U.S. patent application number 15/281212 was filed with the patent office on 2017-07-27 for csliln halide scintillator.
This patent application is currently assigned to Radiation Monitoring Devices, Inc.. The applicant listed for this patent is Radiation Monitoring Devices, Inc.. Invention is credited to Jaroslaw Glodo, Rastgo Hawrami, William M. Higgins, Kanai S. Shah, Urmila Shirwadkar, Edgar V. Van Loef.
Application Number | 20170211203 15/281212 |
Document ID | / |
Family ID | 44066883 |
Filed Date | 2017-07-27 |
United States Patent
Application |
20170211203 |
Kind Code |
A1 |
Shah; Kanai S. ; et
al. |
July 27, 2017 |
CsLiLn HALIDE SCINTILLATOR
Abstract
Li-containing scintillator compositions, as well as related
structures and methods are described. Radiation detection systems
and methods are described which include a Cs.sub.2LiLn Halide
scintillator composition.
Inventors: |
Shah; Kanai S.; (Watertown,
MA) ; Higgins; William M.; (Westborough, MA) ;
Van Loef; Edgar V.; (Watertown, MA) ; Glodo;
Jaroslaw; (Allston, MA) ; Hawrami; Rastgo;
(Watertown, MA) ; Shirwadkar; Urmila; (Watertown,
MA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Radiation Monitoring Devices, Inc. |
Watertown |
MA |
US |
|
|
Assignee: |
Radiation Monitoring Devices,
Inc.
Watertown
MA
|
Family ID: |
44066883 |
Appl. No.: |
15/281212 |
Filed: |
September 30, 2016 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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14603989 |
Jan 23, 2015 |
9459357 |
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15281212 |
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13891771 |
May 10, 2013 |
8969824 |
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14603989 |
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12731003 |
Mar 24, 2010 |
8440980 |
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13891771 |
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12624337 |
Nov 23, 2009 |
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12731003 |
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61230970 |
Aug 3, 2009 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C30B 11/00 20130101;
C01F 17/253 20200101; C09K 11/778 20130101; C30B 29/12 20130101;
G01T 3/06 20130101; C09K 11/772 20130101; C09K 11/7705 20130101;
G01T 1/2023 20130101; C09K 11/7773 20130101; G01T 1/202 20130101;
C30B 15/00 20130101; C09K 11/7733 20130101; G01T 1/2006 20130101;
C09K 11/7791 20130101; C01F 17/271 20200101 |
International
Class: |
C30B 29/12 20060101
C30B029/12; C09K 11/77 20060101 C09K011/77; C01F 17/00 20060101
C01F017/00; G01T 3/06 20060101 G01T003/06; G01T 1/202 20060101
G01T001/202 |
Claims
1. A scintillator comprising a Cs.sub.2LiLn Halide composition,
wherein Ln is selected from one or more of Y, La, Ce, Gd, Lu and
Sc, wherein the scintillator is capable of neutron detection at an
efficiency of greater than 30%, wherein the scintillator is
enriched with Li.sup.-6.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] The present application is a continuation of U.S. patent
application Ser. No. 14/603,989 (now U.S. Pat. No. 9,459,357),
filed Jan. 23, 2015, which is a continuation of U.S. patent
application Ser. No. 13/891,771 (now U.S. Pat. No. 8,969,824),
filed May 10, 2013, which is a continuation of U.S. patent
application Ser. No. 12/731,003 (now U.S. Pat. No. 8,440,980),
filed Mar. 24, 2010 which is a continuation-in-part application of
U.S. patent application Ser. No. 12/624,337, filed on Nov. 23,
2009, which claims the benefit of priority under 35 U.S.C.
.sctn.119(e) of U.S. Provisional Application No. 61/230,970, filed
Aug. 3, 2009, which are incorporated herein by reference in their
entirety.
BACKGROUND OF THE INVENTION
[0002] The present invention relates to scintillator compositions
and related devices and methods. More specifically, the present
invention relates to enriched Li-containing scintillator
compositions suitable for use, for example, in radiation detection,
including gamma-ray spectroscopy, and X-ray and neutron
detection.
[0003] Scintillation spectrometers are widely used in detection and
spectroscopy of energetic photons (e.g., X-rays, gamma-rays, etc.).
Such detectors are commonly used, for example, in nuclear and
particle physics research, medical imaging, diffraction, non
destructive testing, nuclear treaty verification and safeguards,
nuclear non-proliferation monitoring, and geological
exploration.
[0004] Important requirements for the scintillation crystals used
in these applications include high light output, transparency to
the light it produces, high stopping efficiency, fast response,
good proportionality, low cost, and availability in large volume.
These requirements on the whole cannot be met by many of the
commercially available scintillator compositions. While general
classes of chemical compositions may be identified as potentially
having some attractive scintillation characteristic(s), specific
compositions/formulations having both scintillation characteristics
and physical properties necessary for actual use in scintillation
spectrometers and various practical applications have proven
difficult to predict. Specific scintillation properties are not
necessarily predictable from chemical composition alone, and
preparing effective scintillator compositions from even candidate
materials often proves difficult For example, while the
compositions of sodium chloride and sodium iodide had been known
for many years, the invention by Hofstadter of a high light-yield
and conversion efficiency scintillator from sodium iodide doped
with thallium launched the era of modern radiation spectrometry.
