U.S. patent application number 14/989590 was filed with the patent office on 2017-07-06 for combined neutron and gamma-ray detector and coincidence test method.
This patent application is currently assigned to Raytheon Company. The applicant listed for this patent is Raytheon Company. Invention is credited to Siddhartha Ghosh, Kelly Jones, David R. Rhiger, Justin Gordon Adams Wehner.
Application Number | 20170192113 14/989590 |
Document ID | / |
Family ID | 57868388 |
Filed Date | 2017-07-06 |
United States Patent
Application |
20170192113 |
Kind Code |
A1 |
Rhiger; David R. ; et
al. |
July 6, 2017 |
COMBINED NEUTRON AND GAMMA-RAY DETECTOR AND COINCIDENCE TEST
METHOD
Abstract
A method for detecting both gamma-ray events and neutron events
with a common detector, where the detector includes a layer of
semiconductor material adjacent one side of a glass plate and a Gd
layer on an opposite side of the glass plate, between the glass
plate and a layer of silicon PIN material to form an assembly that
is bounded by electrodes, including a semiconductor anode on one
side of the semiconductor layer, a cathode connected to the glass
plate, and a Si PIN anode on a side of the Si PIN layer opposite
the semiconductor anode. The method includes the steps of: (1)
monitoring the electrical signal at each of the semiconductor anode
and the Si PIN anode, and (2) comparing signals from the
semiconductor anode and the SI PIN anode to differentiate between
gamma-ray events and neutron events based on predetermined
criteria.
Inventors: |
Rhiger; David R.; (Santa
Barbara, CA) ; Wehner; Justin Gordon Adams; (Goleta,
CA) ; Jones; Kelly; (Goletta, CA) ; Ghosh;
Siddhartha; (Los Angeles, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Raytheon Company |
Waltham |
MA |
US |
|
|
Assignee: |
Raytheon Company
Waltham
MA
|
Family ID: |
57868388 |
Appl. No.: |
14/989590 |
Filed: |
January 6, 2016 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G01T 3/08 20130101; G01T
1/24 20130101; G01T 1/247 20130101 |
International
Class: |
G01T 3/08 20060101
G01T003/08; G01T 1/24 20060101 G01T001/24 |
Claims
1. A detector for both gamma-rays and neutrons, comprising: a
semiconductor layer including a semiconductor material suitable for
capturing gamma-rays; a glass plate in contact with the
semiconductor layer; a gadolonium (Gd) converter layer in contact
with the glass plate opposite the semiconductor layer for capturing
neutrons; a layer of silicon PIN (p-type/intrinsic/n-type) material
of suitable thickness in contact with the Gd converter layer
opposite the glass plate to detect electrons produced by neutrons
captured in the Gd converter layer; a cathode contact in electrical
contact with the glass plate; a first anode contact in contact with
the semiconductor layer; a second anode contact in contact with the
silicon PIN layer; and a processor in electric contact with the
first and second anode contacts, the processor being configured to
cooperate with the anode contacts and the cathode contacts to
establish electric fields across the semiconductor layer and the Si
PIN layer, and being configured to differentiate between signals
generated by a neutron event and signals generated by a gamma-ray
event.
2. A detector as set forth in claim 1, where the semiconductor
material includes any of cadmium-zinc-telluride (CdZnTe),
high-resistivity gallium arsenide (GaAs), and high purity germanium
(HPGe).
3. A detector as set forth in claim 1, further comprising a printed
circuit board (PCB) in electrical contact with respective ones of
the first and second anode contacts to connect the first and second
anode contacts to respective contacts in respective processors.
4. A plurality of adjacent spaced-apart detectors as set forth in
claim 3, with a plurality of detectors sharing common PCBs.
5. A detector as set forth in claim 1, where the first and second
anode contacts are divided into an array of pixels.
6. An array of detectors as set forth in claim 1.
7. A detector as set forth in claim 1, where the semiconductor
layer includes cadmium-zinc-telluride (CdZnTe) and is separated
into pixels aligned with pixels in the Si PIN layer.
8. A detector as set forth in claim 1, where the semiconductor
layer has a thickness of 0.35 cm, the glass plate has a thickness
of at least 300 .mu.m, the Gd layer has a thickness of 5 .mu.m, and
the Si PIN layer has a thickness of 280 .mu.m.
9. A detector as set forth in claim 1, where the semiconductor
layer has a thickness of 1.0 cm or 1.5 cm or 2.0 cm.
