U.S. patent application number 09/992620 was filed with the patent office on 2002-06-27 for semiconductor radiation detector with enhanced charge collection.
This patent application is currently assigned to Digirad Corporation, a California corporation. Invention is credited to Apotovsky, Boris, Butler, Jack F., Conwell, Richard L., Doty, F. Patrick, Friesenhahn, Stanley J., Lingren, Clinton L..
Application Number | 20020079456 09/992620 |
Document ID | / |
Family ID | 27067171 |
Filed Date | 2002-06-27 |
United States Patent
Application |
20020079456 |
Kind Code |
A1 |
Lingren, Clinton L. ; et
al. |
June 27, 2002 |
Semiconductor radiation detector with enhanced charge
collection
Abstract
A radiation detector for detecting ionizing radiation. The
detector includes a semiconductor having at least two sides. A bias
electrode is formed on one side of the semiconductor. A signal
electrode is formed on a side of the semiconductor and is used to
detect the energy level of the ionizing radiation. A third
electrode (the control electrode) is also formed on the
semiconductor. The control electrode shares charges induced by the
ionizing radiation with the signal electrode, shielding the signal
electrode until the charge clouds are close to the signal
electrode. The control electrode also alters the electric field
within the semiconductor, such that the field guides the charge
clouds toward the signal electrode when the clouds closely approach
the signal electrode. As a result, signal loss due to trapped
charge carriers (i.e., electrons or holes) is minimized, and
low-energy tailing is virtually eliminated. A fourth electrode can
be added to separate the charge-shielding and field shaping
functions of the control electrode. More electrodes can be added to
further enhance both functions. The invention can be used in
several cross-strip detector configurations, in a side-entry
radiation detector, and with liquid/gas ionization detectors.
Inventors: |
Lingren, Clinton L.; (San
Diego, CA) ; Butler, Jack F.; (La Jolla, CA) ;
Apotovsky, Boris; (San Diego, CA) ; Conwell, Richard
L.; (Del Mar, CA) ; Doty, F. Patrick; (San
Diego, CA) ; Friesenhahn, Stanley J.; (Poway,
CA) |
Correspondence
Address: |
JAMES T. HAGLER
Fish & Richardson P.C.
Suite 500
4350 La Jolla Village Drive
San Diego
CA
92122
US
|
Assignee: |
Digirad Corporation, a California
corporation
|
Family ID: |
27067171 |
Appl. No.: |
09/992620 |
Filed: |
November 13, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09992620 |
Nov 13, 2001 |
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09542386 |
Apr 4, 2000 |
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09542386 |
Apr 4, 2000 |
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08943492 |
Oct 3, 1997 |
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08943492 |
Oct 3, 1997 |
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08881175 |
Jun 23, 1997 |
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08881175 |
Jun 23, 1997 |
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08542883 |
Oct 13, 1995 |
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Current U.S.
Class: |
250/370.01 |
Current CPC
Class: |
H01L 27/14603 20130101;
G01T 1/241 20130101; H01L 27/14659 20130101; H01L 27/14676
20130101; G01T 1/243 20130101 |
Class at
Publication: |
250/370.01 |
International
Class: |
G01T 001/24 |
Claims
What is claimed is:
1. A radiation detector, comprising: (a) a semiconductor having a
plurality of sides and a thickness of at least about 0.5 mm; (b) at
least one bias electrode formed on at least one side of the
semiconductor; (c) at least one signal electrode formed on at least
one side of the semiconductor; and (d) at least one control
electrode, formed on at least one side of the semiconductor,
configured so as to form an electric field pattern within the
semiconductor that directs charge clouds resulting from ionizing
events in the semiconductor to the signal electrodes; wherein the
radiation detector is capable of detecting energies greater than
about 20 KeV.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation of U.S. patent
application Ser. No. 09/542,386, filed Apr. 4, 2000, which is a
continuation of U.S. patent application Ser. No. 08/943,492, now
U.S. Pat. No. 6,046,454, which is a continuation of U.S. patent
application Ser. No. 08/881,175, filed Jun. 23, 1997, which is a
continuation-in-part of U.S. patent application Ser. No.
08/542,883, filed Oct. 13, 1995, now U.S. Pat. No. 5,677,539.
BACKGROUND
[0002] 1. Field of the Invention
[0003] This invention relates to a device and method for detecting
ionizing radiation, and more particularly to a semiconductor
radiation detector with enhanced charge collection for reducing
low-energy tailing effects.
[0004] 2. Description of Related Art
[0005] High-resistivity semiconductor radiation detectors are
widely used for detecting ionizing radiation due to their ability
to operate at room temperature, their small size and durability,
and other features inherent in semiconductor devices. Such
detectors are used in a wide variety of applications, including
medical diagnostic imaging, nuclear waste monitoring, industrial
process monitoring, and space astronomy. Ionizing radiation
includes both particulate radiation, such as alpha or beta
particles, and electromagnetic radiation, such as gamma or x
rays.
[0006] Conventional semiconductor radiation detectors are generally
referred to as "planar" detectors. As shown in FIG. 1, the
architecture of such planar detectors 100 typically consists of a
slab of semiconductor crystal 102 with metal covering two opposing
surfaces of the slab to form two electrodes, a cathode 104 and an
anode 106. In one configuration, the anode 106 is connected to
external signal conditioning circuitry 108 and to ground 110, and
the cathode 104 is connected to an external voltage source 111. A
bias voltage across the electrodes 104, 106 creates an internal
electric field. Electron and hole "charge clouds" generated within
the semiconductor crystal 102 by an ionizing radiation 112 absorbed
within the slab of semiconductor crystal 102 are swept toward the
anode 106 and cathode 104 electrodes, respectively. These moving
electron and hole clouds create charge-pulse signals in the
external signal conditioning circuitry 108.
[0007] If all the electrons and holes generated by the ionizing
radiation 112 reach their respective electrodes (i.e., the
electrons reach the anode 106 and the holes reach the cathode 104),
the output charge signal will exactly equal the charge from the
energy deposited within the crystal 102. Because the deposited
charge is directly proportional to the energy of the ionizing
radiation 112, the semiconductor radiation detector 100 provides a
means for measuring the energy of the ionizing radiation 112. The
ability to measure this energy is an important function of
radiation detectors.
[0008] Planar radiation detectors, however, suffer from a serious
drawback: because of limitation in the transport properties of the
bulk semiconductor crystal 102, some of the electrons and holes are
generally lost by being trapped as they sweep toward their
respective electrodes. Thus, the amplitude of the output charge
signal becomes dependent on the position within the crystal at
which the ionizing radiation is absorbed. Generally, the amplitude
is less than the charge deposited by the ionizing radiation 112,
resulting in a corresponding reduction of energy measurement
accuracy as well as poor resolution and reduced peak efficiency.
This loss (or trapping) of charge in a radiation detector results
in asymmetrical spectral peak shapes known as "low-energy
tailing."
[0009] As stated above, in a semiconductor radiation detector, when
an ionizing event occurs, electrons are swept toward the anode 106
and holes toward the cathode 104. In a typical experimental
arrangement, with the cathode 104 facing the source of the
radiation, many ionization events occur over some accumulation
period, and the resulting charge signal pulses are detected and
then displayed in a histogram. In an ideal detector, in which there
is no low-energy tailing, all the pulses would be directly
proportional to the energy of the ionizing radiation 112. This
would result in a histogram like that of FIG. 2, in which counts
per channel are plotted versus charge signal pulse amplitude. As
can be seen in FIG. 2, the energy histogram exhibits no tailing,
because the energy peak (or "photopeak") 202 appears as a straight
vertical line at a single energy level, E, equal to the energy
level of the ionizing radiation 112. Thus, all the charge signal
pulses have an amplitude equal to the energy level E of the
ionizing radiation 112, and no charge is lost in any single
pulse.
[0010] Curves A and B of FIG. 3 illustrate two idealized cases of
low-energy tailing in a non-ideal detector. Curve A represents the
histogram distribution that would result if the ionizing radiation
were absorbed uniformly throughout the crystal, as would occur with
a very low absorption coefficient of the crystal. Curve B
represents the more typical situation, where absorption is large
near the cathode and drops off exponentially as the ionizing event
moves in a direction away from the cathode within the crystal. In
both Curves A and B, there is a maximum signal 302 corresponding to
full charge collection (at amplitude "E") and pronounced low-energy
"tails" 304, 306.
[0011] FIG. 4 shows an energy histogram exhibiting pronounced
low-energy tailing for an actual semiconductor radiation detector
made from Cadmium-Zinc-Telluride (CdZnTe) irradiated with gamma
rays from a cobalt-57 (".sup.u57Co") radiation source. This
detector had area dimensions of 6.1 mm by 6.1 mm and a thickness of
3 mm. Its bias voltage was -500 Volts. The data values in FIG. 4
are spread-out by electronic noise, an effect that was not
considered in plotting the idealized curves of FIG. 3. As with
Curves A and B of FIG. 3, the histogram of FIG. 4 has a pronounced
low-energy tail 404.
[0012] Because of the deleterious effects of low-energy tailing in
semiconductor detectors, much effort has gone into attempting to
solve this problem. One approach to reducing the tailing effect in
semiconductor detectors is to reduce the dependence of the signal
pulse-charge amplitude on the position at which the ionizing
radiation is absorbed. This can be accomplished, in principle, by
contriving to limit to a small distance the region in which charge
is induced on one electrode by a charge cloud in front of that
electrode. If this is accomplished, a charge cloud generated by an
ionizing event induces little charge on the electrode until it
becomes very near the electrode, after which the charge cloud
induces essentially all of its charge on that electrode. This
approach is especially useful for semiconductors in which the
transport properties of one carrier type (e.g., electrons) are much
better than those of the other type (holes in this example). These
transport properties are expressed by a "mobility-lifetime
product." The ratio of the transport properties of one type carrier
(e.g., holes) to those of the other type carrier (e.g., electrons)
is expressed as the "mobility-lifetime-product ratio." Thus, the
general approach described above is useful for all
mobility-lifetime-product ratios, but is most useful for
semiconductors having a large ratio of the larger mobility-lifetime
product divided by the smaller. Semiconductors for which the
mobility-lifetime-product ratio is greater than 10 include
cadmium-zinc-telluride, cadmium-telluride, and mercury-iodide.
