U.S. patent application number 14/047893 was filed with the patent office on 2016-12-01 for barium iodide and strontium iodide crystals and scintillators implementing the same.
This patent application is currently assigned to Fisk University. The applicant listed for this patent is Fisk University, Lawrence Livermore National Security, LLC. Invention is credited to Arnold Burger, Nerine J. Cherepy, Alexander D. Drobshoff, Giulia E. Hull, Stephen A. Payne.
Application Number | 20160349383 14/047893 |
Document ID | / |
Family ID | 41695479 |
Filed Date | 2016-12-01 |
United States Patent
Application |
20160349383 |
Kind Code |
A1 |
Payne; Stephen A. ; et
al. |
December 1, 2016 |
BARIUM IODIDE AND STRONTIUM IODIDE CRYSTALS AND SCINTILLATORS
IMPLEMENTING THE SAME
Abstract
In one embodiment, a material comprises a crystal comprising
strontium iodide providing at least 50,000 photons per MeV, where
the strontium iodide material is characterized by a volume not less
than 1 cm.sup.3. In another embodiment, a scintillator optic
includes europium-doped strontium iodide providing at least 50,000
photons per MeV, where the europium in the crystal is primarily
Eu.sup.2+, and the europium is present in an amount greater than
about 1.6%. A scintillator radiation detector in yet another
embodiment includes a scintillator optic comprising SrI.sub.2 and
BaI.sub.2, where a ratio of SrI.sub.2 to BaI.sub.2 is in a range of
between 0:1 and 1.0, the scintillator optic is a crystal that
provides at least 50,000 scintillation photons per MeV and energy
resolution of less than about 5% at 662 keV, and the crystal has a
volume of 1 cm.sup.3 or more; the scintillator optic contains more
than about 2% europium.
Inventors: |
Payne; Stephen A.; (Castro
Valley, CA) ; Cherepy; Nerine J.; (Oakland, CA)
; Hull; Giulia E.; (Oakland, CA) ; Drobshoff;
Alexander D.; (Livermore, CA) ; Burger; Arnold;
(Nashville, TN) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Fisk University
Lawrence Livermore National Security, LLC |
Nashville
Livermore |
TN
CA |
US
US |
|
|
Assignee: |
Fisk University
Nashville
TN
Lawrence Livermore National Security, LLC
Livermore
CA
|
Family ID: |
41695479 |
Appl. No.: |
14/047893 |
Filed: |
October 7, 2013 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
12255375 |
Oct 21, 2008 |
8580149 |
|
|
14047893 |
|
|
|
|
60988475 |
Nov 16, 2007 |
|
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G21K 4/00 20130101; G01T
1/202 20130101; C30B 11/00 20130101; C09K 11/7791 20130101; G01T
1/2023 20130101; C30B 29/12 20130101; C09K 11/7733 20130101; C09K
11/7772 20130101 |
International
Class: |
G01T 1/202 20060101
G01T001/202; C09K 11/77 20060101 C09K011/77 |
Goverment Interests
[0002] The United States Government has rights in this invention
pursuant to Contract No, DE-AC52-07NA27344 between the United
States Department of Energy and Lawrence Livermore National
Security, LLC for the operation of Lawrence Livermore National
Laboratory.
Claims
1. (canceled)
2. (canceled)
3. (canceled)
4. (canceled)
5. (canceled)
6. (canceled)
7. (canceled)
8. (canceled)
9. (canceled)
10. (canceled)
11. (canceled)
12. (canceled)
13. (canceled)
14. (canceled)
15. A scintillator radiation detector, comprising: a scintillator
optic comprising SrI.sub.2 and BaI.sub.2, wherein a ratio of
SrI.sub.2 to BaI.sub.2 is in a range of between 0:1 and 1:0,
wherein the scintillator optic is a crystal that provides at least
50,000 scintillation photons per MeV and energy resolution of less
than about 5% at 662 keV, wherein the scintillator optic contains
more than 2% europium; and wherein the scintillator optic contains
at least one co-dopant, selected from cerium, praseodymium,
thallium, or lead.