10. A controller for use with a common detector for both gamma-rays
and neutrons including a semiconductor layer including a
semiconductor material suitable for capturing gamma-rays, a glass
plate in contact with the semiconductor layer, a gadolonium (Gd)
converter layer in contact with the glass plate opposite the
semiconductor layer for capturing neutrons, a layer of silicon PIN
(p-type/intrinsic/n-type) material of suitable thickness in contact
with the Gd converter layer opposite the glass plate for detecting
electrons produced from a neutron captured in the Gd converter
layer, a cathode contact in electrical contact with the glass
plate, a first anode contact in contact with the semiconductor
layer, and a second anode contact in contact with the silicon PIN
layer, the controller comprising: a processor configured to
cooperate with the first anode, the second anode and the cathode to
establish an electric field across the semiconductor layer and the
silicon PIN layer; and the processor is configured to differentiate
between signals generated by a neutron event and signals generated
by a gamma-ray event as a function of coincidence testing of
signals received from each of the first and second anodes.
11. A method for detecting both gamma-ray events and neutron events
with a common detector, the detector including a layer of
semiconductor material adjacent a glass plate, a gadolinium (Gd)
converter layer adjacent an opposite side of the glass plate, and a
layer of silicon PIN material in contact with the Gd converter
layer on an opposite side of the glass plate to form a subassembly
that is bounded by electrodes, including a semiconductor anode on
one side of the semiconductor layer, a cathode connected to the
glass plate, and a Si PIN cathode on a side of the Si PIN layer
opposite the semiconductor anode, the method comprising the steps
of: monitoring the electrical signal at each of the semiconductor
anode and the Si PIN cathode; and comparing signals from the
semiconductor anode and the Si PIN cathode to differentiate between
gamma-ray events and neutron events based on predetermined
criteria.
12. A method as set forth in claim 11, comprising the step of
establishing an electric field within the semiconductor layer and
the Si PIN layer.
13. A method as set forth in claim 11, including the step of
providing a common gamma-ray and neutron detector with a controller
connected to the first and second anodes and the cathode, the
controller including a processor and a memory.
14. A detector for both gamma rays and neutrons, comprising: means
for capturing gamma-rays that produces an electron; means for
capturing neutrons that produces an electron; means for separating
electrons generated by the means for capturing gamma rays from the
means from electrons generated by the means for capturing neutrons;
means for establishing an electric field across the means for
capturing neutrons and the means for capturing gamma rays; and
means for differentiating between signals generated by a neutron
event and signals generated by a gamma-ray event.
15. A detector as set forth in claim 14, where the means for
capturing gamma-rays includes a semiconductor layer having a
semiconductor material suitable for capturing gamma-rays.
16. A detector as set forth in claim 15, where the means for
capturing gamma-rays includes a layer of crystallized
cadmium-zinc-telluride (CdZnTe) (CZT).
17. A detector as set forth in claim 14, where the means for
capturing neutrons that produces an electron includes a gadolonium
(Gd) converter layer for capturing neutrons and producing an
electron and a layer of silicon PIN (p-type/intrinsic/n-type)
material of suitable thickness in contact with the Gd converter
layer for detecting the electron.
18. A detector as set forth in claim 14, where the means for
separating electrons generated by the means for capturing gamma
rays from electrons generated by the means for capturing neutrons
includes a glass plate.
19. A detector as set forth in claim 14, where the means for
applying an electric field across the means for capturing neutrons
and the means for capturing gamma rays includes: a cathode contact
in electrical contact with the separating means; a first anode
contact in contact with the gamma-ray capturing means; a second
anode contact in contact with the neutron capturing means; and a
processor in electric contact with the first and second anode
contacts, the processor being configured to cooperate with the
first and second anode contacts and the cathode contacts to
establish an electric field across the means for capturing neutrons
and the means for capturing gamma rays.
20. A detector as set forth in claim 14, where the means for
differentiating between signals generated by a neutron event and
signals generated by a gamma-ray event includes a processor in
electric contact with the means for establishing an electric field
across the means for capturing neutrons and the means for capturing
gamma rays, the processor being configured to differentiate between
signals generated by a neutron event and signals generated by a
gamma-ray event.
Description
FIELD OF THE INVENTION
[0001] This invention concerns a detector for both neutrons and
gamma rays and a corresponding method for improving the detection
of neutrons and gamma rays in a common detector.
BACKGROUND
[0002] Radiation detectors have many important uses in nuclear
energy, physics research, materials science, and radiation safety,
among others. Two types of radiation often of interest are neutrons
and gamma rays.
[0003] One way to detect these types of radiation uses a
scintillator material called CLYC (which is Cs2LiYCl6:Ce3+),
typically in the form of a crystal. Like other scintillators, a
CLYC crystal produces a flash of light when capturing a gamma ray.
