscope
electronics from the x-ray source.
[0175] Calorimeter 2707 surrounds the sides of silicon microstrip
hodoscope 2701. As illustrated, an x-ray photon 2711 that is
unscattered in object 2704 makes a Compton scatter in one of the
silicon microstrip hodoscope planes 2705 and stops in calorimeter
2707. Whether or not a detected photon from a monoenergetic source
is scattered in the test object will be determined by the total
energy of the photon deposited in the detector. The elimination of
the photons scattered in the test object significantly decreases
the background and enhances the signal-to-noise ratio. Calorimeter
2707 can also be shielded from the background photons scattered in
the test object by placing a separate shield 2713 between test
object 2704 and calorimeter 2707, thereby reducing the calorimeter
count rates.
[0176] In this embodiment, source 2703 is monoenergetic in the
range of about 300 to 2,000 keV. Suitable sources include
Cs.sup.137 and Co.sup.60. Energy cuts within the detector energy
resolution are sufficient to eliminate much of the scattered photon
background.
[0177] The preferred embodiment of the munitions inspection system
can be used with both monoenergetic and continuous energy sources
(e.g., 80 to 2,000 keV) for medium to large size test objects. In
this embodiment the calorimeter must be position sensitive to
enable the determination of the incident photon direction. To
achieve sufficiently good direction determination the hodoscope
pixel size is preferably less than 1 square millimeter and the
calorimeter pixel size is preferably less than 1 square centimeter.
The calorimeter preferably absorbs Compton scattered photons with
about 90 percent efficiency.
[0178] In this embodiment the scattered photon background is
eliminated by measuring the incident photon direction. FIG. 28
demonstrates the method of discrimination of the scattered photon
background. The figure shows a x-ray 2801 scattered in test object
2704 as well as an unscattered x-ray 2803. The Compton scatter
angle is calculated using the formula given above from the energies
deposited in the two interaction points, assuming that the photon
is totally absorbed in the second scatter. The scattered photon
vector is determined from the coordinates of the first two
interaction point pixels in the hodoscope. An event ring 2805
determined for the unscattered event passes through the known x-ray
source direction while an event ring 2807 for the scattered event
does not.
[0179] Depending on the energy resolution and the geometry of the
two pixels, some of the event rings from scattered photon
background may overlap the point source direction. Higher energy
and geometric resolution improves scattered photon background. A
much more significant improvement can be made if the first recoil
electron is tracked through the hodoscope. This will limit the
event ring to an arc and the chance probability of a scattered
photon arc with the x-ray source direction diminishes
dramatically.
[0180] Another embodiment of the invention is an energy resolved
tomography imager for use with a continuous x-ray source. In
addition to background-discrimination using the incident photon
direction measurement for a continuous energy x-ray source, the
energies of the unscattered photons are determined. The energy
spectrum for each image pixel can be measured leading to an energy
resolved tomography imaging system, assuming that either the test
object or the detector is rotated. This system can also produce
two-dimensional images with energy spectrum information.
[0181] The Compton scatter depends strongly on the electron content
of the material the x-ray beam traverses. Therefore the energy
spectrum obtained for each pixel represents, with inverse
proportionality, the electron content in the test object along that
path. The different energy spectra can be represented by different
colors and the intensity can be represented as the brightness of
individual colors. This system can be used for real-time imaging of
objects (e.g., munitions), medical imaging, and radiography.
[0182] In this embodiment calorimeter 2707 is position sensitive
and is comprised of CsI(Tl) crystals. The crystals are preferably
rectangular bars with cross-sectional areas in the range of 1
square centimeter to 6.25 square centimeters with a length in the
range of approximately 1.5 to 2.5 centimeters. The length is
primarily driven by the detection energy range. The bottom portion
of the calorimeter requires longer crystals since the forward
scattered gamma rays carry most of the primary photon energy, thus
providing extra thickness to stop the higher energy. The CsI(Tl)
crystals on the top portion of the calorimeter can be short, as the
photons with large Compton scatter angles carry a smaller fraction
of the primary photon energy. Therefore in this embodiment the
calorimeter is comprised of a mosaic of individual CsI(Tl)
crystals. Since each crystal is individually viewed by a
photodiode, the system requires the same number of photodiodes as
crystals.
[0183] Assuming a data rate of 100 kHz to 1 MHz, 1 millimeter thick
hodoscope silicon detector planes with a 25 square centimeter area,
and a factor of 100 photon attenuation at the test object, a 662
keV energy x-ray source with a maximum of 2.times.10.sup.9 photons
per square centimeter per second (unattenuated at the hodoscope)
can be utilized in the present embodiment. At a 1 MHz data
acquisition rate, approximately 10,000 events are accumulated in 1
second per 1 square millimeter pixel, assuming 25 hodoscope planes
while at 10 MHz approximately 100,000 events are accumulated per
pixel. Therefore sufficient information is obtained in seconds for
real-time imaging.
[0184] In one embodiment, the calorimeter modules are positioned
close to the forward direction in order to obtain higher angular
resolution and detector efficiency. FIG. 29 is an illustration of a
configuration in which a plurality CsI(Tl)/photodiode modules 2901
are placed on a hemisphere, preferably with a radius of
approximately 50 centimeters. More detector modules 2901 are placed
in the forward direction with respect to incident beam 2903. A hole
2905 with a 15.degree. solid angle is left at the center of the
calorimeter to allow unscattered photons to pass.
[0185] Positron Imaging System
[0186] The positron is the antiparticle of the electron which has
the same mass but opposite charge. Some radionuclides reduce their
excessive positive nuclear charge by the emission of positrons.