16. The scintillator radiation detector of claim 15, wherein the
crystal provides an energy resolution of less than about 4% at 662
keV.
17. The scintillator radiation detector of claim 15, wherein the
scintillator optic contains more than 2% europium and less than 8%
europium.
18. The scintillator radiation detector of claim 17, wherein the
europium is primarily Eu.sup.2+.
19. The scintillator radiation detector of claim 15, wherein the
scintillator optic provides at least 80,000 photons per MeV.
20. (canceled)
Description
RELATED APPLICATIONS
[0001] This application is a continuation of U.S. application Ser.
No. 12/255,375, filed Oct. 21, 2008 and entitled "BARIUM IODIDE AND
STRONTIUM IODIDE CRYSTALS AND SCINTILLATORS IMPLEMENTING THE SAME,"
which in turn claims priority to Provisional U.S. application Ser.
No. 60/988,475 filed on Nov. 16, 2007, from each of which priority
is claimed and each of which is herein incorporated by
reference.
FIELD OF THE INVENTION
[0003] The present invention relates to scintillator crystals, and
more particularly to ionic iodide-containing crystals and
scintillator detectors employing the same.
BACKGROUND
[0004] Detection and classification of gamma ray emitters has
attained heightened importance in the protection of vulnerable
targets and populaces from high energy explosives. Many nuclear
explosives emit gamma rays, due to radioactive decay of the
materials comprising the explosives. However, many less harmful and
non-explosive materials also emit gamma rays. Therefore, it is
desirable to be able to identify, and whenever possible,
distinguish between the types of gamma ray emitters in an unknown
material, possibly further concealed inside of a container or
vehicle of some type, such as a car, van, cargo container, etc.
[0005] Many types of materials emit gamma rays that appear very
close together on a gamma spectrograph. Scintillator detectors use
crystals that emit light when gamma rays interact with the atoms in
the crystals. The intensity of the light emitted can be used to
determine the type of material that is emitting the gamma rays.
Scintillator detectors may also be used to detect other types of
radiation, such as alpha, beta, and x-rays. High energy resolution
scintillator detectors are useful for resolving closely spaced
gamma ray lines in order to distinguish between gamma emitters
producing closely spaced gamma ray lines.
[0006] Detection sensitivity for weak gamma ray sources and rapid
unambiguous isotope identification is principally dependent on
energy resolution, and is also enhanced by a high effective atomic
number of the detector material. Generally, gamma ray detectors are
characterized by their energy resolution. Resolution can be stated
in absolute or relative terms. For consistency, all resolution
terms are stated in relative terms herein. A common way of
expressing detector resolution is with Full Width at Half Maximum
(FWHM). This equates to the width of the gamma ray peak on a
spectral graph at half of the highest point on the peak
distribution.
[0007] The relative resolution of a detector may be calculated by
taking the absolute resolution, usually reported in keV, dividing
by the actual energy of the gamma ray also in keV, and multiplying
by 100%. This results in a resolution reported in percentage at a
specific gamma ray energy. The inorganic scintillator currently
providing the highest energy resolution is LaBr.sub.3(Ce), with
about 2.6% at 662 keV, but it is highly hygroscopic, its growth is
quite difficult and it possesses natural radioactivity due to the
presence of primordial .sup.138La that produces betas and gamma
rays resulting in interference in the gamma ray spectra acquired
with LaBr.sub.3(Ce). Therefore, it is desirable to have a
scintillator detector that is capable of distinguishing between
weak gamma ray sources that is more easily grown while still
providing high energy resolution.
SUMMARY
[0008] In one embodiment, a material comprises a crystal comprising
strontium iodide providing at least 50,000 photons per MeV, where
the strontium iodide material is characterized by a volume not less
than 1 cm.sup.3.
[0009] A scintillator radiation detector according to another
embodiment includes a scintillator optic comprising europium-doped
strontium iodide providing at least 50,000 photons per MeV, where
the europium in the crystal is primarily Eu.sup.2+, and the
europium is present in an amount greater than about 1.6%.