The flash of light can be turned into an electrical signal for
further analysis. A CLYC crystal also can be used to capture
neutrons through a nuclear reaction with lithium (Li) atoms in the
crystal, and also produces a flash of light due to the energetic
particles from the neutron-lithium reaction. Unfortunately, these
crystals can be difficult to grow and thus are quite expensive, and
it can be difficult to distinguish the flashes of light due to
neutrons from the flashes of light due to gamma rays.
[0004] Another way to detect these types of radiation relies on the
capture of neutrons by cadmium (Cd) in crystals of
cadmium-zinc-telluride (CdZnTe) (often abbreviated CZT). CZT also
is used in detectors for gamma-ray radiation. The neutron-cadmium
reaction produces gamma rays that can be detected by pulses of
electrons from the CZT, but the sensitivity is low and it is
difficult to distinguish whether the pulse of electrons was caused
by a neutron or a gamma ray.
SUMMARY
[0005] The present invention provides a combined neutron and
gamma-ray detector and method that is sensitive to both neutrons
and gamma-rays in the same detector, improves the ability to
distinguish between the two kinds of radiation, is compact,
requires relatively little power, and is relatively inexpensive
compared to current radiation detection devices and methods.
[0006] More specifically, the present invention provides a method
for detecting both gamma-ray events and neutron events with a
common detector that includes a layer of semiconductor material
adjacent a glass plate, a gadolinium (Gd) converter layer adjacent
an opposite side of the glass plate, and a layer of silicon PIN
material in contact with the Gd converter layer on an opposite side
of the glass plate to form an assembly that is bounded by
electrodes, including a semiconductor anode on one side of the
semiconductor layer, a cathode connected to the glass plate, and a
Si PIN anode on a side of the Si PIN layer opposite the
semiconductor anode. The method includes the steps of: (1)
monitoring the electrical signal at each of the semiconductor anode
and the Si PIN anode, and (2) comparing signals from the
semiconductor anode and the SI PIN anode to differentiate between
gamma-ray events and neutron events based on predetermined
criteria.
[0007] The method may further include one or more of the steps of
establishing an electric field within the semiconductor layer and
the Si PIN layer, and providing a common gamma-ray and neutron
detector with a controller connected to the first and second anodes
and the cathode, the controller including a processor and a
memory.
[0008] The present invention also provides a detector for both
gamma-rays and neutrons that includes (a) a semiconductor layer
including a semiconductor material suitable for capturing
gamma-rays, (b) a glass plate in contact with the semiconductor
layer, (c) a gadolonium (Gd) converter layer in contact with the
glass plate opposite the semiconductor layer, (d) a layer of
silicon PIN (p-type/intrinsic/n-type) material of suitable
thickness for detecting electrons emitted from neutrons captured by
the Gd converter layer in contact with the silicon PIN layer
opposite the glass plate, (e) a cathode contact in electrical
contact with the glass plate, (f) a first anode contact in contact
with the semiconductor layer, (g) a second anode contact in contact
with the silicon PIN layer, and (h) a processor in electric contact
with the first and second anode contacts. The processor is
configured to cooperate with the anode contacts and the cathode
contacts to establish electric fields across the semiconductor
layer and the Si PIN layer, and is configured to differentiate
between signals generated by a neutron event and signals generated
by a gamma-ray event.
[0009] The semiconductor material may include any of
cadmium-zinc-telluride (CdZnTe), high-resistivity gallium arsenide
(GaAs), and high purity germanium (HPGe).
[0010] The detector may further include a printed circuit board
(PCB) in electrical contact with respective ones of the first and
second anode contacts to connect the first and second anode
contacts to respective contacts in respective processors. A
plurality of adjacent spaced-apart detectors may share a common
PCB.
[0011] The first and second anode contacts may be divided into an
array of pixels. The semiconductor layer may include
cadmium-zinc-telluride (CdZnTe) and may be separated into pixels
aligned with pixels in the Si PIN layer.
[0012] The semiconductor layer may have a thickness of 0.35 cm, the
glass plate may have a thickness of at least 300 .mu.m, the Gd
layer may have a thickness of 5 .mu.m, and the Si Pin layer may
have a thickness of 280 .mu.m. Alternatively, the semiconductor
layer may have a thickness of 1.0 cm or 1.5 cm or 2.0 cm.