[0013] An early effort aimed at minimizing low-energy tailing using
the above approach employed a semiconductor detector having a
hemispherical configuration. See, e.g., H. L. Malm, et al.,
"Gamma-Ray Spectroscopy with Single-Carrier Collection in High
Resistivity Semiconductors," Appl. Phys. Lett., vol. 26, at 344-46
(1975). In Malm's detector, a large hemispherical surface of the
cadmium-telluride was metallized to form the cathode. The anode
formed a small circle at the center of the flat cross-section of
the hemisphere. A bias voltage applied across these electrodes
produced an internal electric field that varied from a low value
near the cathode to a high value near the small anode. The electric
field lines were thus concentrated near the central point by the
spherical geometry. A result of this electric field concentration
is that electrons move much faster in the close vicinity of the
anode than in the remainder of the detector. Because the charge
induced on the anode is inversely proportional to the square of the
distance from the charge cloud to the anode, most of the charge is
induced when the charge cloud is in the vicinity of the anode. The
signal charge pulse amplitude is thus relatively insensitive to the
position of the ionizing event in the detector.
[0014] The Malm approach, however, has several disadvantages.
First, while energy resolution is improved over the planar
detector, significant tailing remains. Second, it is difficult and
thus costly to fabricate a semiconductor crystal having a
hemispherical configuration. Third, the configuration cannot be
applied to monolithic detector array structures. In consequence,
this detector has achieved little, if any, commercial
acceptance.
[0015] A second approach achieves a reduction in tailing by using a
planar structure in which the anode is in two sections, a 0.5 mm
diameter circle and a ring surrounding and at the same voltage as
the circle, and the cathode covers the opposite surface. See, e.g.,
F. P. Doty, et al., "Pixilated CdZnTe Detector Arrays," Nucl.
Instruments & Methods in Physics Research, vol. A 353, at
356-60 (1994). The charge induced by an electron cloud is shared
between the small circle and the ring, such that the charge induced
on the circle is very small until the charge comes very close to
the circle. The full charge is then induced on the circle within a
distance comparable to the pixel dimensions.
[0016] This second approach also suffers from a significant
disadvantage, in that it results in a very low collection
efficiency. This result stems from the fact that only charge clouds
directly above the small anode are collected.
[0017] A third approach employs a structure in which the anode of a
planar CdZnTe detector is segmented into an array of very small
individual detectors (pixels), with the cathode remaining as a
single, continuous electrode. See H. Barret, et al., "Charge
Transport in Arrays of Semiconductor Gamma-Ray Detectors," Phys.
Rev. Let. (In Press). Here, each pixel is connected to an external
signal conditioning circuit. Charge induced by an electron cloud is
shared among the pixels and is very small on any specific pixel
until the charge is very near the pixel.
[0018] This third approach also suffers from significant
limitations. First, it is only useful for an array of very small
pixels. Thus, this approach cannot be used for single-element
detectors.
[0019] Second, this approach is not applicable to detector arrays
with pixel sizes of a millimeter or more, as used in nuclear
medical imagers.
[0020] A fourth approach employs an anode patterned into an
interleaved grid structure, with the cathode remaining planar. See,
e.g., P.N. Luke, "Unipolar Charge Sensing with Co-Planar
Electrodes-Application to Semiconductor Detectors," IEEE Tran.
Nucl. Science, vol. 42, No. 4, at 207-213 (1995). In the Luke
approach, one set of anode grids is maintained at a slightly higher
voltage than the other. A train of signal conditioning electronics
is connected to each set of grids, and the difference between the
outputs from these trains constitutes the final output signal. With
this arrangement, when the charge cloud is far from the grids, the
difference-signal between the grid outputs is zero. As the cloud
approaches the grids, the induced charge on one grid rises rapidly,
while the charge induced in the other grid drops rapidly. The
difference signal is then a measure of the full charge in the
electron cloud, independent of the position of the ionizing
event.
[0021] The Luke approach, however, also suffers from drawbacks.
First, the grid structure is relatively complex and would be
difficult, if not impossible, to use in detector arrays. Second,
the grids require two separate amplifying chains, plus a difference
amplifier, which add significantly to the complexity and cost of
manufacture. This circuitry would also be very difficult to
implement in multi-channel integrated circuits needed in detector
array structures.
[0022] A final approach to reducing low-energy tailing may be
implicit in the design of silicon drift chambers. See, e.g., E.
Gatti & P. Rehak, "Semiconductor Drift Chamber-An Application
of a Novel Charge Transport Scheme." Nucl. Inst. & Methods in
Physics Research, vol. 225, at 608-614 (1984). A semiconductor
drift chamber is based on the principle that a thin, large area
semiconductor wafer, with rectifying junctions implanted on both
surfaces, can be fully depleted through a small anode contact. The
depletion field confines electrons generated by an ionizing
particle in a buried potential channel parallel to the surface. An
electrostatic field (drift field) parallel to the surface is
independently superimposed and transports the electrons along the
buried potential channel toward a collecting electrode. In
addition, the capacitance of the collecting electrode is very low
and independent of the active area of the detector. It has been
suggested that drift chambers can be made from a variety of
semiconductors. They have been implemented successfully with
300-micron-thick high-resistivity (10.sup.4 to 10.sup.5 ohm-cm)
silicon wafers. Such drift chambers are used as high-resolution
position-sensing detectors for particle physics.
[0023] The silicon drift chamber approach also suffers from several
drawbacks. First, rectifying junction contacts must be used to
generate the depletion field and the drift field. Because of the
limitation of the breakdown voltage of these junction contacts, the
magnitude of usable voltage is limited. This in turn limits the
thickness of the wafer that can be used for the drift detector.
Second, in order to transport charge effectively in the thin
channel, a uniform drift field must be applied. A large number of
junction contacts, each with a carefully controlled, fixed voltage,
is required on the wafer to generate this uniform drift field. This
adds significantly to the manufacturing cost and the complexity of
using the detector. Third, because of the limitation of the
thickness of the detector and the low Z (.about.14) of the
semiconductor material used, the detection efficiency for x rays
and gamma rays is very low for energies above 10-20 KeV.
[0024] Both silicon detectors and detectors made of high
resistivity materials, such as CdTe and CdZnTe, have employed
"guard rings" around the signal electrode. A guard ring is normally
kept at the same potential as the signal electrode and is used
primarily to prevent dark current from the edge of a detector from
reaching the signal electrode, thereby reducing the signal-to-noise
ratio of the measurement. The guard ring does not significantly
reduce low-energy tailing encountered in semiconductor
detectors.
[0025] Therefore, a need exists for a semiconductor radiation
detector that minimizes low-energy tailing and that obviates the
disadvantages and drawbacks of conventional radiation detectors.
The present invention provides such a radiation detector.
SUMMARY
[0026] The invention is a device and method for detecting ionizing
radiation emanating from a source. The ionizing radiation may be
high energy photons, including gamma rays and x-rays, or charged
particles, including beta particles and alpha particles. It should
be recognized, however, that the invention may be used in detecting
any kind of ionizing radiation.
[0027] The invention takes advantage of the principle that a
significant reduction in low-energy tailing in a semiconductor
detector can be attained by a novel arrangement of electrodes that
share induced charge from ionizing events in the detector, that
properly shape the electric field, and that focus charge collection
toward a small electrode. In implementing a semiconductor radiation
detector that follows this principle, the invention employs a
detector structure having a novel arrangement of three electrodes
that virtually eliminates tailing while maintaining high collection
efficiency.
[0028] In accordance with the invention, a radiation detector is
provided that is capable of detecting energies from a few KeV to
several hundred KeV. The detector includes three electrodes formed
on the surface of a semiconductor crystal. The crystal has a
plurality of sides; it preferably has a thickness of at least about
0.5 mm and is preferably formed from a semiconductor material
having a high mobility-lifetime ratio. The first electrode is a
bias electrode, which preferably covers the entire surface of one
side of the crystal. At least one signal electrode having a small
area is preferably formed on the opposing side of the crystal from
the bias electrode. A control electrode is preferably disposed on
the same side containing the signal electrode.
[0029] More particularly, in the invention, the control electrode
is formed on the same side of the semiconductor crystal as the
signal electrode (or anode), and the bias electrode (or cathode)
covers substantially the entire surface of the opposite side of the
crystal. Preferably, the semiconductor crystal is formed from
CdZnTe or CdTe. In the simplest configuration, the anode is a small
contact point located near the center of the
electron-charge-collection side of the crystal. The anode is
coupled to ground through a large-value resistor and to external
signal circuitry. The cathode is coupled to a voltage source that
maintains the cathode at a negative voltage level relative to the
anode. Preferably, the control electrode is much larger in area
than the anode and forms a single ring surrounding the anode. The
control electrode is maintained at a voltage level that is negative
with respect to the anode, but generally not more negative than the
cathode.
[0030] This configuration virtually eliminates low-energy tailing
when measuring the energy of ionizing radiation. When ionizing
radiation is absorbed in the radiation detector, a charge cloud is
generated that induces charge initially on all electrodes. The
amount of charge induced on each electrode is a function of the
distance of the charge cloud from that electrode and the area of
the electrode. Because of its small size, the charge on the anode
is very small until a charge cloud comes close to the anode. In
addition, the control electrode helps shape the electric field to
focus the electron clouds toward the anode. As the electron charge
cloud drifts towards the anode, the charge induced on the anode
remains very small, and the charge on the control electrode builds
up until the charge cloud attains a distance from the anode on the
order of the size of the anode. The charge on the anode then builds
up rapidly to the full value of the charge cloud, while the charge
on the control electrode drops rapidly to zero. Hole charge clouds
drift toward the cathode and away from the anode, and the effects
of hole-trapping in the semiconductor are seen primarily by the
control electrode and the cathode. Thus, the signal charge, which
is the accumulated charge induced on the anode, essentially equals
that of the full electron charge cloud, regardless of its position
of origin. Removing dependence on position of the ionizing event
from the signal virtually eliminates low-energy tailing. Sharing of
induced charge between anode and control electrode, and build-up
and decline of induced charge on the respective electrodes, can be
understood conceptually in terms of capacitances between a charge
cloud and the electrodes and the inter-electrode capacitances.
[0031] Another benefit of the small anode is in establishing a
field concentration that accelerates charge clouds in the vicinity
of the anode. This field concentration is enhanced by the voltage
applied to the control electrode. As a charge cloud drifts from its
point of origin to the anode, the fraction of charge induced on the
anode in any small time increment is a function of the drift
velocity. This velocity is increased substantially near the anode
by the field concentration, with the result that a large fraction
of the total charge is induced on the anode within a small distance
from the anode. Thus, the field concentration further reduces the
dependence of the signal charge on the position of charge cloud
generation.
[0032] The field concentration near the anode results in faster
rise-time pulses in the external circuitry and also more uniformity
of rise-times among pulses. This has the potential benefit of
reducing any gain variations in amplification due to pulse
rise-time.
[0033] In one alternative embodiment, a fourth electrode can be
added to separate the charge-shielding and field shaping functions
of the control electrode. In another alternative embodiment, more
electrodes can be added to further enhance the charge and field
shaping functions.