[0010] A scintillator radiation detector in yet another embodiment
includes a scintillator optic comprising SrI.sub.2 and Bah, wherein
a ratio of SrI.sub.2 to BaI.sub.2 is in a range of between 0:1 and
1.0, where the scintillator optic is a crystal that provides at
least 50,000 scintillation photons per MeV and energy resolution of
less than about 5% at 662 keV, the crystal has a volume of 1
cm.sup.3 or more, and the scintillator optic contains more than
about 2% europium.
[0011] Other aspects and embodiments of the present invention will
become apparent from the following detailed description, which,
when taken in conjunction with the drawings, illustrate by way of
example the principles of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] FIG. 1 illustrates two plots of relative intensity versus
wavelength for several scintillator samples.
[0013] FIG. 2 is a plot of relative intensity versus wavelength for
five scintillator samples.
[0014] FIG. 3 is a plot of counts versus wavelength for undoped
SrI.sub.2 in pure and impure forms.
[0015] FIG. 4A is a plot of gamma ray spectra acquired with
LaBr.sub.3(Ce), SrI.sub.2(5% Eu), and NiI(Tl) scintillators of the
.sup.133Ba source.
[0016] FIG. 4B is a plot of gamma ray spectra acquired with
LaBr.sub.3(Ce), SrI.sub.2(5% Eu), and NiI(Tl) scintillators of the
.sup.137Cs source.
[0017] FIG. 5 is a chart comparing eight different measured or
calculated characteristics for seven different scintillator
materials.
[0018] FIG. 6 is a flow chart of a method according to one
embodiment.
[0019] FIG. 7 illustrates three plots of relative intensity versus
time for several scintillator samples.
[0020] FIG. 8 is a chart of energy resolution as a function of
gamma ray energy acquired for SrI.sub.2(Eu) and LaBr.sub.3(Ce)
crystals.
DETAILED DESCRIPTION
[0021] The following description is made for the purpose of
illustrating the general principles of the present invention and is
not meant to limit the inventive concepts claimed herein. Further,
particular features described herein can be used in combination
with other described features in each of the various possible
combinations and permutations.
[0022] Unless otherwise specifically defined herein, all terms are
to be given their broadest possible interpretation including
meanings implied from the specification as well as meanings
understood by those skilled in the art and/or as defined in
dictionaries, treatises, etc.
[0023] It must also be noted that, as used in the specification and
the appended claims, the singular forms "a," "an" and "the" include
plural referents unless otherwise specified.
[0024] Several crystalline iodides are known to function usefully
as scintillators, including NaI(Tl), CsI(Na), CsI(Tl), and
LuI.sub.3(Ce). NaI(Tl) is by far the most common scintillator,
being grown in large sizes by numerous companies and deployed in
many commercial instruments. However, NaI(Tl) offers a modest light
yield, which limits the gamma ray energy resolution which is
possible (about 40,000 photons/MeV). CaI.sub.2(Eu) and
LuI.sub.3(Ce) both have higher light yields (about 70,000-100,000
photons/MeV) but are exceedingly difficult to grow and the latter
contains Lu as a constituent (which is a natural beta-emitter
leading to an undesirable background count rate). Interestingly,
LaI.sub.3(Ce) has also been tested as a potential scintillator, but
was found to be non-emissive at room temperature.
[0025] Two materials have been found to have great utility as high
energy scintillators: SrI.sub.2(Eu) and BaI.sub.2(Eu), which may
resolve the problems facing the related compounds LuI.sub.3(Ce) and
CaI.sub.2(Eu). Other related compounds currently being used as
scintillators include Ce-doped LaCl.sub.3 and LaBr.sub.3, which
both have very high tight yields but have a lower atomic number (Z)
than the iodides (Z of Cl, Br, I are 17, 35, 53, respectively)--the
Z is a critical parameter since gamma photoelectric absorption goes
approximately as its fourth power, Z.sup.4. As a consequence,
iodides are preferred as constituents in scintillators versus
bromides and chlorides, assuming other features are comparable.