[0013] The present invention further provides a controller for use
with a common detector for both gamma-rays and neutrons, where the
detector includes a semiconductor layer including a semiconductor
material suitable for capturing gamma-rays, a glass plate in
contact with the semiconductor layer, a gadolonium (Gd) converter
layer in contact with the glass plate opposite the semiconductor
layer, a layer of silicon PIN (p-type/intrinsic/n-type) material of
suitable thickness for detecting electrons emitted from neutrons
captured by the Gd converter layer in contact with the silicon PIN
layer, a cathode contact in electrical contact with the glass
plate, a first anode contact in contact with the semiconductor
layer, and a second anode contact in contact with the silicon PIN
layer. The controller includes a processor configured to cooperate
with the first anode, the second anode and the cathode to establish
an electric field across the semiconductor layer and the silicon
PIN layer. The processor is configured to differentiate between
signals generated by a neutron event and signals generated by a
gamma-ray event as a function of coincidence testing of signals
received from each of the first and second anodes.
[0014] The present invention also provides a detector for both
gamma rays and neutrons that includes (a) means for capturing
gamma-rays that produces an electron, (b) means for capturing
neutrons that produces an electron, (c) means for separating
electrons generated by the means for capturing gamma rays from the
electrons generated by the means for capturing neutrons, (d) means
for applying an electric field across the means for capturing
neutrons and the means for capturing gamma rays, and (e) means for
differentiating between signals generated by a neutron event and
signals generated by a gamma-ray event.
[0015] The means for capturing gamma-rays may include a
semiconductor layer having a semiconductor material suitable for
capturing gamma-rays.
[0016] The means for capturing gamma-rays may include a layer of
crystallized cadmium-zinc-telluride (CdZnTe) (CZT).
[0017] The means for capturing neutrons that produces an electron
may include a gadolonium (Gd) converter layer for capturing
neutrons and producing an electron and a layer of silicon PIN
(p-type/intrinsic/n-type) material of suitable thickness in contact
with the Gd converter layer for detecting the electron The means
for separating electrons generated by the means for capturing gamma
rays from electrons generated by the means for capturing neutrons
may include a glass plate.
[0018] The (d) means for applying an electric field across the
means for capturing neutrons and the means for capturing gamma rays
may include (1) a cathode contact in electrical contact with the
separating means, (2) a first anode contact in contact with the
gamma-ray capturing means, (3) a second anode contact in contact
with the neutron capturing means, and (4) a processor in electric
contact with the first and second anode contacts. The processor may
be configured to cooperate with the first and second anode contacts
and the cathode contacts to establish an electric field across the
means for capturing neutrons and the means for capturing gamma
rays.
[0019] The means for differentiating between signals generated by a
neutron event and signals generated by a gamma-ray event may
include a processor in electric contact with the means for
establishing an electric field across the means for capturing
neutrons and the means for capturing gamma rays. The processor is
configured to differentiate between signals generated by a neutron
event and signals generated by a gamma-ray event.
[0020] The foregoing and other features of the invention are
hereinafter fully described and particularly pointed out in the
claims, the following description and the annexed drawings setting
forth in detail one or more illustrative embodiments of the
invention. These embodiments, however, are but a few of the various
ways in which the principles of the invention can be employed.
Other objects, advantages and features of the invention will become
apparent from the following detailed description of the invention
when considered in conjunction with the drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0021] FIG. 1 is a schematic cross-section of a combined neutron
and gamma-ray detector provided by the invention.
[0022] FIG. 2 is a top view of the detector of FIG. 1.
[0023] FIGS. 3A and 3B are graphical representations of gamma ray
energy resolution in the detector.
[0024] FIG. 4 is a graphical representation of the energy spectrum
of internal conversion electrons due to neutrons received in the
detector.
[0025] FIG. 5 is a schematic cross-section of a portion of the
detector illustrating a neutron event.
DETAILED DESCRIPTION
[0026] As noted above, the present invention provides a combined
neutron and gamma-ray detector and method that employs a
coincidence test for pulses in two different detection materials to
detect both thermal or near-thermal neutrons and gamma rays.
[0027] The detector combines two different detector materials in a
common structure, one of which provides a means for capturing
gamma-rays that produces an electron, and the other of which
provides a means for capturing neutrons that produces an electron.
The detector further includes means for separating the electrons
generated by the respective means for capturing gamma-rays and the
means for capturing neutrons. The detector also includes means for
establishing an electric field across each of the means for
capturing gamma-rays and the means for capturing neutrons. And the
detector includes means for differentiating between signals
generated by a neutron event and signals generated by a gamma-ray
event.