[0034] The invention can be used in several cross-strip detector
configurations, in a side-entry radiation detector, and with
liquid/gas ionization detectors.
[0035] The details of the preferred embodiment of the invention are
set forth in the accompanying drawings and the description below.
Once the details of the invention are known, numerous additional
innovations and changes will become obvious to one skilled in the
art.
BRIEF DESCRIPTION OF THE DRAWINGS
[0036] FIG. 1 is a perspective view of a prior art planar radiation
detector.
[0037] FIG. 2 is an energy pulse histogram for an ideal radiation
detector in which counts per channel are plotted versus charge
signal pulse amplitude.
[0038] FIG. 3 is an idealized energy pulse histogram for two
different radiation detectors, in which Curve A represents the
histogram that results if the ionizing radiation are absorbed
uniformly throughout the semiconductor crystal, and Curve B
represents the histogram that results if absorption is large near
the cathode and drops off exponentially as the ionizing event moves
in a direction away from the cathode.
[0039] FIG. 4 is an energy pulse histogram for an actual prior art
CdZnTe planar detector, illustrating low-energy tailing.
[0040] FIG. 5A is a perspective view of the preferred embodiment of
a single element configuration of the invention.
[0041] FIG. 5B is a cut-away side view of the embodiment
illustrated in FIG. 5A taken along line 5B-5B, showing the electric
field created in the semiconductor crystal.
[0042] FIG. 6 is a perspective view of an alternative embodiment of
the invention.
[0043] FIG. 7 is a perspective view of another embodiment of the
invention showing a segmented control electrode.
[0044] FIG. 8 is a perspective view of yet another embodiment of
the invention showing multiple control electrodes and multiple
anodes.
[0045] FIG. 9 is an energy pulse histogram for the preferred
embodiment of the invention illustrated in FIG. 5A, employing a
CdZnTe semiconductor.
[0046] FIG. 10A is a perspective view of an alternative embodiment
of the invention showing a detector array structure.
[0047] FIG. 10B is a cut-away side view of the array embodiment
shown in FIG. 10A taken along line 10B-10B.
[0048] FIG. 11 is a perspective view of a radiation detector having
buried electrodes.
[0049] FIG. 12A is a top view of the preferred embodiment of a
4-electrode configuration of the invention.
[0050] FIG. 12B is a side cross-sectional view the 4-electrode
configuration of FIG. 12A, taken along line A-A of FIG. 12A.
[0051] FIG. 12C is a top view of the preferred embodiment of a
5-electrode configuration of the invention.
[0052] FIG. 12D is a side cross-sectional view the 5-electrode
configuration of FIG. 12C, taken along line A-A of FIG. 12C.
[0053] FIG. 13A is a top view of an embodiment of the invention
with an anode pattern similar to the embodiment shown in FIG. 10A
but configured as an anode-cathode cross-strip detector.
[0054] FIG. 13B is a side view of the embodiment shown in FIG.
13A.
[0055] FIG. 13C is a bottom view of the embodiment shown in FIG.
13A.
[0056] FIG. 14A is a top view of an embodiment of the invention
similar to the embodiment shown in FIG. 10A but configured as an
anode-control electrode cross-strip detector.
[0057] FIG. 14B is a side view of the embodiment shown in FIG.
14A.
[0058] FIG. 15A is a top isometric view of an embodiment of the
invention similar to the embodiment shown in FIG. 10A but
configured as an anode-cathode cross-strip detector.
[0059] FIG. 15B is a bottom view of the embodiment shown in FIG.
15A.
[0060] FIG. 16 is a perspective view of an alternative embodiment
of the invention showing a side-entry radiation detector array
structure.
[0061] FIG. 17 shows an embodiment of the invention in the form of
an ionization gauge.
[0062] Like reference numbers and designations in the various
drawings indicate like elements.
DETAILED DESCRIPTION
[0063] Throughout this description, the preferred embodiment and
examples shown should be considered as exemplars, rather than as
limitations on the invention.
[0064] (1) 3-electrode Radiation Detector
[0065] FIG. 5A is a perspective view of the preferred embodiment of
a single-element (or single-pixel) radiation detector 500 of the
invention. While a single-element detector is shown, it should be
understood that the detector of the invention is not limited to the
single-element embodiment and can be used in any multi-element
configuration. Thus, a number of single-element detectors 500 can
be grouped together to form an array of single-element detectors,
or, in accordance with the principles of the invention, a
monolithic detector array can be fabricated. An example of such a
monolithic detector array is shown in FIG. 10A and described
below.
[0066] The single-element detector 500 of FIG. 5A is preferably
capable of detecting energies in the range from a few KeV to over 1
MeV. The detector 500 includes a semiconductor crystal 502, a bias
electrode 504, a signal electrode 506, and a control electrode 508.
Appropriate biasing of the electrodes creates an electric field
within the crystal 502. Ionizing radiation 112 absorbed in the
detector 500 causes ionizing events within the crystal 502. The
ionizing events, in turn, result in charge clouds that are guided
by the electric field toward the signal electrode 506.
[0067] For the following discussion, it is assumed that the
mobility-lifetime product for electrons is greater than for holes.
For the reverse condition, in which the mobility-lifetime product
for holes is greater than that for electrons, electric polarity
would also be reversed.
[0068] (A) The Semiconductor Crystal
[0069] The semiconductor crystal 502 is a slab or wafer of
high-resistivity, high Z (greater than about 34) semiconductor
material. Preferably, the crystal consists of a slab of
high-resistivity CdZnTe, which can operate at room temperature and
can be fabricated into detectors. Alternatively, the crystal 502
may be formed from CdTe, HgI.sub.2, PbI, or other semiconductor
materials that have high-resistivity and that can be fabricated
into detectors. Of course, those skilled in the art will recognize
that virtually any semiconductor material may be used in the
invention.
[0070] When used in a spectroscopic mode, detectors made from
CdZnTe or CdTe yield many lower-energy pulses in addition to the
characteristic pulse amplitude for any particular energy of
ionizing radiation. That is, such crystals produce significant
low-energy tailing. Thus, it is especially important in radiation
detectors in which the semiconductor slab is fabricated from CdZnTe
or CdTe to design the detector in such a way that minimizes
tailing. The invention provides a detector structure that virtually
eliminates tailing and thus is particularly well-suited to CdZnTe
and CdTe-based semiconductor radiation detectors.
[0071] As shown in FIG. 5A, the semiconductor crystal 502 is
preferably a rectangular parallelepiped. The crystal 502 may,
however, have virtually any desired volumetric shape, including
cubic, hemispherical, cylindrical, conic, or rhombic. In one
experimental embodiment, the crystal 502 was square on sides 510
and 512, with dimensions "c" and "e" each being about 6.1 mm, and
with a thickness "d" of about 3 mm. It should be understood,
however, that the dimensions of FIG. 5A are merely exemplary and
that the dimensions depend primarily on the application in which
the detector is being used and on the measurement conditions. Thus,
the crystal 502 may have a smaller or larger surface area on sides
510 and 512. Typically, the surface area of the sides 510, 512
ranges from about one to several hundred square millimeters.
[0072] The crystal 502 may have a thickness "d" of greater or less
than 3 mm. Preferably, however, the thickness is greater than about
0.5 mm, with the typical range being between approximately 1 mm and
10 mm. (The effect of varying the thickness of the crystal 502 will
be described in greater detail below.) Those skilled in the art
will recognize that the crystal shape and dimensions may be varied
alone or in combination to achieve special performance results or
to improve manufacturability.
[0073] The novel structure of the invention can be used with
virtually any semiconductor or insulating detector material having
a resistivity greater than about 10 megohm-cm. If the semiconductor
resistivity is less than 10 megohms, the resistivity may be
effectively increased to this value by creating a Shottky barrier
or p-n junction at one of the electrodes.
[0074] (B) The Bias Electrode
[0075] The bias electrode 504 is formed as a conductive layer
(e.g., by metallizing) on substantially the entirety of the surface
of side 510 of the crystal 502. (In the embodiment shown in FIG.
5A, the ratio of the electron mobility-lifetime product to the hole
mobility-lifetime product is greater than 1, and the bias electrode
504 thus acts as a cathode and will be referred to as such for the
remainder of this description. Of course, if the mobility-lifetime
ratio were reversed, the polarity would be reversed, and the
cathode 504 would act as an anode.) The cathode 504 may be formed
to cover only a portion of a surface of the crystal 502, or to
cover more than one surface, and/or several cathodes may be
provided on side 510. Further, the cathode 504 may be formed in
different shapes and with various dimensions.
[0076] The cathode 504 is set to a bias (or cathode) voltage,
V.sub.b, which is negative with respect to the anode 506, and which
depends on the thickness "d" of the crystal 502 and on the
application. For the preferred embodiment, in which the crystal 502
is about 3 mm thick, the magnitude of V.sub.b is from about -200
volts to about -1000 volts, and most preferably about -400 volts to
about -500 volts. It should be understood, however, that V.sub.b
may be any suitable voltage level. The cathode may be set to
V.sub.b by coupling the cathode 504 to a constant external voltage
source 514, or by any other suitable means for establishing and
maintaining a substantially constant voltage level. Preferably, the
cathode 504 is coupled to the voltage source 513 via a wire
conductor.
[0077] (C) The Signal Electrode
[0078] In the embodiment of FIG. 5A, the signal electrode 506 is
preferably a small conductive contact located at or near the center
of side 512, which is the surface of the semiconductor crystal 502
opposing the side 510 on which the cathode 504 is formed in the
preferred embodiment. (As explained above, in the embodiment of
FIG. 5A, the electron mobility-lifetime product is greater than the
same product for holes, and the signal electrode 506 thus acts as
an anode and will be referred to as such for the remainder of this
description. If the reverse were true, the polarity would be
reversed, and the anode would act as a cathode.) The anode 506 of
FIG. 5A has a circular shape with a diameter of about 1 mm, the
diameter being less than dimension "a." Preferably, the anode 506
is a "dot" contact formed on the surface of side 512. Nevertheless,
like the cathode 504, the anode 506 may be formed in a variety of
shapes, such as a circle or polygon, and is not fixed
dimensionally.
[0079] The anode 506 is coupled to external signaling circuitry
516, preferably via a wire connection, and via a resistor to an
anode potential, which in the preferred embodiment of FIG. 5A is
ground 514. Therefore, the anode 506 is at a more positive voltage
level, V.sub.a, than the cathode 504, which, as described above, is
set to a negative voltage. Consequently, a bias voltage exists
across the anode 506 and cathode 504 which creates an electric
field within the crystal 502. V.sub.a need not be at ground.