[0026] In one embodiment, a scintillator detector makes use of
SrI.sub.2 or BaI.sub.2 crystals for the purpose of gamma ray
detection, based on measuring the amount of scintillation
luminescence generated by the material For this purpose, the
crystal may be doped or undoped, giving rise to excitonic
(undoped), perturbed excitonic (e.g., Na, Mg, Ca, Sc or other
doping of electronically inactive species), or activator
luminescence (e.g., Eu.sup.2+, Ce.sup.3+, Pb.sup.2+, Tl.sup.+,
Pr.sup.3+).
[0027] Undoped SrI.sub.2 has a useful light yield, but its energy
resolution with standard photomultiplier tubes is only fair, due to
its emission being long wavelength. When a Eu.sup.2+ activator is
used, emissions in the blue region are observed. Contrary to
conventional wisdom and earlier findings, a scintillator optic
comprising SrI.sub.2 doped with Eu, especially Eu.sup.2+, has been
found to provide a high energy resolution. For example, a
SrI.sub.2(Eu) crystal grown by the inventors evidenced an energy
resolution of <2.7% at 662 keV, challenging the performance of
LaBr.sub.3(Ce) obtained under the same conditions.
[0028] An intriguing factor appears relevant to the excellent
performance of SrI.sub.2(Eu). That is, the lattice constants for
SrI.sub.2 and EuI.sub.2 are nearly identical, thus permitting high
doping of Eu in SrI.sub.2. Other favorable aspects of SrI.sub.2
include its tow melting point, 538.degree. C., and its orthorhombic
crystal structure, which will likely be readily grown to large
sizes.
[0029] The alpha particle-induced luminescence of BaI.sub.2(Eu) is
similar to that of Lu.sub.3Al.sub.5O.sub.12(Ce), but shifted to
shorter wavelength.
[0030] A marked advantage of using SrI.sub.2 or BaI.sub.2 crystals
doped with Eu is the relative ease in which the crystals can be
grown in large sizes. Another advantage is that SrI.sub.2(Eu) and
BaI.sub.2(Eu) are less hygroscopic than CaI.sub.2(Eu) which is an
important practical edge in using Sr or Ba instead of Ca.
[0031] In a first general embodiment, a material comprises a
crystal, which is comprised of strontium iodide (doped or undoped)
providing at least 50,000 photons per MeV. In one particularly
preferred embodiment, the energy resolution of the crystal may be
less than about 5.0% at 662 keV, as being enhanced by doping, e.g.,
with Ce or Eu.
[0032] In the first general embodiment, the crystal may be doped
with europium in different percentages, such as containing more
than 1.6% europium, containing between about 0.5% and about 8.0%
europium, and containing more than 2.0% europium.
[0033] In addition, the europium in the crystal may be primarily
Eu.sup.2+. The use of Eu.sup.2+ surprisingly provides excellent
energy resolution, e.g., less than about 2.7% at 662 keV, As noted
above, conventional wisdom and a previous report indicated that
such energy resolution was impossible for such a material. To
exemplify, FIG. 8 is a chart 800 depicting energy resolution as a
function of gamma ray energy acquired for SrI.sub.2(5% Eu) and
LaBr.sub.3(Ce) crystals. These crystals are the same as used below
in Example 4. As shown, the energy resolution of SrI.sub.2(5% Eu)
is comparable or slightly better than that of LaBr.sub.3(Ce).
[0034] Another variation of the first general embodiment is where
the crystal has at least one dopant, selected from: cerium,
praeseodymium, thallium, or lead,
[0035] The first general embodiment may further include barium in
the crystal, or the crystal may provide at least 60,000 photons per
MeV. Further, the resolution of the crystal may be less than about
5%, less than about 4.0%, etc, at 662 keV.
[0036] In a second general embodiment, a scintillator radiation
detector comprises a scintillator optic comprised of strontium
iodide (doped or undoped) providing at least 50,000 photons per
MeV. In one particularly preferred embodiment, the energy
resolution of the crystal may be less than about 5.0% at 662
keV.