[0028] An exemplary means for capturing gamma-rays includes a layer
of semiconductor material configured to capture gamma rays, such as
Cadmium-Zinc-Telluride (CdZnTe) (referred to as CZT), which is
available from eV Microelectronics of Saxonburg, Pa., U.S.A., for
example. An exemplary means for capturing neutrons includes a layer
of a gadolonium (Gd), referred to as a Gd converter layer. The Gd
converter layer produces an electron when a neutron is captured,
and the electron is then detected in a layer of silicon PIN
(p-type/intrinsic/n-type) material of suitable thickness. The
detector also includes a controller connected to the layers of
detector materials that provides a means for establishing electric
fields across the CZT layer and the Si PIN layer, and uses a
coincidence test to check for pulses occurring in the two material
layers and differentiates between signals generated to determine
whether a pulse is the result of a neutron or a gamma ray.
[0029] Turning now to the drawings in detail, and starting with
FIG. 1, an exemplary detector 10 for detecting both gamma-rays and
neutrons is shown. The detector 10 has a compact all-solid-state
structure that includes a series of layered elements that form a
common detector assembly. Although the illustrated embodiment
includes two detector assemblies, the detector 10 could function
with just one or any reasonable number of detector assemblies. The
detector assemblies are identical, and reference to either a
detector 10 or to a detector assembly generally will be
equivalent.
[0030] Each detector 10 includes a layer of semiconductor material
12 suitable for capturing gamma-rays, such as a layer of
Cadmium-Zinc-Telluride (CdZnTe) (CZT), aligned with a layer of
silicon (Si) PIN (silicon p-type/intrinsic/n-type) 14 of the same
area and having a suitable thickness for detecting electrons
produced by neutrons, as further described below. Although the
illustrated embodiment will be described with reference to the CZT
layer 12, other semiconductor materials may be used in place of
CZT, including high-resistivity gallium arsenide (GaAs) and high
purity germanium (HPGe). The CZT layer 12 is relatively thick
compared to the other layers. The CZT layer 12, because of its
composition of relatively heavy atoms and thick configuration, is
sensitive to gamma rays, including those not connected with a
neutron capture, as well as those resulting from a neutron
capture.
[0031] In contrast to the CZT layer 12, the Si PIN layer 14 is
relatively insensitive to gamma rays because of its composition of
relatively light atoms and thin configuration. A glass plate 16 or
other insulating material in contact with the CZT layer 12 isolates
and separates the Si PIN layer 14 from the CZT layer 12. The glass
plate 16 separating the CZT and Si PIN layers 12 and 14 generally
is relatively thin in comparison but has sufficient thickness to
provide means for separating electrons generated in the CZT layer
12 from electrons detected in the Si PIN layer 14. The glass plate
16 prevents electrons from traveling between the CZT layer 12 and
the Si PIN layer 14.
[0032] The means for capturing neutrons includes a gadolinium (Gd)
converter layer (Gd layer 20). The Gd layer 20 is relatively thin
and lies adjacent to the glass plate 16 separating the Gd layer 20
and the CZT layer 12. Thus the Gd layer 20 is in contact with one
side of the glass plate 16 opposite the CZT layer 12, between the
Si Pin layer 14 and the glass plate 16. An exemplary Gd layer 20 is
a 5-.mu.m layer of Gadolinium-157 (.sup.157Gd). The Gd layer 20
absorbs neutrons with high efficiency and consequently emits
energetic electrons and gamma rays. The electrons naturally travel
a much shorter path than the gamma rays. The Si PIN layer 14 is
sensitive to the internal conversion electrons caused by a neutron
capture in the adjacent Gd layer 20. The glass plate 16 prevents
the electrons in the Gd layer 20 from registering in the CZT layer
12, which makes it easier to distinguish between events indicating
neutron capture compared to events indicating the presence of gamma
rays.
[0033] Each CZT/Si PIN assembly 24, including the foregoing layers
12, 14, 16, and 20, has an array of anode contacts that separate
the CZT/Si PIN assembly 24 into pixel areas or pixels 26, as shown
in FIG. 2, for example. The CZT/Si PIN assemblies 24 are sandwiched
between a pair of controllers 30 and 32 with logic instructions
encoded as software or hard-wired. Each controller 30 and 32
typically has at least one processor or Central Processing Unit
(CPU), together with an associated memory for storing an operating
system, application software, and data generated by events in the
detector 10. The controller 30 or 32 may include an
analog-to-digital signal processor or converter, and may be
connected to input and output devices in a well known manner.