Rather, V.sub.a may be any voltage level, provided, however, that
V.sub.a and V.sub.b are set to different levels in order to
establish a bias voltage between the cathode 504 and anode 506.
[0080] (D) The Control Electrode
[0081] A third electrode, the control electrode 508, is formed on
the same side 512 of the crystal 502 as the anode 506 in the
preferred embodiment. As shown in FIG. 5A, the control electrode
508 may be in the form of a conductive circular ring surrounding
the anode 506, having an inside diameter, "a", of about 4 mm and an
outside dimension, "a"+"2b", of about 6 mm. FIG. 6 shows an
alternative embodiment of a radiation detector 600 of the
invention, where the control electrode 608 forms a rectangular ring
at the perimeter of side 612, with the ring surrounding the anode
606. In radiation detector 600, the control electrode 608 has a
width of about 0.5 mm. As defined herein, "ring" means a body or
structure having any shape (for example, circular or polygonal)
that fully or partially encloses or substantially surrounds another
body or structure. Thus, it should be understood that the control
electrode 508 is not limited to a circular or rectangular ring
around the anode 506, as shown in FIGS. 5A and 6, but may be in the
shape of a square, triangle, or other ring shape. Moreover, the
control electrode 508 need not fully surround the anode 506.
Instead, the control electrode 508 may be an "incomplete" circle or
square (rather than a complete ring) or an irregular area formed
adjacent to the anode 506. Some alternative control electrode/anode
configurations are described in detail below.
[0082] The control electrode 508 has a control voltage, V.sub.c,
which can be a constant voltage level, or may be a controlled or
completely variable voltage level. Preferably, V.sub.c, like
V.sub.b, is a negative voltage with respect to the anode. Further,
the magnitude of V.sub.c is typically established so that it lies
between the magnitudes of V.sub.b and V.sub.a. Nevertheless, the
magnitude of V.sub.c may be more negative than V.sub.b.
Accordingly, in the preferred embodiment, the following
relationship exists between V.sub.a and V.sub.c:
V.sub.c<V.sub.a
[0083] For many applications the maximum detector sensitivity will
be at V.sub.c=V.sub.b.
[0084] V.sub.c may be established in a variety of ways. For
example, as shown in FIG. 5A, the control electrode 508 may be
coupled to an external voltage source 522 generating a
substantially constant voltage. The voltage source 522 is used to
maintain the control electrode 508 at V.sub.c. When the external
voltage source 522 is used to establish V.sub.c, current flows into
or out of the control electrode 508, depending on whether the
potential is greater or less than the potential that would be
established by the effective resistances from the control electrode
to both the anode and cathode.
[0085] Alternatively, as shown in FIG. 6, the control electrode 608
may be coupled to a capacitor 624, which is in turn connected to
ground 614. The capacitor 624 can be of any type, including a
discrete, monolithic, thick film, or integrated circuit capacitor,
or the capacitor 624 can be the parasitic capacitance of the
system. The capacitor 624 charges to a voltage determined by
V.sub.b-V.sub.a and by the values of the inherent electrical
resistances from the cathode 604 to the control electrode 608 and
the control electrode 608 to the anode 606. The capacitor 624
prevents V.sub.c from changing significantly as charge is swept
past the control electrode 608 during signal-charge collection.
[0086] In still another alternative embodiment, V.sub.c (and
V.sub.a and V.sub.b, for that matter) can be established by
applying an AC voltage using, for example, voltage source 522.
Similarly, V.sub.a, V.sub.b, and/or V.sub.c can be established by
applying a digital step waveform or other waveform. By applying
such variable voltage sources, V.sub.c can be modulated or actively
varied to modulate or vary the output count rate of charge signal
pulses at the anode 506.
[0087] In the illustrated radiation detectors 500 (FIG. 5A) and 600
(FIG. 600), the position and dimensions of the control electrodes
508, 608 are such that, with only a small capacitor (e.g., about
1000 pF) attached to the control electrodes 508, 608, the control
electrodes can be maintained at about -250 volts, which is an
acceptable operative value of V.sub.c when V.sub.b is about 5-500
volts. Thus, although the various embodiments of the radiation
detector of the invention each require three electrodes, the
addition of capacitor 624 to any one of the detector embodiments
allows the detector to be operated as a two-terminal device with
respect to external circuitry. Alternatively, the control
electrodes 508, 608 and cathodes 504, 604 may be connected
together, and the detectors 500, 500 can be operated as a
two-terminal device with respect to external circuitry, with
V.sub.c equal to V.sub.b. Moreover, as will be described in more
detail below, with the control electrodes 508, 608 held near an
optimum V.sub.c, nearly all electron charge clouds in the
semiconductor crystal 502, 602 are collected by the anodes 506,
606. Thus, nearly all low-energy tailing is eliminated, and the
detection efficiency is very high. The control electrodes 508, 608
also minimize effects from defects in the edges of the
semiconductor crystal 502, 602.
[0088] (E) Electrode Configurations and Features
[0089] FIG. 7 illustrates an alternative embodiment of the
invention having a segmented anode 706 and control electrode 708.
The anode 706 of FIG. 7 is segmented into a plurality of segments
730, and the control electrode 708 is also segmented into a
plurality of segments 732. Both the anode 706 and control electrode
708 may have any number of segments 730, 732, and the segments may
take virtually any shape. Moreover, all of the segments 730, 732 of
a single anode 706 or control electrode 708 need not have the same
shape or the same dimensions. Each segment in a single electrode,
therefore, may have its own shape and size.
[0090] The different segments 730, 732 may be set at different
voltages in order to optimize the electric field distribution
within a semiconductor crystal 504. Those skilled in the art will
recognize that, through simulation and/or experimentation, such
optimizing voltages can be selected empirically.
[0091] Alternatively, as shown in FIG. 8, more than one control
electrode 808 and anode 806 may be formed on a semiconductor
crystal 504. The various control electrodes 808 can take any shape
and size, and may be placed in various positions relative to the
anode 806. As with the segments 730, 732, the different control
electrodes 808 can be set at different voltages to optimize
electric field distribution. In addition, the control electrodes
808 can be formed at various locations on the crystal 504 to
optimize the electric field distribution.
[0092] As is also shown in FIG. 8, neither the control electrodes
808 nor the anodes 806 need be located on a surface of the detector
800 opposite to that of the cathode 804. For example, in the
detector 800 of FIG. 8, the cathode 804 is formed on a first
surface 810 of the cubic semiconductor crystal 802. A first control
electrode 808a and the anode 806 are both formed on a second
surface 840 adjacent to the first surface 810. A second set of
control electrodes 808b and anodes 808b are formed on a third
surface 812 opposite to the first surface 810. Alternatively, the
crystal 802 could have a control electrode 808 on adjacent surface
840 and the anode 806c on another adjacent surface 842. It should
be understood from the above that any distribution of the anode and
control electrodes on the crystal is possible, so long as the
electric field in the crystal is formed to focus the electron
charge cloud toward the anode and to shield the anode from the
effects of hole trapping.
[0093] Further, the cathode, anode, and control electrode need not
be restricted to a single surface of the crystal. For example, as
shown in FIG. 8, control electrodes 808b extend from one surface of
the crystal, around the edges to adjacent surfaces. The electrodes
can even extend to more than one adjacent surface. Alternatively,
an electrode may be in the form of a band fully or partially
encircling the crystal.
[0094] The electrodes can be formed on or in the crystal 502 using
a variety of techniques. Preferably, the electrodes are gold films,
commonly used in CdZnTe detector manufacture, electrochemically
deposited on the surface of the crystal 502. Alternatively, other
conducting materials, including platinum, rhodium, and other
metals, can be electrochemically deposited on the crystal surface
502 to form the electrodes. Those skilled in the art will recognize
that nearly any conductor may be used for the electrodes. As an
alternative to electrochemical deposition, the electrode material
may be deposited on the crystal 502 via evaporation, sputtering, or
surface alloying. The electrodes may be formed by other techniques
as well, such as ion beam deposition and chemical vapor deposition
of conductive materials. The electrodes may be formed in a variety
of configurations, including mesa, trenched, and buried
configurations. For example, FIG. 11 illustrates a buried anode
1110 and control 1112 electrode in a radiation detector 1114.
[0095] For perfect trapped charge shielding in the detector, the
ratio of the anode capacitance to the total capacitance from every
point inside the detector would be zero. This is approximated at
most points because of the relative sizes of the anode and other
electrodes and their relative distances. However, near the anode
the anode capacitance becomes large because the distance to the
anode is smaller than the distance to other electrodes. This effect
is minimized by bringing the control electrode very close to the
anode. But the difference between the control electrode voltage
potential and the anode voltage potential is typically several
hundred volts. That voltage potential and the dielectric strength
and voltage breakdown characteristics of the materials (including
air) between the two electrodes determines how close they can be
located to each other. By insulating the control electrode from the
crystal and from other materials with a high-dielectric-strength
insulator that has good breakdown characteristics and provides a
high capacitance value from the control electrode to the crystal,
the distance from the anode to the control electrode can be
minimized and the detector performance optimized.
[0096] Separation between the electrodes can be achieved in a
variety of ways. For example, the electrodes can be separated by
modifying the surface of the crystal 502, and/or by a bulk material
on the surface of the crystal (i.e., by passivation or resistive
layers formed by any process).
[0097] In order to provide proper electric field shaping, the
control electrode should be in ohmic contact with the crystal near
the perimeter of the detector. In general, all of the electrodes
are preferably ohmic contacts. Nevertheless, the electrodes of the
invention need not be ohmic; they could be rectifying, a p-n
junction, or some other type of contact.
[0098] (F) Theory of Operation of the 3-electrode Radiation
Detector
[0099] The operational aspects of the radiation detector of the
invention will be described with reference to the preferred
embodiment of FIG. 5A. Nevertheless, it should be recognized that
the principles described below apply to any embodiment of the
invention.
[0100] Following is a discussion of what is believed to be the
physical basis for the operational characteristics of 3-electrode
embodiments of the invention. While sound theoretical
considerations indicate that this theory of operation of the
invention is correct, it should be understood that the utility of
the invention does not rest on the correctness of the following
discussion. Further, although the following description refers to
radiation detector 500 of FIG. 5A, it should be understood that the
principles and theories described are applicable to any
configuration of the invention.