More than half a century later, thallium doped sodium iodide, in
fact, still remains one of the most widely used scintillator
materials. Since the invention of NaI(Tl) scintillators in the
1940's, for half a century radiation detection applications have
depended to a significant extent on this material. The fields of
nuclear medicine, radiation monitoring, and spectroscopy have grown
up supported by NaI(Tl). Although far from ideal, NaI(Tl) was
relatively easy to produce for a reasonable cost and in large
volume. With the advent of X-ray CT in the 1970's, a major
commercial field emerged as did a need for different scintillator
compositions, as NaI(Tl) was not able to meet the requirements of
CT imaging. Later, the commercialization of positron emission
tomography (PET) imaging provided the impetus for the development
of yet another class of detector materials with properties suitable
for PET.
[0005] As the methodology of scintillator development evolved, new
materials have been added, and yet, specific applications are still
hampered by the lack of scintillators suitable for particular
applications. Today, the development of new scintillator
compositions continues to be as much an art as a science, since the
composition of a given material does not necessarily determine its
properties as a scintillator, and because scintillation properties
are strongly influenced by the history (e.g., fabrication process)
of the material as it is formed.
[0006] Thus, there is continued interest in the search for new and
useful scintillator compositions and formulations, as well as
corresponding detection systems, with both the performance and the
physical characteristics needed for use in various
applications.
BRIEF SUMMARY OF THE INVENTION
[0007] The present invention provides Li-containing scintillator
compositions, as well as related structures and methods.
Compositions include Cs2LiLn Halide (Z.sub.6) scintillator
compositions.
[0008] In one aspect, structures and methods of the present
invention include a scintillator comprising a Cs.sub.2LiLn Halide
composition. Ln is selected from one or more of Y, La, Ce, Gd, Lu
and Sc, and the halide comprises at least Cl. The scintillator is
capable of neutron detection at an efficiency of greater than 30%.
In some of these embodiments. In some embodiments, the lithium
content of the composition is enriched to include a Li-6 content
above that which is found in naturally occurring lithium sources.
In some embodiments, the scintillator compositions disclosed herein
can include a dopant or a mixture of dopants.
[0009] In one aspect, a detection system is provided. The system
comprises a scintillator comprising a Cs.sub.2LiLn Halide
composition. Ln is selected from one or more of Y, La, Ce, Gd, Lu
and Sc. The Halide comprises at least Cl, wherein the scintillator
is capable of neutron detection at an efficiency of greater than
30%. The system further comprises a detector assembly coupled to
the scintillator to detect a light pulse luminescence from the
scintillator as a measure of a neutron scintillation event.
[0010] In one aspect, a method of radiation detection is provided.
The method comprises providing a detection system comprising a
scintillator comprising Cs.sub.2LiLn Halide composition. Ln is
selected from one or more of Y, La, Ce, Gd, Lu and Sc, wherein the
Halide comprises at least Cl. The detection system further
comprises a detection assembly coupled to the scintillator to
detect a light pulse luminescence from the scintillator as a
measure of a scintillation event. The method further comprises
positioning the system such that a radiation source is within a
field of view of the system so as to detect emissions from the
source and measuring a scintillation event luminescence signal from
the scintillator with the detection assembly. The method further
comprises processing the measured luminescence signal using pulse
shape discrimination analysis over a time of greater than 50 ns to
differentiate between gamma emissions and neutron emissions from
the source.
[0011] In some aspects, a method of radiation detection is
provided. The method comprises providing a detection system
comprising a scintillator comprising Cs.sub.2LiLn Halide
composition. Ln is selected from one or more of Y, La, Ce, Gd, Lu
and Sc, wherein the Halide comprises at least Cl. The detection
system further comprises a detection assembly coupled to the
scintillator to detect a light pulse luminescence from the
scintillator as a measure of a scintillation event. The method
further comprises positioning the system such that a radiation
source is within a field of view of the system so as to detect
emissions from the source and measuring a scintillation event
luminescence signal from the scintillator with the detection
assembly. The method further comprises processing the measured
luminescence signal comprising comparing the measured luminescence
signal from a first window of time to the measured luminescence
signal from a second window of time.
[0012] Excellent scintillation properties, including high light
output, good proportionality, response, and good energy resolution
have been measured for certain compositions of the present
invention. Scintillator compositions of the present invention have
demonstrated emission characteristics indicating suitability for
use in various applications. For example, scintillation properties
of the compositions can include peak emission wavelengths from
about 380 nm, which is well matched to PMTs as well as silicon
diodes used in nuclear instrumentation and a peak wavelength for
gamma-ray spectroscopy.
[0013] The present invention, in some aspects, advantageously
provides high-efficiency neutron detection compositions and
structures. Thus, compositions of the present invention may be used
in a variety of radiation detection structures and
applications.
[0014] Scintillator compositions demonstrated suitability for
gamma-ray spectroscopy and neutron emission detection, including
differential gamma-ray/neutron detection. Surprisingly good energy
resolution of the compositions make the scintillators of the
present invention particularly attractive for combined or
simultaneous gamma and neutron detection. Additionally, timing
characteristics such as rise time and decay time for gamma-ray and
neutron may be utilized in differential detection of gamma-ray
scintillation events and neutron events. In one embodiment,
detection includes measuring and/or processing a scintillation
luminescence signal including comparing different timing windows so
as to identify a scintillation event as a gamma event or neutron
event.