[0034] Multi-layer printed circuit boards (PCBs) 34 and 36
cooperate with the associated controller 30 and 32 to connect each
pixel area 26 of the CZT/Si PIN assembly 24 to a respective contact
on the respective processor. The processor may include two or more
silicon-based application-specific integrated circuits (ASICs) that
drive the CZT and Si PIN layers 12 and 14. To that end, first and
second anode contacts are coupled to the processors or PCBs 34 and
36, in electric contact with respective outer surfaces of the CZT
layer 12 and the Si PIN layer 14. Traces on the PCBs 34 and 36
connect the anodes to the corresponding unit cells of the ASICs or
other processors. The processors also are coupled to and in
electric contact with cathode contacts, adjacent to and in electric
contact with the glass plate 16. Thus the electrodes, the anodes
and the cathodes, are connected to the processors in respective
controllers 30 and 32. The controller 30 and 32 controls the
application of the electric field to the CZT/Si PIN assembly 24
through the anodes and the cathodes. The potential difference
between the anode and cathode contacts, controlled by the
controller 30 and 32, establishes an electric field within each
detection material (the CZT and Si PIN layers 12 and 14). The
controller 30 and 32, and more specifically the associated
processors, also receives and analyzes electrical signals received
at the anodes and the cathodes using coincidence criteria to
identify neutron events and gamma-ray events and to distinguish
between them. Put another way, the processor of each controller 30
and 32 is configured to differentiate between signals generated by
a neutron event and signals generated by a gamma-ray event as a
function of coincidence testing of signals received from each of
the first and second anodes.
[0035] Although the detector 10 is not primarily intended for
imaging, advantages of the pixel approach include:
[0036] 1) the ability to turn off defective pixels 26, greatly
increasing the yield of usable CZT material and reducing cost;
[0037] 2) the use of multiple independent detectors 10 operating in
parallel reduces dark current, capacitance, and pulse pile-up;
[0038] 3) aligned CZT/Si PIN pixel pairs facilitate coincidence
testing; and
[0039] 4) the CZT/Si PIN assembly 24 topologically conforms to an
existing ASIC that can perform depth correction calculations that
improve gamma-ray energy resolution in the CZT layer 12. In the
exemplary embodiment described here, as many as 12 pixels (worst
case) out of 242 can be turned off and still meet a desired minimum
efficiency, thereby increasing the yield of usable CZT
material.
[0040] In an exemplary embodiment, each CZT layer 12 may be formed
of a 3 cm.times.3 cm square chip, providing a total sensitive area
of A=18 cm.sup.2, separated into pixels 26. Each CZT/Si PIN
assembly 24 may have an 11.times.11 array of anode contacts such as
is illustrated in FIG. 2, creating 121 pixels per chip (242 pixels
total). This arrangement forms 242 pixels 26 in each CZT/Si PIN
assembly 24, and each pixel 26 in the Si PIN layer 14 is aligned
with a pixel 26 in the CZT layer 12. Each pixel 26 independently
detects neutrons and gamma rays. Individual defective pixels can be
turned off via the controllers 30 and 32, significantly increasing
the yield of usable CZT material, which is typically more
expensive. An exemplary pixel pitch is 2.6 mm, with a 0.7 mm
setback at the outer edge (around the periphery).
[0041] As mentioned above, neutron detection is achieved through
the gadolinium conversion layer (Gd layer 20) on the glass plate
16. An exemplary Gd layer 20 has a thickness of about 5 .mu.m. For
a 254,000 barn cross-section of .sup.157Gd (where a barn is defined
as 10.sup.-28 m.sup.2), the absorption rate in enriched elemental
Gd is 7682 cm.sup.-1. Thus a 5 .mu.m Gd layer thickness will be
expected to capture 98% of all impinging thermal neutrons.
Efficiency can be increased by increasing the thickness of the Gd
layer 20, but that increased thickness comes with a corresponding
increase in cost.
[0042] And as mentioned above, gamma-ray detection is achieved via
the CZT layer 12. For a thickness of d=0.35 cm, the CZT layer 12
has a total volume of 6.30 cm.sup.3. Using Harmonex software from
Aprend Technology, available via www.aprendtech.com/OverView.html,
the cross-section for photoelectric interaction of a 1.17 MeV gamma
ray in a 0.35 cm-thick CZT layer 12 is about 0.002456 cm.sup.2/g.
Given a density of 5.845 g/cm.sup.3, the absorption rate in the CZT
layer 12 is then about 0.01436 cm.sup.-1. Although this example
uses a CZT layer 12 that has a thickness of 0.35 cm, the
sensitivity to gamma rays can be significantly improved by
increasing the thickness to 1.0 cm, or 1.5 cm, or 2.0 cm, for
example.