[0101] The semiconductor crystal 502 has electrons and holes. When
an ionizing event occurs within the crystal 502, electrons and
holes are formed into electron and hole charge clouds that are
transported in a particular manner within the crystal 502. As
explained above, the essential electron and hole parameters
describing transport properties are (1) mobility, .mu., which
determines how fast an electron or hole travels in a particular
electric field, and (2) trapping lifetime, .tau., which is the
average time an electron or hole generated by an ionizing radiation
remains free and able to participate in the transport process. The
mobility-lifetime product (.mu..tau.) is a critical parameter to
consider in analyzing semiconductor radiation detectors, and there
is a mobility-lifetime product for electrons, (.mu..tau.).sub.e,
and a mobility-lifetime product for holes, (.mu..tau.).sub.h. The
mobility-lifetime product generally differs significantly for
electrons and holes in a particular semiconductor. If electrodes
are formed on the semiconductor and a voltage potential established
between the electrodes, an electric field, E, in the semiconductor
will cause the electrons to drift to the anode and the holes to the
cathode.
[0102] In semiconductors such as CdTe and CdZnTe, the
mobility-lifetime product for electrons is generally much larger
than that for holes, with the mobility-lifetime ratio,
(.mu..tau.).sub.c/(.mu..tau.).sub.h, typically being greater than
10. (The principles described below, however, also apply to
semiconductor detectors for which
(.mu..tau.).sub.h/(.mu..tau.).sub.e is smaller or larger.) The
mobility-lifetime ratio may be even higher in other high
resistivity semiconductors, such as HgI.sub.2 and PbI. For
conceptual purposes, assume that the mean-free path (.mu..tau.E) is
large for electrons and small for holes with respect to the
detector thickness. Essentially all electrons generated by ionizing
radiation reach the anode, and many of the holes are trapped before
reaching the cathode. Under these conditions, the amount of charge
transported in a planar detector will depend strongly on the
position within the crystal at which the ionizing event occurs. If
the event occurs very near the cathode, electrons will be swept all
the way across the crystal to the anode, holes will be swept to the
cathode, and the full charge deposited by the ionizing radiation
will be sensed in the external circuit. If the event occurs near
the anode, the electrons will be swept to the anode, but holes will
be trapped, and the net effect will produce a much reduced output
signal. In between, the fraction of deposited charge induced in the
external circuit is a function of the distance from the anode at
which the event occurs. The dependence of charge response on the
position at which the event occurs is the source of unwanted
low-energy tailing in planar detectors.
[0103] With the three-electrode detector of the invention, at least
three factors contribute to the elimination of low-energy
tailing.
[0104] First, low-energy tailing is reduced by approximately the
ratio of the anode area to the control-electrode and cathode areas.
When a charge cloud is generated by an ionizing event in the
detector 500, charge is induced on all electrodes. The electric
field 518 established by the voltages on the electrodes guides the
electron cloud to the anode 506 and the hole cloud to the cathode
504. The charge induced on the control electrode 508 is, in
general, initially much larger than the charge on the anode 506
because of the relative capacitances between the charge clouds and
the anode 506 and the control electrode 508 (approximately the
ratio of their areas). This condition is true except when a charge
cloud is closer to the anode 506 than to the control electrode 508.
As an electron cloud arrives at the anode 506, it accelerates as it
gets very near the anode 506 because of the high concentration of
electric field 518 at the anode 506, and the charge on the anode
506 builds up very rapidly to the full value of the electron cloud.
Conversely, as hole clouds move to the cathode 504, their effect on
the anode 506 diminishes. Thus the effects that produce low-energy
tailing in planar detectors are reduced from the anode signal by an
amount that is approximately the ratio of the anode area to the
control-electrode and cathode areas. Thus, the signal charge, which
is the total charge collected by the anode 506, is essentially the
full charge of the ionizing event, regardless of the position at
which the event occurred within the crystal 502. It can be seen
that this method of detection removes from the anode charge
response its dependence on the position of the ionizing event,
which is the source of unwanted low-energy tailing. Furthermore,
the negative bias on the control electrode 508 inhibits the
collection of electron charge clouds by the control electrode 508,
thus maintaining high collection efficiency.
[0105] Second, electric field shaping helps reduce low-energy
tailing. FIG. 5B is a cut-away side view of the radiation detector
500 of FIG. 5A, showing a calculated electric field 518 that is
believed to be created by the preferred three-electrode
configuration. As can be seen in FIG. 5B, the electric field 518 is
characterized by field paths 520 within the crystal 502 that are
uniformly parallel for most of the distance between cathode 504 and
anode 506. The field paths become highly concentrated as they
closely approach the anode 506. Because the fraction of charge
induced on the anode 506 is a function of the velocity of the
charge clouds, and because the electric field 518 concentration
causes the drift velocity to increase as the clouds approach the
anode 506, a large fraction of the total charge is induced on the
anode 506 within a short time. Enhancement of field concentration
at the anode 506 by the control electrode 508 results in a charge
signal at the anode 506 that has a faster rise time and minimizes
the variance in rise times that are normally encountered in
semiconductors such as CdTe and CdZnTe. This in turn results in
more uniform amplification by external circuits 516 that have
finite clipping times.
[0106] Third, the control electrode 508 also removes some of the
charge clouds that have poor transport properties as a result of
being generated near the perimeter of the crystal 502 where
structural defects may be more frequently encountered. Such charge
clouds cause low-energy tailing. Nevertheless, by adjusting the
voltage of the control electrode 508, such charge clouds can be
collected by the control electrode 508 instead of the anode 506,
thereby preventing such charge clouds from causing low-energy
tailing.
[0107] (G) Example of Operation
[0108] FIG. 9 shows a signal pulse histogram of the gamma radiation
from Tc-99 mm for an actual radiation detector configured in
accordance with the 3-electrode detector 500 of FIG. 5A. The actual
detector employed a rectangular parallelepiped CdZnTe semiconductor
crystal. The dimensions of the crystal were about 6.1 mm by 6.1 mm
on the sides, with a thickness of about 3 mm. The detector had a
cathode covering one surface of the crystal and an anode and
control electrode on the opposite surface, with V.sub.a=0 volts,
V.sub.b=-450 volts, and V.sub.c=-250 volts.
[0109] FIG. 9 clearly demonstrates the virtual elimination of
low-energy tailing that can be achieved with the invention. A large
photopeak 902 exists at the energy level of the ionizing radiation,
and only a small amount of low-energy pulses were detected,
indicated by tail 904. The reduction in tailing achieved with the
detector of the invention is evident in comparing the histogram
FIG. 9 with that of FIG. 4. The histogram of FIG. 4 was acquired
with a conventional CdZnTe planar detector with the same crystal
dimensions and quality as that of FIG. 9. As can be seen, FIG. 4
has a broadened photopeak 402 at the target energy, indicating a
degradation of resolution, and a substantial amount of low-energy
tailing 404, indicating a reduction in peak efficiency. It is
interesting to note the relative amplitudes of the three energy
peaks. The total counts in the peaks should be proportional to the
emission probabilities for those peaks. For cobalt-57, those
probabilities are: 1) 14.413 KeV--9.14%, 2) 122.06 KeV--85.68%, and
3) 136.45 KeV--10.67%. In FIG. 9, the relative magnitudes of counts
in the peak are very close to those emission probabilities, while,
in FIG. 4, it appears that at least half of the counts that should
be in the peaks are in the low-energy tails of the higher energy
peaks. (The two curves of FIGS. 4 and 9 were obtained with
detectors of identical dimensions and identical surrounding
materials; therefore, the two curves can be compared for relative
peak amplitudes, although direct comparison of peak amplitudes
cannot be made without photon absorption data for each peak.)
[0110] Accordingly, as evidenced by the histogram of FIG. 9, the
addition of the control electrode 508 and its affect on the shape
of the electric field 518 results in virtual elimination of
low-energy tailing. The detector of the invention therefore
achieves high-resolution and collection efficiency, despite the
charge transport problems inherent in high-resistivity,
large-mobility-lifetime-ratio semiconductor materials. Furthermore,
the invention is simple and inexpensive to manufacture.
[0111] (H) Additional Aspects and Features of Operation
[0112] The magnitude of V.sub.c can be established by experiment.
If the magnitude of V.sub.c is too small, the anode 506 will
collect only some of the electron clouds, and the collection
efficiency of the radiation detector 500 will be low. The optimum
value for V.sub.c is dependent on electrode geometry. In the
radiation detector 500 of FIG. 5A, the preferred value of V.sub.c
is in the range from (V.sub.a+V.sub.b)/2 to V.sub.b. The value of
V.sub.b-V.sub.a is chosen based on the thickness of the
semiconductor crystal and the requirement of the application. For a
3 mm thick crystal, V.sub.b-V.sub.a may be about -400 volts.
[0113] The resistances between the cathode 504 and control
electrode 508 and between the control electrode 508 and anode 506
can be tailored to achieve specific performance results. This is
done by varying the electrode geometry and by changing bulk or
surface resistivity by ion damage, ion implantation,
thermo-chemical treatment, and/or other means.
[0114] The radiation detector of the invention can be used with
nearly any crystal thickness. Preferably, however, the thickness is
at least about 0.5 mm. The only limitation on the thickness is that
the larger .mu..tau. product (i.e., (.mu..tau.).sub.e or
(.mu..tau.).sub.h) must be sufficiently large for most of the
charge carriers to traverse the thickness of the crystal. For
state-of-the-art CdZnTe, this thickness is approximately 10 mm. For
a single anode on a crystal, the useful area of the detector may be
limited by the maximum anode capacitance that can be accommodated
by the electronics and by the ability to form an electric field
that will guide the electron clouds to the anode. Large areas may
be achieved by appropriate geometries for the anode and the control
electrode. Larger areas may also be used by forming grid structures
similar to the detector array configuration described below, but
with the anodes connected together.
[0115] Thus, the thickness of the semiconductor crystal of the
invention can be substantial and, thereby, can provide high
sensitivity and high detection efficiency for a wide range of
radiation energies.
[0116] If an embodiment of the new device is maintained in a
constant flux of ionizing radiation, varying the voltage (V.sub.c)
of the control electrode 508 below its optimum value will cause the
count rate to vary in a corresponding manner. Thus, the radiation
detector of the invention can be used to modulate the signal from a
beam of ionizing radiation.
[0117] (2) 4-electrode Radiation Detector
[0118] An alternative embodiment provides an improved structure for
achieving enhanced charge collection in the photopeak of a
solid-state radiation detector that exhibits charge carrier
trapping, particularly holes. This embodiment is particularly
advantageous with large volume radiation detectors used for
achieving high-efficiency detection and for high-energy gamma-rays.
As the volume of a radiation detector increases, it is more
difficult to simultaneously achieve good energy resolution, good
detection efficiency, and good peak-to-valley efficiency in the
spectrum. The purpose of this embodiment of the invention is to
minimize the compromise among these parameters.
[0119] The 3-electrode radiation detector described above uses a
control electrode positioned around the anode to focus the charge
from the total detector volume to the anode and to shield from the
anode the effects of induced positive charge of trapped holes.