[0015] In another aspect, the invention further includes systems
and devices making use of the scintillator compositions of the
present invention. A system or device can include, for example, a
radiation detection device having a scintillation composition as
described herein, and a detector assembly optically coupled to the
scintillator composition. A detector assembly can include, for
example, a photomultiplier tube, a photo diode, or a PIN detector.
The detector assembly may further include a data analysis, or
computer, system for processing and analyzing detected signals.
Exemplary devices or assemblies can include an X-ray and/or neutron
detector assembly, as well as imaging systems. For example, the
device can include electronics configured for performing pulse
shape and pulse height analysis to differentiate gamma ray from
neutron emissions. Scintillator compositions of the present
invention can further find use in a variety of detector or imaging
system configurations commonly using scintillator compositions, and
methods of the present invention can include radiation detection
and/or imaging applications using the aforementioned
devices/systems.
[0016] In yet another aspect, the invention includes a method of
performing radiation detection. Such a method can include, for
example, providing a detection system or device having a
scintillator composition of the present invention, and the system
or a target/radiation source such that the source is within a field
of view of the scintillator for detecting emissions from the target
or source. Emissions can include, for example, gamma-ray, X-ray, or
neutron emissions. A target can include various potential sources
of detectable emissions including neutron emitters and gamma-ray
sources (e.g., uranium and the like), X-ray sources, etc.
[0017] For a fuller understanding of the nature and advantages of
the present invention, reference should be made to the ensuing
detailed description taken in conjunction with the accompanying
drawings. The drawings represent embodiments of the present
invention by way of illustration. The invention is capable of
modification in various respects without departing from the
invention. Accordingly, the drawings/figures and description of
these embodiments are illustrative in nature, and not
restrictive.
BRIEF DESCRIPTION OF THE DRAWINGS
[0018] FIG. 1 shows neutron absorption efficiency as a function of
scintillator thickness for a non-enriched Cs.sub.2LiYCl.sub.6 and a
Li-6 enriched Cs.sub.2LiYCl.sub.6 scintillator composition.
[0019] FIG. 2 shows time profiles for gamma-ray and neutron
detection for a Cs.sub.2LiYCl.sub.6 scintillator composition,
according to an embodiment of the present invention.
[0020] FIG. 3 shows time profiles for gamma-ray and neutron
detection for a scintillator composition, according to an
embodiment of the present invention, with a first timing window and
a second timing window for differentiation of gamma-ray and neutron
scintillation event signal.
[0021] FIG. 4 illustrates comparison of different portions of
scintillation event signal, as in FIG. 3, for differentiation of
gamma-ray and neutron events, according to one embodiment of the
present invention.
[0022] FIG. 5 shows an optical emission spectrum for a Ce doped,
Cs.sub.2LiYCl.sub.6 composition upon gamma-ray irradiation,
according to an embodiment of the present invention.
[0023] FIG. 6 illustrates proportionality for a Ce doped,
Cs.sub.2LiYCl.sub.6 composition, according to an embodiment of the
present invention. The figure shows light output of the composition
measured under excitation from isotopes such as .sup.241Am,
.sup.57Co, .sup.22Na, and .sup.137Cs.
[0024] FIG. 7 illustrates an energy spectrum for a Ce doped, Li-6
enriched Cs.sub.2LiYCl.sub.6 composition, according to an
embodiment of the invention. The figure shows excellent energy
resolution measured for the composition.
[0025] FIG. 8A is a conceptual diagram of a radiation detection
system of the present invention.
[0026] FIG. 8B is a diagram of a scintillator composition disposed
on a substrate, according to an embodiment of the present
invention.
DETAILED DESCRIPTION OF THE INVENTION
[0027] This invention will be better understood with resort to the
following definitions:
[0028] A. Rise time, in reference to a scintillation crystal
material, shall mean the speed with which its light output grows
once a gamma-ray has been stopped in the crystal. The contribution
of this characteristic of a scintillator contributes to a timing
resolution.
[0029] B. A Fast timing scintillator (or fast scintillator)
typically requires a timing resolution of about 500 ps or less. For
certain PET applications (e.g., time-of-flight (TOF)), the fast
scintillator should be capable of localizing an annihilation event
as originating from within about a 30 cm distance, i.e., from
within a human being scanned.
[0030] C. Timing accuracy or resolution, usually defined by the
full width half maximum (FWHM) of the time of arrival differences
from a point source of annihilation gamma-rays. Because of a number
of factors, there is a spread of measured values of times of
arrival, even when they are all equal. Usually they distribute
along a bell-shaped or Gaussian curve. The FWHM is the width of the
curve at a height that is half of the value of the curve at its
peak.
[0031] D. Light Output shall mean the number of light photons
produced per unit energy deposited by the detected gamma-ray,
typically the number of light photons/MeV. For neutrons, the light
output is typically measured in photons/neutron.
[0032] E. Stopping power and attenuation refer to the penetration
range of the incoming X-ray or gamma-ray in the scintillation
crystal material. Attenuation is the gradual loss of intensity of a
flux through a medium. The attenuation length, in the context of a
scintillator, is the length of scintillator material needed to
reduce the incoming beam flux to 1/e.sup.-. For neutrons, the
attenuation length and the useful attenuation length may differ.
For instance, for Cs.sub.2LiYZ.sub.6 scintillator compositions,
neutrons are stopped by all of the elements in the composition,
with those stopped by Li-6 provide a useful signal.
