U.S. patent application number 11/513944 was filed with the patent office on 2007-03-15 for neutron and gamma-ray detection system.
Invention is credited to Ulisse Bravar, John R. Macri, Mark L. McConnell, James M. Ryan.
Application Number | 20070057194 11/513944 |
Document ID | / |
Family ID | 37854149 |
Filed Date | 2007-03-15 |
United States Patent
Application |
20070057194 |
Kind Code |
A1 |
Ryan; James M. ; et
al. |
March 15, 2007 |
Neutron and gamma-ray detection system
Abstract
The present invention is a radially symmetric imaging detector
that measures an incident neutron's or gamma-ray's energy and
identifies its source on an event-by-event basis.
Inventors: |
Ryan; James M.; (Lee,
NH) ; Macri; John R.; (Durham, NH) ;
McConnell; Mark L.; (Newmarket, NH) ; Bravar;
Ulisse; (Omaha, NE) |
Correspondence
Address: |
DEVINE, MILLIMET & BRANCH, P.A.
111 AMHERST STREET
BOX 719
MANCHESTER
NH
03105
US
|
Family ID: |
37854149 |
Appl. No.: |
11/513944 |
Filed: |
August 31, 2006 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
60713104 |
Aug 31, 2005 |
|
|
|
Current U.S.
Class: |
250/390.11 ;
250/367 |
Current CPC
Class: |
G01T 1/2907 20130101;
G01T 1/20 20130101; G01T 3/06 20130101 |
Class at
Publication: |
250/390.11 ;
250/367 |
International
Class: |
G01T 3/06 20060101
G01T003/06; G01T 1/20 20060101 G01T001/20 |
Goverment Interests
GOVERNMENT SPONSORSHIP
[0002] The development of the present invention was funded in part
by the DoE of the United States Government under Contract No.
DE-FG52-04NA25687 and by the NASA of the United States Government
under Contract No. NAG5-13519.
Claims
1. A neutron detector to determine the direction and energy of each
incident neutron comprising multiple substantially parallel
scintillator bars, said bars being positioned such that the bars
are radially symmetric.
2. The neutron detector of claim 1 wherein said bars contain
organic scintillating material.
3. A neutron detector to determine the direction and energy of each
incident neutron comprising multiple substantially parallel
scintillator bars, said bars each having a first end and a second
end and said bars being positioned such that the bars are radially
symmetric.
4. The neutron detector of claim 3 further comprising a light
sensing means, attached at the first end and second end of each of
said bars, for detecting n-p scatter events in each of said
bars.
5. The neutron detector of claim 4 further comprising electronic
signal processing means to process the light-sensing signals and
measure the sequence, positions, energies, relative times and pulse
shapes of each coincident n-p interaction.
6. A method of detecting neutrons by determining the direction and
energy of each incident neutron comprising positioning multiple
scintillator bars so that said bars are substantially parallel;
further positioning said bars so that the bars are radially
symmetric; attaching a light sensing means to the ends of each of
said bars; detecting successive n-p scatters in different bars
caused by an incident neutron; determining the relative times of
the successive n-p scatters; and determining the sequence,
positions, relative times and energies of a recoil proton and a
scattered neutron resulting from each successive scatters.
7. A neutron detector to determine the direction and energy of each
incident neutron comprising multiple substantially parallel
scintillator bars, said bars being positioned such that the bars
are radially symmetric providing a 360.degree. wide scan without
changing the detector's orientation.
8. A gamma-ray detector to determine the direction and energy of
each incident gamma-ray comprising multiple substantially parallel
scintillator bars, said bars being positioned such that the bars
are radially symmetric.
9. The gamma-ray detector of claim 8 wherein said bars contain
organic scintillating material.
10. A gamma-ray detector to determine the direction and energy of
each incident gamma-ray comprising multiple substantially parallel
scintillator bars, said bars each having a first end and a second
end and said bars being positioned such that the bars are radially
symmetric.
11. The gamma-ray detector of claim 10 further comprising a light
sensing means attached at the first end and second end of each of
said bars, for detecting Compton interactions in each of said
bars.