However, the ideal electrical potential of the control electrode
for focusing electron charge to the anode may be approximately the
potential of the cathode, while optimum shielding is achieved when
the control electrode covers the entire surface not covered by the
anode. In the latter case, there must be sufficient distance
between the anode and the control electrode to avoid voltage
breakdown at the anode. These requirements seem to be mutually
exclusive, each requiring some compromise for achieving overall
acceptable performance.
[0120] An alternative embodiment of the invention separates the
charge focusing function and anode shielding functions of the
control electrode by having a shield electrode separate from a
focus control electrode--in essence adding a fourth electrode to
the three-electrode configuration described above. FIG. 12A is a
top view of the preferred embodiment of a 4-electrode configuration
of the invention. FIG. 12B is a side cross-sectional view the
4-electrode configuration of FIG. 12A, taken along line A-A of FIG.
12A. In this embodiment, the radiation detector 1200 includes a
semiconductor crystal 1202, a bias electrode (e.g., a cathode)
1204, a signal electrode (e.g., an anode) 1206, a focus control
electrode 1208, and a shield electrode 1210 shown insulated from
and overlying the focus control electrode 1208.
[0121] In a typical configuration, the bias electrode 1204 is a
cathode that provides the negative electrical potential for biasing
the radiation detector 1200, the signal electrode 1206 is an anode
that collects the signal charge, the focus control electrode 1208
focuses the electron charge to the anode, and the shield electrode
1210 minimizes charge at the anode induced by trapped charges
within the radiation detector 1200. In most applications where this
embodiment is applicable, the charge transport properties are
better for electrons than for holes. In any application where the
reverse is true, the same structure can be applied with the bias
voltage reversed. Thus, the signal electrode 1206 would become the
cathode and the bias electrode 1204 would become the anode. The
other electrodes would be the same as described with appropriate
bias voltages applied.
[0122] The focus control electrode 1208 can be strategically placed
on the surface of the radiation detector 1200 for best shaping of
the electrical field without the need for high capacitance, and
thus can maintain adequate distance from the anode to avoid
injection of pulse-breakdown noise into the anode signal. In the
illustrated embodiment, the focus control electrode 1206 is formed
as a narrow band around the perimeter of the signal electrode 1206
side of the radiation detector 1200.
[0123] The shield electrode 1210 preferably covers most of the
anode-side of the radiation detector 1200, except for an area large
enough for the anode. The shield electrode 1210 can be held at any
convenient electrical potential. In the preferred embodiment, the
shield electrode 1210 is directly or capacitively connected to
ground (perhaps via the cathode) in order to provide good shielding
characteristics, and is isolated from the anode to prevent charge
injection into the signal.
[0124] The shield electrode 1210 should have a high capacitance
with respect to points within the radiation detector 1200. Thus, in
the preferred embodiment, the shield electrode 1210 is insulated
from the surface of the radiation detector 1200 by a material that
has a high dielectric constant for maximum capacitance to points
within the detector, and high dielectric strength to avoid voltage
breakdown. One such material is epoxy-based solder mask.
[0125] In this embodiment, performance of the radiation detector
1200 relies primarily on the focus control electrode 1208 for
achieving maximum detection efficiency and on the shield electrode
1210 for eliminating the effects of charge trapping (typically hole
trapping). By assigning the charge-shielding function to the shield
electrode 1210 and making the function of the focus control
electrode 1208 principally that of shaping electrical field within
the radiation detector 1200, both functions can be performed
better. Use of a shield electrode 1210 may also be applicable to
any device where it is beneficial to shield an electrode from the
effects of induced charge from trapped charge carriers.
[0126] (3) 5-electrode Radiation Detector
[0127] Another embodiment of the invention adds a supplement
shielding fifth electrode 1212 to the radiation detector 1200 shown
in FIG. 12B. FIG. 12C is a top view of the preferred embodiment of
a 5-electrode configuration of the invention. FIG. 12D is a side
cross-sectional view the 5-electrode configuration of FIG. 12C,
taken along line A-A of FIG. 12C. A typical configuration of such a
radiation detector 1200 is a parallelopiped or a right circular
cylinder with a fifth electrode 1212 covering most or all of the
side surfaces orthogonal to the anode and cathode surfaces. The
fifth electrode 1212 may be plated on the side surfaces or
insulated from those surfaces. If the fifth electrode 1212 is
ohmically connected to the side surfaces, a bias voltage is applied
that helps in shaping the electric field in the radiation detector
1200 for optimum charge collection at the anode. If the fifth
electrode 1212 is not ohmically connected to the detector, it must
be connected, at least capacitively, to reference ground (which
could be through some power supply or by direct connection). The
fifth electrode 1212 may be patterned into different shapes if
desired.
[0128] The fifth electrode 1212 increases the capacitance in the
denominator of the capacitance ratio that determines charge induced
on the anode. In other words, the fifth electrode 1212 serves to
increase the shielding of induced charge from the anode, and thus
shape the electric field in the detector. The function of shaping
electric field may be important in some geometries.
[0129] In similar fashion, additional electrodes can be added, each
with its own voltage for shaping electric field and, thereby,
optimizing charge collection, in addition to providing additional
charge shielding.
[0130] (4) Timing Correction for Electron Trapping
[0131] When a gamma ray is absorbed in a radiation detector made in
accordance with the invention, electrons and holes are formed at
the site of the event and, as they drift toward the collecting
electrode(s), a signal is induced on the electrode(s). With the
electrode configurations described above (see, for example, FIGURE
SA and 12A), the signal generally is initially induced on the
cathode and on the control electrode but does not show up on the
anode until the electrons have drifted close to the anode.
[0132] By measuring the signals on the cathode, control electrode,
and anode, a timing relationship can be determined for the
occurrence of the initiating event and for the arrival of the
drifting electrons at the anode. For example, the time from when
the cathode signal exceeds a selected threshold, T.sub.c, to the
time when the anode signal exceeds a selected threshold, T.sub.a,
can be measured using any of many conventional means. As another
example, the time from when the signal on the control electrode
exceeds a selected threshold, T.sub.ct, to the time when the anode
signal exceeds a selected threshold, T.sub.a, can be measured using
any of many conventional means. Such timing information is
important for a number of reasons.
[0133] (1) Knowing the time of absorption is important for positron
emission tomography (PET) for determining coincidence with a
similar event in another detector. Coincident timing on the order
of 10 nsec may be required for many PET applications.
[0134] (2) Knowing the drift time of the electrons can allow signal
compensation for the effects of electron trapping in large
detectors. In prior art detectors, the effects of hole trapping
predominate and the effects of electron trapping are small. The
effects of hole trapping are so severe that no meaningful
improvement can be achieved by compensation for electron trapping.
However, in a radiation detector made in accordance with the
invention in which the effects of hole trapping have been nearly
eliminated, electron trapping becomes a more prominent factor.
Also, electron trapping may be somewhat greater in such radiation
detectors because the electric field is non-uniform and is
intensified near the small anode and correspondingly decreased away
from the anode. Where the electric field is low, the transit time
should be longer, with a corresponding increase in charge trapping.
In a planar detector, the electric field is expected to be uniform.
In a 3 mm cubic planar radiation detector, typical electron
trapping varies from 0% to about 3% depending on the location of
gamma-ray absorption and the corresponding drift-path length. As
the detector size gets larger and path lengths get longer or
electron mean-free-path lengths get shorter, energy resolution is
more affected by electron trapping.
[0135] By determining the drift time of electrons in the detector,
an improvement in energy resolution can be realized by correcting
for electron trapping. This can be done by measuring the electron
drift time and adding to the signal a small amount of charge that
is a function of the drift time. The amount of charge added for the
maximum drift time would be set equal to the maximum
electron-charge loss corresponding to that maximum drift time.
Though the charge loss is a negative exponential function of the
drift time, for systems in which the maximum charge loss is a few
percent, the charge loss can be closely approximated with 1 q e (
trapped ) = q o ( 1 - - 1 e ) = q o [ 1 - ( 1 - t e + t 2 2 e 2 - t
3 6 e 3 + ) ]
[0136] linear proportionality. The equation for electron charge
trapping is: 2 q o ( trapped ) t e 10
[0137] for t much smaller than .tau..sub.e.
[0138] The system would be calibrated using conventional techniques
to obtain the best energy resolution. Adding lost charge to delayed
events will narrow the energy peak; however, adding more charge
than was lost will again widen the energy peak.
[0139] Another method for compensating for electron charge loss is
by measuring the charge induced on the cathode and control
electrode(s) by trapped hole charge and trapped electron charge,
and using the measured magnitudes to determine the amount of
compensation to provide.
[0140] (3) The depth of interaction of an ionizing event in the
semiconductor crystal can be determined from timing information.
For example, for the embodiment shown in FIGS. 10A and 10B, an
ionizing event occurring closer to the cathode than the anode will
cause the time from when the cathode signal exceeds a selected
threshold, T.sub.c, to the time when the anode signal exceeds a
selected threshold, T.sub.a, to be different than if the ionizing
event occurs closer to the anode than the cathode. Calibration
using conventional techniques can map the timing information to
depth of interaction.
[0141] Depth of interaction is very important in any imaging
application where the angle of incidence of a photon with the
detection plane of the imager is other than normal to the surface
and the detector thickness is comparable to or greater than the
detection element size. In such cases, the photon may be absorbed
deep in the detector in a detection element other than the element
where the photon entered the imager surface. Applications where
measurement of depth of interaction is important include coded-mask
imagers and positron-emission tomography.
[0142] Another method of determining depth of interaction involves
measuring the magnitude of signals on the cathode, control
electrode, and/or shield electrodes. It is known that the amplitude
of a detected gamma-ray-absorption event in a prior art
room-temperature, semiconductor detector is dependent on the depth
of interaction in the detector, i.e., the correct magnitude is
decreased by a factor that is a function of the depth at which
holes are trapped, which in turn is a function of the location at
which the initiating photon is absorbed. In the present invention,
the full amplitude signal is obtained at the anode because the
anode is shielded from the effects of trapped charge by the other
electrodes, while the signals on the other electrodes are each a
function of the position of the trapped charge. For any given
detector geometry and material, a relationship between the relative
magnitude of an electrode signal and the depth of photon absorption
can be established by analysis or by testing. In a planar detector,
the charge induced on the cathode by trapped charge is a linear
function of the distance of the trapped charge between the anode
and cathode, and the mean location of the trapped charge is a fixed
distance from the depth of photon absorption, i.e., it is
established by the mean-free path (.mu..tau.E). In the present
invention, if the electrodes on the anode side of the detector
cover essentially the entire anode side of the detector, the
relationship between the magnitude of the cathode signal and depth
of photon interaction is essentially identical to that of a planar
detector. The depth of interaction can be determined by dividing
the cathode signal by the anode signal and multiplying the product
by the appropriate function. Likewise, a relationship can be
determined between depth of interaction and the magnitudes of each
of the other electrodes.