[0033] F. Proportionality of response (or linearity). For some
applications (such as CT scanning) it is desirable that the light
output be substantially proportional to the deposited energy. For
applications such as spectroscopy, non-proportionality of response
is an important parameter. In a typical scintillator, the number of
light photons produced per MeV of incoming gamma-ray energy is not
constant. Rather, it varies with the energy deposited by the
stopped gamma-ray. This has two deleterious effects. The first is
that the energy scale is not linear, but it is possible to
calibrate for the effect. The second is that it degrades energy
resolution. To see how this occurs, consider a scintillator that
produces 300 photons at 150 keV, 160 photons at 100 keV and 60
photons at 50 keV. From statistics alone, the energy resolution at
150 keV should be the variability in 300 photons, which is 5.8%, or
8.7 keV. If every detected event deposited 150 keV in one step this
would be the case. On the other hand, if, as it occurs, an event
deposited 100 keV in a first interaction and then another 50 keV in
a second interaction, the number of photons produced would not be
300 on the average, but 160+60=220 photons, for a difference of 80
photons or 27%. In multiple detections, the peak would broaden well
beyond the theoretical 8.7 keV. The smaller the non-proportionality
the smaller this broadening and the closer the actual energy
resolution approaches the theoretical limit.
[0034] The present invention includes compositions and related
radiation detection systems incorporating a Cs.sub.2LiLn Halide
composition. The compositions may be represented by the formula
Cs.sub.2LiLnZ.sub.6, where Z is a halide. Suitable halides can
include, for example, F, Cl, Br, or I, or a mixture of two or more
halides. Lanthanides (or "Ln") can include lanthanides such as Y,
La, Ce, Gd, Lu, Sc, etc. In some embodiments, the composition
includes a mixture of lanthanide elements. In some embodiments, a
scintillator includes a Cs.sub.2LiYCl.sub.6 composition.
[0035] In some embodiments, the lithium content of the composition
is enriched to include a Li-6 content above that which is found in
naturally occurring lithium sources. It should be understood,
however, that not all compositions of the invention are enriched.
Enrichment refers to a change through processing of a nuclear
species mixture found on Earth or as naturally occurring so that
the resultant material has a different mix of nuclear species. In
naturally occurring sources of lithium, 93% of the lithium is in
the form of Li-7 or .sup.7Li, having an atomic weight of
approximately 7 and includes a nucleus with three protons (defining
the chemical species) and four neutrons. Approximately 7% of
naturally occurring lithium is Li-6 or .sup.6Li, which has an
atomic weight of approximately six, including three protons and
three neutrons. Although the chemical properties are substantially
similar, the physical (weight) and nuclear properties are
significantly different. Of interest in the compositions of the
present invention is that the neutron interaction cross-section of
Li-6 is larger than that of Li-7.
[0036] Thus, Li-6 enriched compositions of the present invention
will include compositions where the Li-6 content is higher or above
that which is found in naturally occurring lithium sources.
Compositions can include lithium with a Li-6 content that is at
least about 10% or higher, and will typically include lithium with
a Li-6 content of about 50% or more, and in some instances about
80%, 90%, 95% or more (as well as any integral number in the
specified ranges).
[0037] Most neutron detection applications demand high detection
efficiency. In neutron radiography is important to keep exposure to
neutrons low, e.g., since the interaction of neutrons with many
materials results in chemical transformations that alter the
properties of these materials by transmuting elements, and some can
become radioactive for long times, this latter factor complicating
handling and disposal. In certain applications, radiation sources
of interest may have low-level neutron emissions or low ratio of
neutron to gamma-ray emission, thereby requiring a high-efficiency
detector for meaningful analysis or effective detection. In the
nuclear security or monitoring applications, e.g., radiation
sources or materials of interest have a low ratio of neutron
emissions relative to gamma-rays, such as one neutron per thousand
or more gamma-rays, thus making neutron detection efficiency and
neutron discrimination from gamma rays key factors for detection
and/or analysis applications.
[0038] While the detection efficiency of unenriched material might
seemingly be augmented simply by making thicker detectors, this is
not necessarily the case. In practice, detector efficiency is not
found to be directly proportional to scintillator thickness, and
merely increasing thickness produces diminishing return in terms of
efficiency. As an example illustrating limitations in merely
increasing thickness as a measure to increase efficiency, the
following example is provided. For gamma rays, on first order, for
a given scintillator, if a thickness 1.times. of material stops 30%
of the incoming gamma-rays, then 2.times. stops 51% and 3.times.66%
and so on. The reason is that substantially all gamma rays
interacting in the material produce detectable light. Limitations
on size/volume of scintillator that can practically be incorporated
into a device restricts the usefulness of certain compositions that
are unable to provide a desired detection efficiency with a
thickness that is practical or useful.
[0039] In the case of non-enriched Cs.sub.2LiYCl.sub.6, for
instance, the Cs and Cl (mainly the latter) do stop some of the
neutrons but do not correspondingly produce useful neutron
detection signal, so as the detector is made thicker it becomes
opaque to neutrons. Thus, as scintillator thickness increases the
Li component does decreasing proportion of useful work because the
Li-6 component competes for neutrons with the Cl, which as noted
above does not interact with neutrons so as to produce a useful
signal. As a consequence, because the neutron absorption per unit
thickness/length of scintillator material for non-enriched
Cs.sub.2LiYCl.sub.6 is limited, the maximum neutron detection
efficiency of such material is limited even as the thickness
increases. For example, a scintillator composition of non-enriched
Cs.sub.2LiYCl.sub.6 may gain some increased neutron detection
efficiency as thickness is increased up to about 10 cm, but further
doubling the thickness from 10 cm to 20 cm produces minimal further
gains in neutron detection efficiency (see FIG. 1). The maximum
detection efficiency for any thickness is approximately 27%. It is
noted that, separate for the issue of detection efficiency, as
scintillator thickness increases to a range of about 10 cm to about
20 cm, practically application of such thick scintillators,
particularly in more portable or hand held detector configurations,
becomes more limited and even precludes some applications.