12. The gamma-ray detector of claim 11 further comprising
electronic signal processing means to process the light-sensing
signals and measure the sequence, positions, energies, relative
times and pulse shapes of each coincident Compton interaction.
13. a method of detecting gamma-rays by determining the direction
and energy of each incident gamma-ray comprising. positioning
multiple scintillator bars so that said bars are substantially
parallel; further positioning said bars so that the bars are
radially symmetric; attaching a light sensing means to the ends of
each of said bars; detecting successive Compton interactions in
different bars caused by an incident gamma-ray; determining the
relative times of the successive Compton interactions; and
determining the sequence, positions, relative times and energies of
a recoil electron and a scattered gamma-ray resulting from each
successive interaction.
14. A gamma-ray detector to determine the direction and energy of
each incident gamma-ray comprising multiple substantially parallel
scintillator bars, said bars being positioned such that the bars
are radially symmetric providing a 360.degree. wide scan without
changing the detector's orientation.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] The present application claims the benefit of provisional
patent application Ser. No. 60/713,104 filed Aug. 31, 2005, which
is incorporated herein by reference.
TECHNICAL FIELD
[0003] The present invention relates to a system for neutron and
gamma-ray detection. More specifically, it relates to a radially
symmetric imaging detector that directly measures the incident
radiation.
BACKGROUND INFORMATION
[0004] Because they are electrically neutral, neutrons and
gamma-rays have been traditionally detected using indirect means.
However, typical indirect techniques of the prior art, for
neutrons, for example, while able to measure count rate, provide
little, if any, information on the neutron's energy or the location
of the neutron's source. This lack of information limits the
usefulness of prior art detectors in a number of applications,
including the detection of special nuclear material (SNM). These
materials--specifically uranium and transuranics--emit neutrons via
spontaneous or induced fission, which neutron emissions are unique
to fissionable material.
[0005] While position sensitive neutron detectors have been
described in the prior art, such as the COMPTEL as described in J.
Ryan, et al., "COMPTEL as a Solar Gamma-Ray and Neutron Detector,"
presented at Data Analysis in Astronomy; 1992, the active areas of
these prior art detectors were typically a flat surface, with a
limited field of view. The radial symmetry of the detection of the
present invention is a desirable feature in several applications.
In space-based solar observations, the detector is typically
installed on a spacecraft spinning around an axis orthogonal to the
direction to the Sun. Therefore, a flat-surface detector has a
time-dependent sensitivity to solar events, which is undesirable
when detecting time-varying neutron or gamma-ray fluxes, such as
the ones from solar flares. In another important application, the
search for SNM emitting neutrons, a radially symmetric detector
placed in any area (e.g. a storage warehouse or loading dock)
provides a complete 360.degree. wide scan with no need to change
its orientation.
[0006] A cylindrically symmetric imaging neutron detector described
in the prior art is described in U.S. Pat. No. 5,345,084. However,
the detector therein is based on count rate rather than measurement
of individual neutrons and, as a result, provides no information on
neutron energy and no means to identify gamma-rays. With respect to
the coordinate system in FIG. 1, the detector of the prior art can
determine the azimuthal angle .phi. of the neutron source (with
relatively poor accuracy,
.DELTA..phi..about.30.degree.-45.degree.), but is unable to measure
the zenith angle .theta., making it impossible to locate point
sources.
SUMMARY OF THE INVENTION
[0007] The present invention is a system comprising a radially
symmetric imaging detector that directly measures an incident
neutron's or gamma-ray's energy and identifies the point source of
the neutron or gamma-ray on an event-by-event basis through an
event circle analysis.