[0043] In optimizing the design of the detector, multiple
parameters may be adjusted, including Gd layer 20 thickness, Si PIN
layer 14 characteristics, CZT layer 12 thickness, glass plate 16
thickness, bias voltages, controller 30 and 32 operating
conditions, and pixel coincidence criteria. The exemplary example
described here is but one configuration.
[0044] Turning now to a description of the method of detecting
neutrons and gamma rays using a common detector 10, where the
detector includes a layer of semiconductor material (such as CZT)
12 adjacent a glass plate 16 a layer of silicon PIN material 14 on
an opposite side of the glass plate 16 and a Gd layer 20 between
the glass plate 16 and the silicon PIN layer 14 to form a
subassembly that is bounded by electrodes, including a
semiconductor or first anode on one side of the semiconductor
material, a cathode connected to the glass plate, and a Si PIN or
second anode on a side of the Si PIN layer opposite the
semiconductor anode. The method includes detecting electrons in the
CZT layer 12 or the Si PIN layer 14, and determining whether
electrons were detected in both the CZT and the Si PIN layers 12
and 14 at about the same time, i.e. coincidentally. Consequently
the method may be referred to as including a coincidence test. More
particularly, the method includes the steps of (1) monitoring the
electrical signal at each of the semiconductor anode and the Si PIN
anode, and (2) comparing signals from the semiconductor anode and
the SI PIN anode to differentiate between gamma-ray events and
neutron events based on predetermined criteria, described in
further detail below. The method further includes establishing an
electric field within each of the CZT and the Si PIN layers 12 and
14 by generating a potential difference between the anode and
cathode contacts. The method also may include the step of providing
a common gamma-ray and neutron detector with a controller connected
to the anodes and the cathode, the controller including a processor
and a memory.
[0045] Through the photoelectric effect, a gamma ray deposits all
of its energy in the CZT layer 12, generating a proportional number
of electron-hole pairs. In an applied electric field, the electrons
and holes drift toward their respective anode and cathode contacts,
ideally generating a signal proportional to the gamma-ray energy.
Because of trapping effects, however, not all of the charge is
collected. This problem has been substantially overcome with the
3-D position-sensing technology that has been implemented in the
HPL2 ASIC, developed at Brookhaven National Laboratory in Long
Island, N.Y., US, making it an exemplary controller 30 and 32.
[0046] Each individual gamma event is corrected in amplitude
according to the depth in the CZT layer 12 at which the charge was
generated. FIGS. 3A and 3B show an example of how the 662 keV peak
of .sup.137Cs has been sharpened from 2.2% to 0.72% full width at
half maximum (FWHM) using the 3-D technique mentioned above. An
estimate of the width expected at 1.17 MeV, scaling this by the
square root of the ratio of the gamma ray energies, leads to a
linewidth of 0.54%. To be conservative, this can be doubled to
predict a linewidth of 1.15%. Most of the other gamma-ray events
will consist of Compton scattering, creating a broad background,
especially if the ambient gamma field contains a variety of
energies. Photoelectric gamma events, given reasonable statistics,
will stand out above this background because of their narrow
lines.
[0047] Neutrons are detected by catching internal conversion
electrons in the Si PIN layer 14. The neutrons are first captured
in the Gd layer 20, producing the electrons that escape to the Si
PIN layer 14. Neutron capture by .sup.157Gd is represented by
.sup.157Gd(n,.gamma.+e.sup.-+x).sup.158Gd, in which the final
nucleus, .sup.158Gd, relaxes to its ground state by releasing 7.9
MeV of energy in a complex mixture of gamma rays, internal
conversion (IC) electrons, and x-rays. Typically more than three
gamma rays emerge in each neutron capture. The most probable
energies occur at 79.51 and 181.94 keV, with others between 0.606
and 6.75 MeV. All of these are unlikely to register in the Si PIN
layer 14 because of the low atomic number of the Si PIN material
and the relatively thin (typically 280 .mu.m) thickness of the Si
PIN layer 14. In comparison, prominent energies of the IC electrons
are 79.51, 181.94, 255.67, and 277.55 keV, all of which have a
small enough projected range to be fully stopped in the Si PIN
layer 14. On average they will lose minimal energy emerging from
the Gd layer 20 before entering the Si PIN layer 14.
[0048] FIG. 4 is an actual spectrum of the IC electrons produced
due to neutrons being captured in the Gd layer 20. (The angstrom
notations represent the kinetic energy of the thermal neutrons.)