[0037] Also, in the second general embodiment, the scintillator
optic may contain more than 1.6% europium, may contain between
about 0.5% and about 8.0% europium, or may contain more than 2.0%
europium. In addition, the europium may be primarily Eu.sup.2+.
Further, the scintiliator optic may include barium and/or
calcium.
[0038] With continued reference to the second general embodiment,
the scintillator optic may provide at least 60,000 photons per MeV,
and may have a resolution of less than or about 4.0% at 662
keV.
[0039] In a third general embodiment, a scintillator radiation
detector comprises a scintillator optic comprised of SrI.sub.2 and
BaI.sub.2, wherein a ratio of SrI.sub.2 to BaI.sub.2 is in a range
of between 0:1 and 1:0.
[0040] In the third general embodiment, the scintillator optic may
provide at least 50,000 photons per MeV and energy resolution of
less than about 5.0% at 662 keV,
[0041] Further, the scintillator optic may contain europium, and
the europium may be primarily Eu.sup.2+. In addition, the
scintillator optic may provide at least 80,000 photons per MeV, and
may contain at least one dopant, selected from: cerium,
praeseodymium, thallium, or lead.
[0042] In a fourth general embodiment, a scintillator radiation
detector comprises a scintillator optic comprising barium
iodide.
[0043] In the fourth general embodiment, the scintillator optic may
be doped with at least one of cerium, praeseodymium, thallium,
lead, indium, or a transition metal ion. Also, the scintillator
optic may be doped with an activator that luminesces in response to
gamma radiation.
[0044] In the fourth general embodiment, the activator may include
an ion which luminesces via a 5d.fwdarw.4f transition or the
activator may include an s.sup.2 ion or a closed shell ion.
Further, the activator may be a transition metal ion.
[0045] In a fifth general embodiment, an iodide crystal comprises a
single metal ion (M, M' or M'') with the formula MI.sub.2,
M'I.sub.3, or M''I.sub.4, where M or M' has an atomic number
>40, but is not Y, Sc, La, Lu, Gd, Ca, Sr or Ba. M'' may or may
not have an atomic number greater than 40.
[0046] Any of the general embodiments may include further
limitations as directed below. In addition, combinations of the
additional limitations directed below may be combined to create
even more permutations and combinations of features.
EXAMPLES
[0047] To demonstrate various embodiments of the present invention,
several examples are provided bellow, It should be appreciated that
these are presented by way of nonlimiting example only, and should
not be construed as limiting.
Example 1
[0048] Strontium iodide and barium iodide crystals were grown in
quartz crucibles using the Bridgman method. The melting points of
SrI.sub.2 and BaI.sub.2 are 515 and 711.degree. C., respectively;
both possess orthorhombic symmetry while calcium iodide is
hexagonal. All crystals described in this section were doped with
0.5 mole % europium and were several cubic centimeters per boule,
then cut into .about.1 cm.sup.3 pieces for evaluation. Barium
iodide as-supplied powder, 99.995% pure ultradry (Alfa Aesar) was
yellowish in color (thought to be due to oxide or oxyiodide
contamination). Crystals grown directly from as-supplied powders
retained a dark coloration (referred to henceforth as "first
crystal"), Zone refining rendered the starting powders colorless,
and the resulting pure powders were used to grow several crystals
(referred to as "second crystal," although several were grown
following this procedure). Finally, an ultrapurificafion method was
used to grow a BaI.sub.2(Eu) crystal, referred to as "third
crystal."
[0049] Radioluminescence spectra were acquired using a
.sup.90Sr/.sup.90Y source (average beta energy .about.1 MeV) to
provide a spectrum expected to be essentially equivalent to that
produced by gamma excitation, Radioluminescence spectra were
collected with a spectrograph coupled to a thermoelectrically
cooled camera and corrected for spectral sensitivity. The
beta-excited luminescence of SrI.sub.2(0.5% Eu) compared to that of
a standard scintillator crystal, CsI(Tl), is shown in FIG. 1 in the
upper plot 102, along with a SrI.sub.2(Eu) crystal grown at RMD. It
possesses a single band centered at 435 nm, assigned to the
Eu.sup.2+ d.fwdarw.f transition, and an integrated light yield of
93,000 photons/MeV. FIG. 1 in the lower plot 104 shows beta-excited
luminescence spectra of three BaI.sub.2(Eu) crystals compared to a
CsI(Tl) standard crystal. The Eu.sup.2+ luminescence at 420 nm is
enhanced in the second BaI.sub.2(Eu) crystal, while the .about.550
nm band is reduced, and for the third crystal, the .about.550 nm
band is entirely absent. It is notable that the overall light yield
is highest for the first crystal; its integral tight yield
(including both the 420 nm and the 550 nm bands) is 60,000
photons/MeV. The weak band at 550 nm may be assigned to an
impurity-mediated recombination transition.