BRIEF DESCRIPTION OF THE DRAWINGS
[0008] These and other features and advantages of the present
invention will be better understood by reading the following
detailed description, taken together with the drawings wherein:
[0009] FIG. 1 is a schematic diagram of one preferred embodiment of
the present invention;
[0010] FIG. 2 is a side view of one of the plastic scintillator
bars used in the preferred embodiment of the present invention
shown in FIG. 1;
[0011] FIG. 3 is an end view of one preferred embodiment of the
present invention;
[0012] FIG. 4 is a side view of the preferred embodiment of the
present invention shown in FIG. 3;
[0013] FIG. 5 is a top view of one preferred embodiment of the
present invention;
[0014] FIG. 6 is a side view of the preferred embodiment of the
present invention shown in FIG. 5; and
[0015] FIG. 7 is a schematic diagram of a double n-p scatter events
detected by a preferred embodiment of the present invention.
DETAILED DESCRIPTION OF THE INVENTION
[0016] The present invention relates to a radially symmetric
imaging detection system for neutrons or gamma-rays.
[0017] With respect to neutrons, the present invention measures the
energy of an incident neutron, and through scattering kinematics
determines the point or extended sources of the neutron. This
technique is based on a known detection mechanism--fast neutron
scattering off ambient hydrogen (n-p scattering). Such a detection
system is configured to locate n-p scatter sites within its volume
using the scintillation light generated by recoil protons, highly
ionizing particles. For the neutrons that undergo at least two
successive n-p scatters, an image revealing the location of a
source can be constructed.
[0018] FIG. 1 shows one preferred embodiment of the imaging
detection system 2 of the present invention. The design consists of
a radially symmetric array 4 of multiple (in this embodiment 13)
parallel scintillator bars 6. The scintillator bars 6 are
preferably organic scintillator bars, either plastic or liquid,
selected for their relative abundance of protons. The dimensions of
each scintillator bar 6 used in a particular embodiment may vary,
from case to case, depending on the desired application. For
example, as shown in FIG. 2, typical values for a plastic
scintillator bar 20, are 30.0 cm length 22 and 1.5 cm diameter 24.
The total number of the scintillator bars and the requisite
cylindrical symmetric array of the bars may also vary, from
embodiment to embodiment, depending on the desired application, as
is discussed in more detail below. For example, as shown in FIG. 3,
the detection system 30 comprises a cylindrical symmetric array 34
of 19 scintillator bars 36 in which each scintillator bar has a
diameter of 1.5 cm and a length of 30 cm. As shown in FIG. 4, this
detection system 30 has resulting overall dimensions of 11.5 cm
diameter 38 and 51.0 cm length 40.
[0019] It is to be understood that the term "scintillator bar," as
used herein, includes optically separated chambers, filled with
scintillation material, in a unitary housing. Thus, FIGS. 5 and 6
show a preferred embodiment of the imaging detection system 52
present invention in which the design consists of a radially
symmetric array 54 of multiple (in this case 37) scintillator bars,
or chambers 56, in a liquid scintillator tank 58 divided by baffles
into optically separated chambers 56.
[0020] Referring again to the preferred embodiment of FIG. 1, a
photomultiplier tube (PMT) 8, or other light sensing device, known
to those skilled in the art, is connected to the first and the
second end of each scintillator bar 6. Each PMT 8 is, in turn,
connected 10 to signal processing electronics 12, to which it sends
light-sensing signals, consisting of pulse processing electronics
known to those skilled in the art. Fast discriminators and
coincidence circuits are employed to initiate the measurement of
the required parameters for each incident particle registering two
or more interactions. The required parameters, interaction
locations, energies, relative times and pulse shapes are digitized
and registered as a series of detector IDs, pulse heights, and
times of flight for each detected particle. A data acquisition
system records these parameters for each detected particle for
subsequent imaging and energy analysis.
[0021] Referring still to FIG. 1, the technique for detecting a
neutron 11 impinging the detection system 2 is based on the
measurement of the energies, positions, sequence and relative times
of interaction of recoil protons resulting from multiple,
successive, neutron-proton)(n-p) scatters 13 and on the kinematics
of n-p scattering for reconstruction of the incident neutron energy
and direction. In the case of gamma-rays, the same technique
applies, but Compton-scatter electrons are used instead of protons.