[0040] The scintillator compositions of the present invention may
include a lithium component that is enriched with Li-6, e.g.,
compared to Li-7. The enriched compositions advantageously allow
high-efficiency neutron detection with relatively thin scintillator
configurations, thereby allowing practical application of the
compositions in a variety of detection devices that would not
otherwise be available with a corresponding non-enriched
composition. Since in one embodiment, lithium enriched to the level
of 95% Li-6 is available, high-efficiency scintillators can be
produced using enriched material, and can optionally include a thin
scintillator profile. As will be recognized, neutron detection
efficiency increases when the ratio of neutrons incident to the
scintillator that generate a scintillation event or detectable
signal compared to neutrons incident to the scintillator not
generating a scintillation event or signal is higher. A
"high-efficiency" detector, as referred to herein, can include a
detector where the scintillator composition is capable of neutron
detection at an efficiency of about 30% or more, and/or an
incorporating device/system will be configured for detection at an
efficiency about 30% or more. In some embodiments, the scintillator
composition is capable of neutron detection at an efficiency of
about 50% or more; in some embodiments, the scintillator
composition is capable of neutron detection at an efficiency of
about 75% or more. Often, a detector device or system of the
present invention will include a neutron detection efficiency in a
range of about 20% to about 80% or greater (and any integral number
therebetween). For 95% enriched material the maximum detection
efficiency for any thickness is approximately 82%, approximately 3
times higher than for unenriched material, and this is attained for
a thickness that is approximately 1/10th of the thickness of
unenriched material.
[0041] The terms "thick" and "thin" are used herein in reference to
a scintillator thickness or distance from one surface to an
opposing surface (see, also, FIG. 8B, below). In some instances,
reference to thickness or thinness refer to scintillators having a
thickness of about 20 cm or less (e.g., 0.01 cm to about 20 cm, or
any integral number therebetween), typically less than about 10 cm.
In some instances, scintillators are less than about 1 cm and may
have portion of about 1 mm to about 5 mm in thickness. In some
cases, the scintillators have a thickness of greater than 1 cm; in
some cases, greater than 5 cm; in some cases, greater than 10 cm;
and, in some cases, greater than 20 cm. In some cases, the terms
thick or thin are used in a relative manner, such as referring to
the thickness/thinness of an enriched scintillator composition
compared to a corresponding non-enriched composition.
[0042] Enriched compositions of the present invention in some
embodiments may provide significantly greater neutron detection
efficiency per unit thickness compared to a corresponding
non-enriched composition. In many instances, enriched scintillator
compositions will provide thin scintillators that are
high-efficiency neutron detecting scintillators, and the present
invention includes high-efficiency neutron detection
systems/structures making use of relatively thin enriched
scintillator compositions.
[0043] Increased neutron detection efficiency per unit thickness in
enriched scintillator compositions of the present invention is
described with reference to FIG. 1. As can be seen in FIG. 1, in
one example, a Li-6 enriched Cs.sub.2LiYCl.sub.6 of 1 cm in
thickness is capable of detecting through scintillation about 80%
of the neutrons reaching the scintillator, with a maximum detection
efficiency of about 82% is reached at about 1.5 cm of material
thickness. In contrast, a 10 cm thick corresponding unenriched
Cs.sub.2LiYCl.sub.6 material detects approximately 23% of the
neutrons, and 20 cm of unenriched material detect 27%. Remarkably,
the about maximum detection efficiency (about 27%) of the
unenriched Cs.sub.2LiYCl.sub.6 that is reached at about 20
cm-thickness, is matched by the Li-6 enriched Cs.sub.2LiYCl.sub.6
of the present invention at a thickness of just over 1 mm.
[0044] One of the valuable characteristics of the scintillator of
the present invention is the ability to differentiate neutrons from
gamma rays. The principle behind discrimination is described with
reference to FIGS. 2-4. FIG. 2 shows the time course of light
emission by gamma rays and neutrons obtained from a small
scintillator crystal of Cs.sub.2LiYCl.sub.6 doped with Ce. As can
be seen, timing profile of a gamma-ray scintillation event differs
compared to neutron scintillation event. For incident gamma-rays,
scintillation is very fast, including a fast light decay where 1/e
was reached in less than 100 nsec. Neutron scintillation event
exhibits a relatively slower timing profile, the 1/e point being
reached at about 500 ns. The difference in the timing profile
between gamma-ray scintillation events and neutron scintillation
events can facilitate differentiation between gamma-ray detection
and neutron detection. In particular, such differences enable
gamma-ray detection and neutron detection to be differentiated
using pulse shape discrimination (PSD) analysis. PSD analysis, in
general, involves comparing the luminescence signal pulse shape
resulting from gamma-ray detection to the luminescence signal pulse
shape resulting from neutron detection. In some embodiments, it may
be advantageous to use PSD analysis over relatively long time
periods to differentiate gamma-ray detection and neutron detection.