Example 2
[0050] Calcium iodide and strontium bromide crystals were grown via
the Bridgman method, with 0.5% Europium doping. The CaI.sub.2(Eu)
crystal is substantially opaque due to optical scatter, considered
unavoidable due to its platelet crystal structure. Its
radioluminescence spectrum was measured at 110,000 Ph/MeV, and is
shown in the chart 200 of FIG. 2. SrBr.sub.2(Eu) is an orthorhombic
crystal with good optical properties, however, its light yield so
far is low (.about.25,000 Ph/MeV). AU radioluminescence spectra
reported herein were acquired with a .sup.90Sr/.sup.90Y source
(.about.1 MeV average beta energy) and emission spectra were
collected using a Princeton Instruments/Acton Spec 10 spectrograph
coupled to a thermoelectrically cooled CCD camera.
Example 3
[0051] Undoped strontium iodide was grown and zone-refined. The
luminescence spectrum, shown in FIG. 2, is unchanged between pure
and impure segments of the bottle, however, the pulse height
spectrum of the purer section is slightly higher, Pulse height
measurements, shown in the chart 300 of FIG. 3, were acquired using
a Hamamatsu R329EGP PMT (QE at 550 nm of 15%). The signals from the
PMT anode were collected on a 500 .OMEGA. resistor, shaped with a
Tennelec TC 244 spectroscopy amplifier (shaping time of 8 .mu.s)
and then recorded with the Amptek MCA8000-A multi-channel analyzer,
The emission is likely due to self-trapped excitons, as it is
present for all un-doped samples.
Example 4
[0052] A scintillator crystal of strontium iodide doped with 5%
europium, a scintillator crystal of LaBr.sub.3(Ce), and a
scintillator crystal of NiaI(Tl) were acquired and exposed to a
.sup.133Ba source. Acquisition parameters (e.g., shaping time,
gain) were optimized for each crystal to give the best results for
the particular crystal. The resulting gamma ray spectra are shown
in the chart 400 of FIG. 4A. As shown, the energy resolution of the
SrI.sub.2(Eu) is better than the energy resolution of the
LaBr.sub.3(Ce) in the low energy region.
Example 5
[0053] The same crystals used in Example 4 were exposed to a
.sup.137Cs source, which is primarily monoenergetic. Again,
acquisition parameters (e.g., shaping time, gain) were optimized
for each crystal to give the best results for the particular
crystal. The resulting gamma ray spectra are shown in the chart 402
of FIG. 4B. As shown, the energy resolution of the SrI.sub.2(Eu) is
comparable to the energy resolution of the LaBr.sub.3(Ce).
Example 6
[0054] The same LaBr.sub.3(Ce) and SrI.sub.2(Eu) crystals used in
Example 4 were exposed to a .sup.137Cs source, which is primarily
monoenergetic. Again, acquisition parameters (e.g., shaping time,
gain) were optimized for each crystal to give the best results for
the particular crystal. The resulting gamma ray spectra are shown
in FIG. 4B. As shown, the energy resolution of the SrI.sub.2(Eu) is
comparable to the energy resolution of the LaBr.sub.3(Ce).
Example 7
[0055] Several crystals of barium iodide were grown and
characterized. The radioluminescence of BaI.sub.2(Eu) typically
shows both a long-wave band, similar to that seen in undoped
SrI.sub.2, as well as the BaI.sub.2(Eu) band shown in FIG. 2. The
long-wave band, thought to be related to self-trapped exciton
luminescence, is reduced as the Eu doping level is increased.