With respect to neutrons, scintillator bar 6 material and diameter
are chosen to maximize the probability of single n-p scatters
occurring within one scintillator bar 6, with the scattered charged
particle being fully contained within the boundaries of the
scintillator bar 6. At the same time, the scintillator bars 6
should be sufficiently thin for a scattered neutron to exit the bar
after the first n-p scatter 13 and to produce successive n-p
scatters 13 in other scintillator bars 6.
[0022] Energy information on a recoil proton, or Compton electron
in the case of gamma-rays, resulting from an elastic n-p scatter in
a given scintillator bar 6, is obtained from the amplitude of the
signals measured by the PMTs 8 at the first and second ends of the
scintillator bar 6. Position information on the proton in the x-y
plane is determined from the position in the x-y plane of the
scintillator bar 6 in which the interaction occurs. Position
information on the proton along the z-axis is measured by analyzing
the arrival time differences and/or the amplitude differences of
signals measure by PMTs 8 at the ends of the scintillator 6 in
which the interaction occurs. The signals measured by the PMTs at
the ends of the scintillator bars 6 in which successive n-p
scatters occur also provide a measure of the relative times of the
successive scatters.
[0023] Referring to FIG. 7, a neutron 71, whose incident direction
is unknown, undergoes at least two n-p scatters 72, 73. By
measuring the coordinates of the two interactions, the relative
times of the two interactions and the energy of the recoil proton
of the first interaction, one can determine the energy and
direction (i.e. vector velocity) of this particle. The neutron
scatter angle, shown as .theta..sub.n, 74 is given by: sin 2
.times. .theta. ^ n = E p .times. .times. 1 E n ##EQU1## where
E.sub.p1 and E.sub.n are the energies of the first recoil proton
and the incident neutron, respectively. Once E.sub.p1 and E.sub.n
are known, one can determine .theta..sub.n.
[0024] However, measurement of the sequence, energies and positions
of the protons resulting from two successive n-p scatters of an
incident neutron is not sufficient to localize an unknown source of
neutrons. One more piece of information is needed, the energy of
the incident neutron, En. In the present invention, the energy of
the incident neutron is determined by measuring the time difference
between the two successive n-p scatters 72, 73. This time
difference provides the velocity and thus the energy of the neutron
scattered after the first recoil. The incident neutron energy,
E.sub.n, is the sum of this scattered neutron's energy and the
energy of the first scattered proton, E.sub.p1. In addition, this
time difference allows for the separation of 1-100 MeV neutrons
from gamma-rays.
[0025] FIG. 5 shows a schematic diagram of the basic kinematics of
event reconstruction for two successive n-p scatter events 72, 73
caused by a neutron 71. Again, .theta..sub.n 74 the neutron scatter
angle is given by sin 2 .times. .theta. n = E p .times. .times. 1 E
n ##EQU2## where E.sub.p1 and E.sub.n are the energy of the first
recoil proton and the incident neutron, respectively. Hard-sphere
scattering implies that the scattered neutron and proton momenta
will lie at right angles to one another and that the incident
neutron direction must lie on a cone 75 about the recoil neutron
velocity vector. The projection of this cone on the image plane or
the celestial sphere is an event circle 76. The superposition of
event circles from many incident neutrons provides the statistical
information necessary to locate an unknown source of neutrons,
event circles from a point source intersect, but unrelated, for
example background event circles, do not. This procedure has been
demonstrated successfully on the COMPTEL experiment by imaging MeV
gamma-ray and neutron sources.
[0026] If certain liquid scintillator materials are used instead of
plastic scintillator materials, pulse shape discrimination (PSD)
techniques can be employed to further discriminate neutron form
gamma-ray interactions.
[0027] Finally, by preferably augmenting the structure of the
detection system of the present invention with an anticoincidence
shield, unrelated charged particles can be excluded.
[0028] While the principles of the invention have been described
herein, it is to be understood by those skilled in the art that
this description is made only by way of example and not as a
limitation as to the scope of the invention. Other embodiments are
contemplated within the scope of the present invention in addition
to the exemplary embodiments shown and described herein.
Modifications and substitutions by one of ordinary skill in the art
are considered to be within the scope of the present invention,
which is not to be limited except by the following claims.
* * * * *