For example, in some embodiments, methods of differentiating
gamma-rays from neutrons involve analyzing the luminescence signal
over a time of greater than 50 ns; in some cases, over a time of
greater than 100 ns; or, in some cases, over a time of greater than
150 ns. Relatively long PSD times are particularly useful in
embodiments when the scintillator is relatively thick, for example,
greater than 1 cm, greater than 5 cm, etc.
[0045] FIG. 3 shows a method to use rise time to effect gamma
ray/neutron discrimination in larger crystals by placing two time
windows from which to accumulate (integrate) or process the
luminescence signal. In the illustrative embodiment, window 1 is on
the rise and window 2 on the decay sides of the time course. In
some embodiments, window 1 has a time duration of at least 50 ns,
or at least 100 ns. In some cases, the time duration for window 1
is less than 150 ns, less than 125 ns, or less than 100 ns. Window
1 may be between 0 and 100 ns, as measured from the start of the
luminescence signal. In some embodiments, window 2 has a time
duration of at least 50 ns, at least 100 ns (e.g., between 100-125
ns), at least 200 ns, or at least 300 ns. In some cases, the time
duration for window 2 is less than 400 ns, less than 300 ns or less
than 200 ns. Window 2 may be between 100 ns and 500 ns, as measured
from the start of the luminescence signal. Analysis can include a
comparison of windows 1 and 2 so as to identify a scintillation
event as a gamma event or neutron event. The analysis may include
assessing the ratio of the value of the integrated signal within
the first time window and the second time window. The ratio will be
different for events due to gamma-ray and neutrons and, thus, can
be used to differentiate. FIG. 4 shows comparison according to one
embodiment, where a plot of window 2/window 1 vs. window one shows
the neutron and gamma-ray events well identified.
[0046] FIG. 5 shows light emission from of Cs.sub.2LiYCl.sub.6
doped with Ce under gamma ray irradiation. There is core valence
luminescence (CVL) in the 250-350 nm range and light from the
dopant in the 350-450 nm range from the gamma rays. Neutrons
produce light in the higher range only. A large detector results in
absorption by the dopant of the CVL produced by gamma rays, and
re-emission in the 350-450 run range (FIG. 5, main graph). As
described herein, it has been discovered that the light produced by
gamma rays, in the 250-350 nm range, is strongly absorbed by a
large (e.g., thick) scintillator (light output decreases with
crystal volume)(FIG. 5, inset), and re-emitted at the 350-450 nm
range (light output increases with crystal volume). This being the
case, a thick detector cannot take advantage of conventional
methods for neutron/gamma ray discrimination based on decay time
measurements, which are gamma/neutron discrimination. Because the
light is being absorbed by the Ce dopant, we have discovered that
to make large volume detectors Ce levels will preferably be low.
The non-limiting examples shown here contain 0.05% of Ce as a
dopant.
[0047] Because of its relatively low light yield for gamma rays
(22,000 photons/MeV for Cs2LiYCl6:Ce) compared to scintillators
such as LaBr3 which yield 60,000 to 90,000 photons/MeV, it would be
expected that the energy resolution of Cs.sub.2LiYCl.sub.6 for
gamma rays will be poor, making the material less attractive for
simultaneous gamma and neutron detection. Indeed, energy resolution
of 7-11% for non-enriched Cs.sub.2LiYCl.sub.6 for the 662 KeV peak
of Cs-137 has been reported previously (Bessiere et al, Nuclear
Instruments and Methods in Physics Research A 537 (2005) pp.
242-246). Thus, while Cs.sub.2LiYCl.sub.6 has been suggested for
use in differential detection of gamma-rays and neutrons, factors
such as modest to poor energy resolution has limited applicability
in this context. According to the present invention, however, it
has been discovered that the proportionality of response of
Cs.sub.2LiYCl.sub.6:Ce is very high (+/-5%) (e.g., FIG. 6) and as a
consequence its energy resolution is surprisingly good, 4% for 662
keV (FIG. 7). As such, the Li-6 enriched Cs.sub.2LiYCl.sub.6
compositions of the present invention demonstrate excellent energy
resolution suitable for simultaneous or differential detection of
gamma-rays and neutrons.
[0048] As indicated above, scintillator compositions disclosed
herein can include a dopant or a mixture of dopants. Dopants can
affect certain properties, such as physical properties (e.g.,
brittleness, etc.) as well as scintillation properties (e.g.,
luminescence, etc.) of the scintillator composition. The dopant can
include, for example, cerium (Ce), praseodymium (Pr), lutetium
(Lu), lanthanum (La), europium (Eu), samarium (Sm), strontium (Sr),
thallium (Tl), chlorine (Cl), fluorine (F), iodine (I), and
mixtures of any of the dopants. Where certain halides are included
as dopants, such dopants will be present in the scintillator
composition in addition to those halide(s) already otherwise
present in the scintillator compound. The amount of dopant present
will depend on various factors, such as the application for which
the scintillator composition is being used; the desired
scintillation properties (e.g., emission properties, timing
resolution, etc.); and the type of detection device into which the
scintillator is being incorporated. For example, the dopant is
typically employed at a level in the range of about 0.01% to about
20%, by molar weight. In certain embodiments, the amount of dopant
is in the range of about 0.01% to less than about 100% (and any
integral number therebetween), or less than about 0.1%, 1.0%, 5.0%,
or 20% by molar weight.