After a positron is emitted from a radionuclides it loses its
kinetic energy through a series of ionizations and excitations in
matter until it slows down and forms an atom-like structure called
"positronium" with an electron. Positronium has a very short life
time, 10.sup.-10 seconds for the para state, and annihilates into a
pair of photons in the para state or into 3 photons in the ortho
state. Approximately 99.7 percent of the annihilation radiation
comes from the para state since the ortho state has to go through a
forbidden transition. Since both the electron and the positron are
nearly at rest, to conserve energy and momentum the two photons fly
off in opposite directions in a nearly perfect straight line, each
carrying an energy of about 511 keV which is the rest mass of
electron and positron. There is a slight non-collinearity of about
180.degree..+-.0.25.degree. due to the small but finite energy of
the positronium just before annihilation.
[0187] The present invention can be used to observe the presence of
positrons in tissues by detecting the generated annihilation
radiation. The simultaneous emission of two photons traveling in
opposite directions renders this process extremely effective.
[0188] The table below shows some of the positron sources that can
be used as radio-tracers with the present invention. Since most of
these sources are radioactive counterparts of normal constitutes of
living beings, they are excellent for the production of
radiopharmaceuticals. Their short life-times are also useful in
administration of large doses without the risk of significant
radiation exposure to the patient. Fluorine-18 is the preferred
radio-tracer since it has the shortest range in body tissue (i.e.,
approximately 2.4 millimeters) and the longest life-time (i.e.,
approximately 109.7 minutes).
1 Half-Life End-Point Max Range Radionuclide (min) Energy (MeV) %
.beta..sup.+Decay (mm) Carbon-11 20.4 0.97 99.8 4.1 Nitrogen-13
9.96 1.19 100.0 5.1 Oxygen-15 2.07 1.7 99.9 7.3 Fluorine-18 109.7
0.635 96.9 2.4 Gallium-68 68.1 1.88 90.0 8.1 Bromine-75 101.0 1.70
76.0 7.3 Strontium-82 1.3 3.15 96.0 15.6
[0189] An embodiment of positron imaging system according to the
present invention is shown in FIG. 30. As illustrated, an object
3001 such as a human skull is surrounded by a plurality of
detection systems 3003. Each detection system 3003 is comprised of
strip detectors 3005 and a calorimeter 3007. Preferably the systems
use CdZnTe strip detectors and CdZnTe calorimeters.
[0190] The first interaction point in CdZnTe strip detectors 3005
gives the coordinates of the Compton interaction within a 4 square
millimeter pixel size, assuming that double-sided strip detectors
are used is orthogonal strips on each side and a pitch of 2
millimeters. The coincident detection in two different detector
banks defines a positron annihilation photon pair chord, such as
pair 3009. By positioning strip detector planes 3005 extremely
close to one another, the detector depth can be reduced. The size
of the detector planes can be increased progressively in the radial
direction to reduce the gap between each detector bank or the
CdZnTe strip detectors can be placed overlapping each other by
increasing the plane separation.
[0191] As illustrated, photon pairs are created inside the tissue
of object 3001 from positron annihilations and detected in CdZnTe
strip detectors 3005 by undergoing Compton scattering and
photoelectric absorption. Photons can also be scattered inside the
tissue, thereby producing the scattered photon background. Other
photons are absorbed in the body and produce the gamma ray
attenuation that can be corrected by well known methods. These
background photons have lower energies because of initial
scatterings and are rejected as described above. Occasionally
photons will make more than one Compton scatter in a CdZnTe strip
detector bank. These are legitimate events as the full energy of
the photon is measured by adding the energies observed at each
interaction point. Some photons may escape with significant energy
after the last interaction. These events will be rejected as
scattered photon background. The stopping power of the whole
detector can be matched to that of BGO crystals by either
increasing the number of CdZnTe strip detectors or putting a
calorimeter plane at the back of the CdZnTe strip detectors as
illustrated in FIG. 30. The present invention can also be made in
an elliptical or curved shape to fit the contours of the human body
since the 8 cubic millimeter voxel size virtually eliminates the
radial elongation. The detector can be protected from photons
coming from different parts of the patient by using a shield 3011
(e.g., lead) around the outer perimeter of each detection system
3003.
[0192] The coincidence setup follows the fan angle technique. Fan
tangents 3013 are drawn from each detector bank to the organ under
study as shown in FIG. 30. The banks which are covered by the fan
beam are put in coincidence with the originator. A smaller number
of banks (e.g., 16) make this complex technique feasible.
[0193] In an alternate embodiment illustrated in part in FIG. 31,
each detection system 3003 is separated by a calorimeter septa
3101. If individual hodoscope/calorimeter modules are used as shown
in FIG. 30, the modules can either completely surround the patient
or some even number of diametrically opposed modules can be used.
In the latter case, the detector modules may be rotated to form the
desired tomographic images.
[0194] In another embodiment of the detection modules illustrated
in FIG. 32, the calorimeter is built from two sections. Surrounding
hodoscope 3201 is a thin inner detector 3203. Surrounding detector
3203 is a thick outer detector 3205. Inside detector 3203 can be
made from CdZnTe strip or pad detectors to produce fine position
resolution. Outer detectors 3205 can be thicker and may have fine
or even low spatial resolution. The calorimeter and hodoscope are
shown in rectangular structure but in actual implementation they
can have any shape, for example they can be curved into spherical
or parabolic configurations to maximize the efficiency,
sensitivity, angular and energy resolutions.
[0195] As will be understood by those familiar with the art, the
present invention may be embodied in other specific forms without
departing from the spirit or essential characteristics thereof.
Accordingly, the disclosures and descriptions herein are intended
to be illustrative, but not limiting, of the scope of the invention
which is set forth in the following claims.
* * * * *