[0143] Determination of depth of interaction in some detector
designs may require combining signals from more than one electrode
in order to take into account geometric considerations. For some
applications, although the use of all signals might be required for
best accuracy, sufficient accuracy may be achieved with the signal
from only a single electrode.
[0144] (5) Detector Array Embodiment
[0145] FIGS. 10A and 10B show an embodiment of a detector array
1000 in accordance with the invention. FIG. 10A is a perspective
view of the detector array 1000. FIG. 10B is a cut-away side view
of the detector array 1000 showing the electric field 1018 within
the semiconductor crystal 1002.
[0146] A cathode 1004 is preferably formed to cover substantially
all of one side 1010 of a semiconductor crystal 1002. The cathode
1004 need not fully cover such side 1010 of the crystal 1002,
however, and may be nearly any size and shape desired (e.g. a
square grid). The semiconductor crystal 1002 is substantially
similar to the crystal 502 described above for the single-element
detector 500 of FIG. 5A, with the exception that it may have a
larger surface area on the electrode sides 1010, 1012 to
accommodate an array of anodes 1006 and a control grid 1008. The
crystal 1002 can be formed in a single block of monolithic or tiled
semiconductor material.
[0147] The detector array 1000 is produced by replacing the single
anode of a 3-electrode structure with a plurality of anodes 1006
and forming the control electrode 1008 as a grid within which the
anodes 1006 are situated on the top side 1012 of the crystal 1002,
as shown. Thus, each anode 1006 and its surrounding section of the
control grid 1008 forms a pixel. The anodes 1006 and control grid
1008 can be formed using conventional semiconductor processing
techniques. Such pixel arrays are particularly useful for radiation
cameras, such as are used in industrial and medical
applications.
[0148] In addition to the advantages of reducing low-energy tailing
and improving resolution and collection efficiency, the detector
array structure of the invention, as illustrated in FIG. 10B,
establishes an electric field pattern 1018 that isolates each pixel
from its neighbor, thereby suppressing cross-talk. Further, because
the anodes 1006 can be made much smaller than the control grid
1008, substantial separation can be achieved between the anodes
1006 and control grid 1008. This has the effect of reducing
inter-grid leakage current, which can be a source of unwanted noise
in detector array devices.
[0149] The detector array 1000 operates under the same principles
described above with respect to the various embodiments of the
3-electrode single-element detector. Thus, the control grid 1008
and the anodes 1006 share the charge induced by electron clouds,
but, because the control grid 1008 is much larger than the anodes
1006, low-energy tailing is mostly eliminated from the anodes 1006.
In addition, the control grid 1008 is preferably set at a voltage
level V.sub.c that lies near the voltage level V.sub.b of the
cathode 1004. Again, for the detector array 1000, the following
voltage relationship exists: V.sub.c<V.sub.a. As noted above,
this relationship acts to shape the electric field 1018 into field
paths that guide the electron clouds toward the anodes 1006. In
consequence, the electron clouds induce their full charge on the
small anodes 1006. The result is a significant reduction in
low-energy tailing for all the anode elements 1006 of the detector
array 1000.
[0150] Although a single control grid 1008 is shown, zone control
grids could be formed to control zones or subsets of the anodes
1006, or a control grid could be formed for each anode.
[0151] In one experimental unit fabricated as an array, a plurality
of 3 mm.times.3 mm square pixel detectors were formed by patterning
a control grid and centered anodes on a suitable semiconductor
crystal (CdZnTe). Such an array is well suited for use in an imager
for nuclear medicine.
[0152] All or some of the anodes 1006 may be connected together
electrically in parallel. As a result, the structure of FIG. 10A
can be used to produce single detectors of much larger area than
would be possible with a single anode. In addition, such large area
detectors have low overall capacitance, allowing the detector 1000
to have high sensitivity proportional to its area or volume and to
have high resolution similar to smaller detectors.
[0153] One useful embodiment is a configuration similar to FIG. 10A
but with all of the anodes 1006 connected together (either by wire
or through logic circuitry) to the same measurement circuit. Such a
configuration provides a large-area, high-sensitivity radiation
detector with the good energy resolution of a much smaller device.
The invention eliminates or significantly reduces effects of
trapped charge from the signal, thus eliminating low-energy tailing
due to hole trapping. However, when such a radiation detector gets
large, the path lengths for electron collection at the anodes 1006
become significant. If the mean-free path for electrons is high
enough that essentially all electrons are collected at the anodes
1006, the full electron charge is seen at the anodes 1006 for each
gamma-ray-absorption event. However, if the path lengths are
appreciable with respect to the mean-free path, only part of the
electron charge will be measured and the portion measured will vary
with path length, yielding a broadened peak in the energy spectra.
Segmenting the radiation detector into smaller response areas and
connecting the anodes 1006 together reduces the peak broadening due
to electron trapping.
[0154] Another advantage of such a configuration occurs when
multiple detector modules of the type shown in FIG. 10A are "tiled"
together to form a larger detector array. An example of such tiling
is shown in U.S. patent application Ser. No. 08/372,807, entitled
"Semiconductor Gamma-ray Camera And Medical Imaging System",
assigned to the assignee of the present invention, which is hereby
incorporated by reference.
[0155] The anodes 1006 of each module can be logically combined so
that each module in effect becomes a single "big pixel". If the
signals of groups of detector elements within a module are
combined, the number of effective pixels in the array decreases and
the size of each pixel increases. In doing so, the efficiency of
each "pixel" increases (there are many detector elements
contributing to the signal) and the spatial resolution decreases
(there are fewer pixels). For example, individual
8.times.8-detector-element modules could be tiled into a 7.times.9
array of modules. If each detector element were 1 mm.times.1 mm in
size, the detector would then have 56 mm.times.72 mm resolution at
maximum pixel sensitivity (i.e., all modules combined into a single
"giant" pixel), 8 mm.times.8 mm resolution at "medium" pixel
sensitivity (i.e., each detector module being a single pixel), and
1 mm.times.1 mm resolution at minimum pixel sensitivity (i.e., each
detector element of each module being a pixel).
[0156] This arrangement allows the detector array to be used in a
"rapid scan" mode of few, sensitive pixels to locate a region of
interest at low resolution. The detector elements of the modules in
the array can then be separately measured to view the region of
interest with many lower sensitivity pixels but at higher
resolution, providing the ability to "zoom in" on the region.
[0157] Further, if detector elements are connected together through
logic circuitry, each detector element may have its own
signal-conditioning circuit, and the gains of the detector elements
with their signal-conditioning circuits can be corrected before
being combined. This can be important in order to eliminate
variations in detector performance due to detector responses, lead
lengths, cross-coupling effects, or circuit values that might lower
the energy resolution of the combined signal.
[0158] (6) Cross-strip Radiation Detectors
[0159] The invention can be used to enhance charge collection in
various types of cross-strip detectors by providing a control
electrode on the same side of a radiation detector on which the
anode is located.
[0160] In cross-strip radiation detectors, the location of a
detected photon is determined by identifying the intersection(s) of
orthogonal strips that simultaneously provide a signal from the
absorption of a photon in the detector. Cross-strip detectors
typically have been made with orthogonal strips on opposing
surfaces. The strips on the two surfaces have typically been of the
same strip-width and spacing. Systems that have deviated from this
configuration typically have done so because the same spatial
resolution was not required in both directions.
[0161] Conventional cross-strip detectors exhibit charge loss from
hole trapping that is manifest as spectral degradation and
low-energy tailing in the measured energy spectra. This
significantly reduces the photopeak efficiency for gamma-ray
energies above about 30 KeV. Applying the concepts of the invention
can significantly reduce these problems. In general, the invention
is useful in any situation where a radiation detector in a
cross-strip configuration may suffer from signal loss due to charge
trapping. Described below are several cross-strip detector
embodiments of the invention.
[0162] (A) Anode-cathode Cross-strip Detector
[0163] FIGS. 13A, 13B, and 13C show an embodiment of the invention
similar to the embodiment shown in FIG. 10A but configured as a
simple anode-cathode cross-strip detector. FIG. 13A is a top view,
FIG. 13B is a side view, and FIG. 13C is a bottom view. The anodes
are connected in rows 1300. The cathode is formed into columns 1302
that are orthogonal to the anode rows 1300. Each row 1300 and
column 1302 is coupled to amplification circuitry 1304 and
measurement circuitry (not shown). A control electrode 1308
surrounds the anodes.
[0164] The benefits of this configuration include a reduction in
the number of channels of electronics and finer spatial resolution
than the pitch of the columns 1302. That is, spacial resolution on
one side of the detector may be better than the other side with the
same electrode-strip spacing. For example, while electrons are, in
general, collected totally on one strip, trapped charge induces
signal on all strips. The two or three strips nearest the trapped
charge may have sufficient induced charge to be useable signals.
Their relative magnitudes would provide finer spatial resolution
than the pitch of the strips.
[0165] (B) Anode-control Electrode Cross-strip Detector
[0166] FIGS. 14A and 14B show an embodiment of the invention
similar to the embodiment shown in FIG. 10A but configured as a
simple anode-control electrode cross-strip detector. FIG. 14A is a
top view, and FIG. 14B is a side view. The control electrode is
patterned into columns 1400, each encasing a column of anodes 1402.
The anodes are connected in rows 1402 that are orthogonal to the
control electrode columns 1400. Each column 1400 and row 1402 is
coupled to amplification circuitry 1404 and measurement circuitry
(not shown).
[0167] This configuration allows a reduction in electronic
circuitry by measuring the signals from radiation absorption events
with the anode rows 1402 and determining which anode(s) received
the resultant charge cloud by measuring signals from the control
electrode columns 1400. Further, this configuration allows both
sets of signals to be on the same side of the detector, which has
major significance in packaging and manufacturing a detection
module so that it is "all-sides" buttable.
[0168] (C) Anode/Control Electrode-cathode Cross-strip Detector
[0169] FIG. 15A is a top isometric view of an embodiment of the
invention similar to the embodiment shown in FIG. 10A but
configured as an anode/control electrode-cathode cross-strip
detector. FIG. 15B is a bottom view of the embodiment shown in FIG.
15A. In the illustrated embodiment, the detector 1500 includes a
plurality of parallel cathode strips 1502 on one surface of a
semiconductor crystal 1504, and a plurality of alternating or
interdigitated anode strips 1506 and control electrode strips 1508
on the opposite surface of the semiconductor crystal 1504,
orthogonal to the cathode strips 1502.