[0049] The scintillator compositions of the invention may be
prepared in several different forms. In some embodiments, the
composition is in a crystalline form (e.g., monocrystalline).
Scintillation crystals, such as monocrystalline scintillators, have
a greater tendency for transparency than other forms. Scintillators
in crystalline form (e.g., scintillation crystals) are often useful
for high-energy radiation detectors, e.g., those used for gamma-ray
or X-ray detection. However, the composition can include other
forms as well, and the selected form may depend, in part, on the
intended end use of the scintillator. For example, a scintillator
can be in a powder form. It can also be prepared in the form of a
ceramic or polycrystalline ceramic. Other forms of scintillation
compositions will be recognized and can include, for example,
glasses, deposits, vapor deposited films, and the like. It should
also be understood that a scintillator composition might contain
small amounts of impurities. Also, minor amounts of other materials
may be purposefully included in the scintillator compositions to
affect the properties of the scintillator compositions.
[0050] Scintillator compositions can be substantially pure (e.g.,
about 99% scintillator composition or greater) or may contain
certain amounts of other compounds or impurities. In some cases,
impurities may originate, for example, with starting materials for
composition preparation. Typically, impurities constitute less than
about 0.1% by weight of the scintillator composition, and often
less than about 0.01% by weight of the composition. In some
instances, minor amounts of other materials may be purposefully
included in the scintillator compositions. For example, minor
amounts of other rare earth metals, oxides can be added to affect
scintillation properties, such as reduce afterglow, and the like.
Scintillator compositions can include single halide compositions as
well as mixed halide compositions, e.g., where the term halide
includes a mixture of two or more halides.
[0051] Methods for making crystal materials can include those
methods described herein and may further include other techniques.
Typically, the appropriate reactants are melted at a temperature
sufficient to form a congruent, molten composition, with operative
melting temperature(s) at least partially depending on the identity
of the reactants themselves (see, e.g., melting points of
reactants). Non-limiting examples of the crystal-growing methods
can include certain techniques of the Bridgman-Stockbarger methods;
the Czochralski methods, the zone-melting methods (or "floating
zone" method), the vertical gradient freeze (VGF) methods, and the
temperature gradient methods. See, e.g., (see also, e.g.,
"Luminescent Materials", by G. Blasse et al, Springer-Verlag (1994)
and "Crystal Growth Processes", by J. C. Brice, Blackie & Son
Ltd (1986)).
[0052] In the practice of the present invention, attention is paid
to the physical properties of the scintillator material. In
particular applications, properties such as hygroscopy (tendency to
absorb water), brittleness (tendency to crack), and crumbliness
should be minimal.
TABLE-US-00001 TABLE I Properties of Scintillators Wavelength Light
Output Density Of Emission Rise-time Material (Photons/MeV)
(g/cm.sup.3) (nm) (ns) NaI(T1) 38,000 3.67 415 >10 CsI(T1)
52,000 4.51 540 >10 LSO 24,000 7.4 420 <1 BGO 8,200 7.13 505
>1 BaF.sub.2 10,000 4.88 310, slow <0.1 ~2,000 220, fast GS0
7,600 6.7 430 ~8 CdW0.sub.4 15,000 8.0 480 YAP 20,000 5.55 370
<1
[0053] Table I provides a listing of certain properties of a number
of scintillators. As shown, Li-6 enriched Cs.sub.2LiLn:Z.sub.6
compositions of the present invention demonstrate a useful light
emission spectrum comparable to other commercially available
scintillators. Table II further provides certain properties for a
Cs.sub.2LiY:Cl.sub.6. doped with 0.05% of Ce scintillator
composition, according to an embodiment of the present
invention.
TABLE-US-00002 TABLE II RbGd.sub.2Br.sub.7:Ce Li- Li.sub.6Gd
Property Cs.sub.2LiYCl.sub.6:Ce (RGB) Lil:Eu Glass:Ce
(BO.sub.3).sub.3:Ce .lamda. .sub.em, nm 373 420 470 395 400 Light
yield, 1 neutron 73,000 <5,000 50,000 ~6,000 50,000 photons per
1 MeV .gamma.-ray 22,000 56,000 12,000 ~4,000 14,000 decay time
constants, ns 1*, 25, 2000 45, 400 1,400 75 200, 700 density .rho.,
g/cm.sup.3 3.31 4.8 4.1 2.5 3.5 Pulse shape .gamma.-.sup.1n yes no
no no no discrimination
[0054] As set forth above, scintillator compositions of the present
invention may find use in a wide variety of radiation detection and
processing applications and structures. Thus, the present invention
includes methods and structures for detecting energy radiation
(e.g., gamma-rays, X-rays, neutron emissions, and the like) with a
scintillation detector including the scintillation composition of
the invention.
[0055] FIG. 8A is a diagram of a radiation detection system or
apparatus of the present invention. The detector system 10 includes
a scintillator 12 optically coupled to detector assembly including
a light photodetector assembly 14 or imaging device. The detector
assembly of system 10 can include a data analysis or computer
system 16 (e.g., data acquisition and/or processing device) to
process information from the scintillator 12 and light
photodetector 14. In use, the detector 10 detects energetic
radiation emitted form a source 18.