Energy spectroscopy of the IC electrons can be performed in the Si
PIN layer 14, in the same manner as gamma rays in the CZT layer 12,
but without any need to correct for trapping. One electron-hole
pair is created in the Si PIN layer 14 for each 3.6 eV of IC
electron energy, so each pulse will be much greater than the dark
current charge collected within pulse shaping time.
[0049] FIG. 5 illustrates the capture of a neutron in the detector
10. The approximate cathode-to-anode biases are 600V and 20V across
the CZT and Si PIN layers 12 and 14, respectively.
[0050] The glass layer 16, generally at least 300 .mu.m thick,
functions: (1) to stand off the potential difference between the Si
PIN and CZT cathodes, (2) to form a substrate for the Gd layer 20,
(3) to stop IC electrons emitted by the Gd layer 20 from entering
the CZT layer 12, and (4) to reduce the number of Compton-scattered
electrons from the CZT layer 12 that enter the Si PIN layer 14.
Additionally, a thinner (5 .mu.m) enriched, rather than a thicker
natural Gd layer 20 is advantageous because it will interact less
with the gamma ray field and will permit easier escape of the IC
electrons. The enriched Gd layer 20 also emits a simpler IC
electron spectrum than the natural isotopic mixture.
[0051] For each event, the detector 10 provides six pieces of
information: (1) pulse amplitude in the CZT layer 12 (if any); (2)
pulse amplitude in the Si PIN layer 14 (if any); (3,4) timing of
the CZT layer 12 and Si PIN layer 14 signals; and (5,6) the pixel
locations in the CZT and Si PIN arrays. The spectroscopy performed
with the Si PIN layer 14 is not done to reveal the neutron's
kinetic energy, but to identify the IC electrons. The manner in
which these quantities are interpreted may be optimized before
assigning an event to a neutron or to a gamma ray with the highest
confidence. For example, a pulse occurring in the Si PIN layer 14
with no CZT layer 12 coincidence is most likely generated by a
neutron, as long as the energy is compatible with the known IC
electron spectrum. A coinciding event in the CZT layer 12,
especially in a nearby pixel 26, assumed to represent a gamma ray
event from the Gd layer 20, would confirm a neutron event. Because
of timing, the amplitude could be ignored, allowing either Compton
scattering or photoelectric capture of the Gd-originated gamma ray
in the CZT layer 12 to help confirm the neutron.
[0052] For gamma detection, a pulse in the CZT layer 12 with the
absence of a coincident signal in the Si PIN layer 14, especially
in neighboring pixels 26, would most likely indicate a gamma
ray.
[0053] Accordingly, when the anodes are at zero volts, the glass
plate 16 stands off the potential difference between the two
cathodes (negative voltages indicated). Internal conversion
electrons easily escape from the Gd layer 20 and deposit their
energy in the Si PIN layer 14. If emitted in the other direction,
the glass plate 16 prevents the IC electrons from registering in
the CZT layer 12. Gamma rays from the same neutron event, if
emitted downward, can be detected by the CZT layer 12. Because of
the CZT-Si PIN coincidence test, both Compton scattering and
photoelectric absorption of gamma rays in the CZT layer 12 can be
used to help confirm the presence of a neutron event.
[0054] In summary then, the present invention provides a method for
detecting both gamma-ray events and neutron events with a common
detector, where the detector includes a layer of semiconductor
material adjacent one side of a glass plate and a Gd layer on an
opposite side of the glass plate, between the glass plate and a
layer of silicon PIN material to form an assembly that is bounded
by electrodes, including a semiconductor anode on one side of the
semiconductor layer, a cathode connected to the glass plate, and a
Si PIN anode on a side of the Si PIN layer opposite the
semiconductor anode. The method includes the steps of: (1)
monitoring the electrical signal at each of the semiconductor anode
and the Si PIN anode, and (2) comparing signals from the
semiconductor anode and the SI PIN anode to differentiate between
gamma-ray events and neutron events based on predetermined
criteria.
[0055] Although the invention has been shown and described with
respect to a certain preferred embodiment, it is obvious that
equivalent alterations and modifications will occur to others
skilled in the art upon the reading and understanding of this
specification and the annexed drawings. In particular regard to the
various functions performed by the above described components, the
terms (including a reference to a "means") used to describe such
components are intended to correspond, unless otherwise indicated,
to any component which performs the specified function of the
described component (i.e., that is functionally equivalent), even
though not structurally equivalent to the disclosed structure which
performs the function in the herein illustrated exemplary
embodiments of the invention. In addition, while a particular
feature of the invention can have been disclosed with respect to
only one of the several embodiments, such feature can be combined
with one or more other features of the other embodiments as may be
desired and advantageous for any given or particular
application.
* * * * *
References