[0170] In a radiation imaging system, the anode signals from the
anode strips 1506 are used to identify both incident radiation
energy and the anode strip 1506 on which the electron signal is
collected. The cathode signals from the cathode strips 1502 are
used only to identify the orthogonal position of the radiation
interaction. The only requirement for the cathode signals is that
they be large enough to be distinguished above noise levels.
[0171] By making the cathode strips 1502 wide and the anode strips
1506 narrow, the relative capacitances from the point of
interaction of a radiation event in the detector 1500 to the anode
strips 1506 can be minimized and to the cathode strips 1502 can be
maximized. This results in an electron charge signal at the anode
strips 1506 that has minimum charge loss from trapped holes, and in
a hole charge signal at the cathode strips 1502 that is maximized
because the trapped-hole charge induced on the anode strips 1506
has been minimized and is, therefore, induced on the cathode strips
1502.
[0172] As in the embodiments of the invention described above, the
control electrode strips 1508 placed between the anodes enhance
electron collection by the anode strips 1506. The width of the
control electrode strips 1508 is kept small for optimum separation
of detection rows. A detection row is the volume of detector
material between planes orthogonal to the cathode side of the
detector and passing through the center lines of consecutive
control electrode strips. Optionally, capacitance on the anode side
of the detector for shielding trapped charge from being induced on
the anode can be provided by a planar electrode 1510 that is as
close as possible to--but insulated from--the surface of the
detector. Such a structure is essentially the same as the fourth
control electrode 1210 shown in FIG. 12B. The planar electrode may
be connected to the control electrodes or to ground directly or
through coupling capacitance. In one embodiment, the planar
electrode 1510 can be provided with openings through which the
anodes 1506, electrically isolated from the planar electrode 1510,
can be coupled to measurement circuitry. Alternatively, contacts
can be made to the anodes at one end of the anode structures.
[0173] The planar-electrode capacitance 1510 enhances the
elimination of anode-signal-charge loss that results from trapped
charge, but also reduces somewhat the signal at the cathode strips
1502. However, the benefits from enhancing the anode signal more
than offset any disadvantage from degrading the cathode
signals.
[0174] Preferred embodiments of cross-strip radiation detectors
made in accordance with the invention typically follow the
following general rules:
[0175] 1. Anode strips should be as narrow as can be reliably
manufactured with high strip conductivity.
[0176] 2. Cathode strips should be as wide as can be reliably
manufactured with good resistance between strips.
[0177] 3. An interleaved set of control strips on the anode side
should be as narrow as can be reliably manufactured.
[0178] 4. A capacitance plane, tightly coupled to the detector and
covering the areas between the anode strips and the
control-electrode strips should be reliably connected to the
control strips or to circuit ground, either directly or through a
coupling capacitance.
[0179] 5. The control strips should be connected to the control
voltage supply with very low resistance to eliminate signal
coupling into the anode strips.
[0180] 6. The anode strips should be connected to preamplifiers
with very low resistance to eliminate charge coupling into other
electrodes or conductors.
[0181] 7. The cathode strips should be connected to preamplifiers
via a coupling capacitor with very low series resistance.
[0182] (7) Side-entry Radiation Detector
[0183] FIG. 16 is a perspective view of an alternative embodiment
of the invention showing a side-entry radiation detector array
structure. The structure is essentially identical to the embodiment
shown in FIG. 10A. However, in this embodiment, radiation enters
the detector 1600 from an "illumination" side 1602, parallel to the
cathode 1604 surface and anode 1606/control electrode 1608 surface,
rather than through the cathode 1604. This configuration allows
high detection efficiency of high energy gamma rays while
permitting dimensions from anode 1606 to cathode 1604 that are
smaller than would be otherwise required for such high energy gamma
rays. For example, the anode-to-cathode thickness for the
embodiment shown in FIG. 10A required to absorb very high energy
gamma rays may require a bias voltage sufficiently high so as to
cause pulse breakdown noise to be generated between conductors, or
the thickness may be so great that electron-charge trapping may
significantly degrade energy resolution. However, by using a
side-entry embodiment, any desired depth of penetration can be
achieved while allowing an optimum anode-to-cathode thickness.
[0184] The signals at the anodes 1606 can be individually measured
by electronic circuitry, or the anodes 1606 can be tied together to
form one or more "big pixel" detection elements. If the anodes 1606
are connected together in the direction of the incoming radiation,
all of the events collected by a group of anodes 1606 can be
measured with a single channel of electronics, reducing the cost of
such a detector.
[0185] Another advantage of the side-entry radiation detector array
structure is that if the anodes 1606 are measured individually or
are connected together in a pattern orthogonal to the direction of
the incoming radiation, the depth of interaction of the radiation
within the semiconductor crystal 1610 can also be measured. As
noted above in discussing measuring depth of interaction using
timing information, measurement of depth of interaction is
important for such applications as image reconstruction with coded
masks or positron emission tomography.
[0186] (8) Liquid/Gas Ionization Detectors
[0187] The principles of the invention are also applicable to other
types of detectors, such as liquid ionization detectors and gas
ionization detectors. Semiconductor radiation detectors of the type
described above are just one member of a class of radiation
detectors known as ionization detectors. In such detectors,
radiation is absorbed in an appropriate radiation interaction
material to produce mobile electric charges which are collected by
electrodes, thereby producing electrical output signals. In
addition to semiconductor materials, the radiation interaction
material may be a solid insulator (which can be considered as
semiconductors with very wide bandgaps), a liquid, or a gas.
[0188] Liquid and gas ionization detectors (also known as
ionization gauges) have been commercially available for many years
and are widely used in nuclear technology. Such detectors
essentially comprise an enclosed liquid or gaseous substance and
two electrodes, the cathode and anode. Gamma rays or x-rays
absorbed in the gas produce electrons and positive ions which, when
a bias voltage is applied, sweep to the anode and cathode,
respectively. Thus, while conceptually analogous to a semiconductor
radiation detector, a central difference is that positive charge is
carried by positive ions instead of holes.
[0189] Charge carrier trapping is generally not an issue in
ionization gauges as it is in semiconductor detectors. However,
because of their large mass, the ions travel very slowly and
generally take several milliseconds to reach the cathode (compared
to microseconds for electrons to reach the anode). When, as is
often the case, the average time between absorbed photons is less
than the ion transit time, "pulse pileup" results. To reduce or
eliminate pulse pileup, electronic shaping circuitry may be applied
that effectively cancels much or all of the contribution to the
output pulse from the positive ions.
[0190] However, with the loss of the contribution from the positive
ions, the magnitude of the output charge becomes dependent on the
position within gauge at which the electrons are produced, i.e.,
the position of the radiation event. This dependence of the output
pulse on position of the radiation event causes tailing in the
energy distribution spectrum similar to the tailing seen with
planar semiconductor detectors.
[0191] One solution to the tailing problem employed in ion gauges
is the well-known Frisch grid (O. Frisch British Atomic Energy
Report BR-49 (1944); G. F. Knoll, Radiation Detection and
Measurement (John Wiley & Sons, N.Y., 1979), pp. 178-9). The
Frisch grid consists of a screen grid placed between the cathode
and anode that is biased to a voltage intermediate between the
cathode and anode. The grid screens the electric field between
electrons and the anode when the electrons are in the region
between the grid and the cathode, but is effectively transparent to
electrons. Electrons generated in the region between the grid and
the cathode are swept through the grid to the anode, but do not
begin inducing charge on the anode until they pass the grid. Thus,
the effective "event position" for such electrons becomes that of
the grid, which is the same for all such electrons.
[0192] Problems with the Frisch grid include:
[0193] Radiation events can occur in the region between the grid
and the anode and contribute to tailing. Thus, to completely
eliminate tailing from the output, the ionization gauge must be
specially configured to prevent gamma rays from entering the
grid-anode space. This complicates and increases the cost of
operation as well as reducing overall collection efficiency.
[0194] The grid can never be completely transparent to electrons.
It therefore further reduces efficiency by collecting some fraction
of electrons.
[0195] The inclusion of the grid adds significantly to the overall
manufacturing cost of the ionization gauge.
[0196] These problems may be overcome by applying the concepts
described above for semiconductor radiation detectors. FIG. 17
shows an embodiment of the invention in the form of an ionization
gauge. In the illustrated embodiment, a closed chamber 1700, which
may be made from glass, includes a cathode 1702 covering the inner
surface of one end, an anode 1704 covering a very small area on the
inner surface of the other end, and a control electrode 1706 on an
inner surface surrounding the anode 1704. A gas, such as argon at
about 4 atmospheres, fills the interior of the chamber 1700.
External leads connect to the electrodes through conventional
glass-metal seals.
[0197] Alternative ways of implementing an ionization gauge
include: all three electrodes may be wires internal to a glass
enclosure that contains the gas; one or more of the electrodes may
consist of coatings on a glass containment vessel (with the control
electrode(s) on the internal or external surface); the cathode may
constitute a wall of the containment structure; other gases may be
used; liquids may be used; other shapes may be used; multiple
electrodes of any of the three types may be used.
[0198] As in the semiconductor radiation detectors described above,
the control electrode 1706 serves to focus the electric field. The
small size of the anode 1704 and the field focusing action of the
control electrode 1706 eliminate tailing in the same manner as they
do in a semiconductor device. The same principles apply except that
the performance limiting factor is a difference in mobilities
between electrons and ions instead of a .mu..tau. product
difference between electrons and holes. Such a detector is less
costly to produce than a Frisch grid and is applicable in
configurations where a Frisch grid cannot be effected.
[0199] (9) Conclusion
[0200] An essential feature of the invention is to employ a
combination of control electrode(s), anode(s) and cathode(s) in
such a way that essentially the full electron charge from a
radiation absorption event is collected at the anode, while the
effects of hole trapping or loss of positive ion signal are
shielded from the anode and most low-energy tailing is eliminated
from the signal. The electrodes are also configured so as to form
an electric field pattern within the detector that directs electron
clouds produced by ionizing radiation efficiently to the
anode(s).
[0201] A number of embodiments of the invention have been
described. Nevertheless, it will be understood that various
modifications may be made without departing from the spirit and
scope of the invention. For example, different independent aspects
of the various embodiments described above may be combined into new
embodiments. As one example, shielding control electrodes can be
added to the embodiments shown in FIGS. 13-17. As another example,
supplemental shielding control electrodes can be added to such
combinations of shielding control electrodes and the embodiments
shown in FIGS. 13-17. Accordingly, it is to be understood that the
invention is not to be limited by the specific illustrated
embodiment, but only by the scope of the appended claims.
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