[0056] A system as in FIG. 8A containing the scintillator
composition (scintillator 12) of the present invention is optically
coupled to the detector assembly (e.g., photodetector 14) and can
include an optical window that can be disposed, e.g., at one end of
the enclosure-casing. The window permits radiation-induced
scintillation light to pass out of the scintillator composition
assembly for measurement by the photon detection assembly or
light-sensing device (e.g., photomultiplier tube, etc.), which is
coupled to the scintillator assembly. The light-sensing device
converts the light photons emitted from the scintillator into
electrical pulses or signal that are output and may be shaped,
digitized, or processed, for example, by the associated
electronics.
[0057] A data analysis, or computer system thereof can include, for
example, a module or system to process information (e.g., radiation
detection data or signals) from the detector/photodetectors can
also be included in an invention assembly and can include, for
example, a wide variety of proprietary or commercially available
computers, electronics, or systems having one or more processing
structures, a personal computer, mainframe, or the like, with such
systems often comprising data processing hardware and/or software
configured to implement any one (or combination of) the method
steps described herein. Any software will typically comprise
machine readable code of programming instructions embodied in a
tangible media such as a memory, a digital or optical recording
media, optical, electrical, or wireless telemetry signals, or the
like, and one or more of these structures may also be used to
transmit data and information between components of the system in
any of a wide variety of distributed or centralized signal
processing architectures.
[0058] The detector assembly typically includes material formed
from the scintillator composition described herein (e.g., one or
more scintillator crystals). The detector further can include, for
example, a light detection assembly including one or more
photodetectors. Non-limiting examples of photodetectors include
photomultiplier tubes (PMT), photodiodes, CCD sensors, image
intensifiers, and the like. Choice of a particular photodetector
will depend in part on the type of radiation detector being
fabricated and on its intended use of the device. In certain
embodiments, the photodetector may be position-sensitive.
[0059] FIG. 8B shows a scintillator as in scintillator 12
illustrated in FIG. 8A. Scintillator 12 includes a
Cs.sub.2LiLn:Z.sub.6 composition as described above. Various
sizing, shapes, dimensions, configurations of scintillator 12 may
be selected depending on intended use and/or system in which the
scintillator 12 is incorporated. Scintillator 12 includes a top
side 18 and an opposing side (not shown) with a thickness ("T")
measuring between the top side 18 or surface of the scintillator 12
and the opposing side or surface. Scintillator 12 is shown coupled
to a substrate 20, which may be selected from a variety of
substrates. Non-limiting substrate composition examples may include
amorphous carbon, glassy carbon, graphite, aluminum, sapphire,
beryllium, or boron nitrate. A substrate may include a fiber optic
plate, prism, lens, scintillator, or photodetector. The substrate
can be a detector device or portion or surface thereof (e.g.,
optical assembly, photodetector, etc.). The substrate can be
separate from a detector device and/or comprise a detector portion
(e.g., scintillator panel) that can be adapted to or incorporated
into a detection device or assembly. In one embodiment, the
scintillator is optically, but not physically, coupled to a
photodetector.
[0060] The detector assemblies themselves, which can include the
scintillator and the photodetector assembly, can be connected to a
variety of tools and devices, as mentioned previously. Non-limiting
examples include nuclear weapons monitoring and detection devices,
well-logging tools, and imaging devices, such as nuclear medicine
devices (e.g., PET). Scintillator compositions of the present
invention, e.g., due to high-detection efficiency and/or relatively
thin profile or sizing described above, can be incorporated into
smaller or more compact devices or systems, including hand-held
probes, detectors, or dosimeters, portal monitoring structures, and
the like. Various technologies for operably coupling or integrating
a radiation detector assembly containing a scintillator to a
detection device can be utilized in the present invention,
including various known techniques.
[0061] The detectors may also be connected to a visualization
interface, imaging equipment, or digital imaging equipment. In some
embodiments, the scintillator may serve as a component of a screen
scintillator. For example, powdered scintillator material could be
formed into a relatively flat plate, which is attached to a film,
such as photographic film. Energetic radiation, e.g., X-rays,
gamma-rays, neutron, originating from a source, would interact with
the scintillator and be converted into light photons, which are
visualized in the developed film. The film can be replaced by
amorphous silicon position-sensitive photodetectors or other
position-sensitive detectors, such as avalanche diodes and the
like.
[0062] Neutron radiographic devices represent another important
application for invention scintillator compositions and radiation
detectors. Furthermore, geological exploration devices, such as
well-logging devices, represent an important application for these
radiation detectors. In such an embodiment, gamma-rays can be
detected, which in turn provides an analysis of geological
formations, such as rock strata surrounding the drilling bore
holes.
[0063] Specific aspects or dimensions of any of the compositions,
devices, systems, and components thereof, of the present invention
may readily be varied depending upon the intended application, as
will be apparent to those of skill in the art in view of the
disclosure herein. Moreover, it is understood that the examples and
embodiments described herein are for illustrative purposes only and
that various modifications or changes in light thereof may be
suggested to persons skilled in the art and are included within the
spirit and purview of this application and scope of the appended
claims. Numerous different combinations of embodiments described
herein are possible, and such combinations are considered part of
the present invention. In addition, all features discussed in
connection with any one embodiment herein can be readily adapted
for use in other embodiments herein. The use of different terms or
reference numerals for similar features in different embodiments
does not necessarily imply differences other than those which may
be expressly set forth. Accordingly, the present invention is
intended to be described solely by reference to the appended
claims, and not limited to the preferred embodiments disclosed
herein.
* * * * *