However, even for crystals exhibiting only Eu luminescence, gamma
light yields and energy resolution so far are modest (see the chart
500 of FIG. 5).
Example 8
[0056] Barium Bromide crystals were grown doped with Eu, but the
light yields are <30,000 Ph/MeV. While it may be possible for
the performance of BaI.sub.2 and BaBr.sub.2 to be improved, but
energetic considerations, such as relative positions of the
Eu.sup.2+ states within the bandgap, may limit light yields. For
example, the Eu.sup.2+ excited state in BaI.sub.2 may be too close
to the conduction band to compete effectively with residual shallow
traps, while this matter is resolved in SrI.sub.2 since the
Eu.sup.2+ excited state is slightly lower with respect to the
conduction band.
[0057] Therefore, of the alkaline earth halides. SrI.sub.2(Eu)
appears most promising due to its very high light yield, good
optical properties, ease of growth, high achievable doping with
Eu.sup.2+, Z.sub.eff higher than LaBr3(Ce), excellent light yield
proportionality and demonstrated energy resolution of <2.7% at
662 keV. CaI.sub.2 has not been effectively grown in large sizes
and SrBr.sub.2 has a low Z.sub.eff, while BaI.sub.2 and BaBr.sub.2
have not demonstrated adequate light yields for high energy
resolution.
Example 9
[0058] Decay times were acquired using a flashlamp-pumped Nd:YAG
laser using the 4.sup.th harmonic at 266 nm, and 20 ns FWHM pulses.
Luminescence was collected with a monochromator coupled to an R928
Hamamatsu PMT and read out by an oscilloscope. in SrI.sub.2(Eu),
the Eu.sup.2+ band decays with a 1.2 microsecond time constant as
shown in FIG. 7, top plot 702. FIG. 7, middle plot 704, shows that
for the BaI.sub.2(Eu) crystal grown with as-received powder, the
Eu.sup.2+ decay is about 450 ns while the impurity-mediated
luminescence is slower, and cannot be fully integrated within an 8
.mu.s shaping time. It is interesting that a component of the
impurity-mediated recombination pathway is prompt
(pulsewidth-limited) proceeding, directly by trapping carriers from
the conduction and valence band, but there is also a component that
forms by depopulating the Eu.sup.2+ excited state (possibly
electrons trapped initially at Eu.sup.2+ are able to thermally
de-trap to the conduction band), as revealed by a rise-time
component observed for 600 nm detection. Also, the
impurity-mediated decay is very slow, on the tens of microseconds
timescale (perhaps due to an exciton experiencing a triplet to
singlet spin-forbidden transition). For ZR BaI.sub.2(Eu), the
Eu.sup.2+ decay is about 770 ns, as shown in FIG. 7, bottom plot
706, effectively lengthened due to the reduction of de-trapping and
excitation transfer to the impurity-mediated recombination
pathway.
ILLUSTRATIVE METHOD
[0059] Now referring to FIG. 6, a method 600 according to one
embodiment is shown. As an option, the present method 600 may be
implemented in the context and functionality architecture of the
preceding descriptions. Of course, the method 600 may be carried
out in any desired environment. It should also be noted that the
aforementioned definitions may apply during the present
description.
[0060] With continued reference to FIG. 6, in operation 602,
strontium iodide-containing crystals are mixed with a source of
Eu.sup.2+. Any type of strontium iodide-containing crystals may be
used, and the source of Eu.sup.2+ may be of any type.
[0061] In operation 604, the mixture is heated above a melting
point of the strontium iodide-containing crystals. The melting
point may be different than that of Eu.sup.2+ alone or It may be
different than a melting point of strontium iodide-containing
crystals alone.
[0062] In operation 606, the heated mixture is cooled near the seed
crystal for growing a crystal. The grown crystal may contain more
than 1.6% europium, more than 2.0% europium, or between about 0.5%
and about 8.0% europium. Further, the europium in the grown crystal
may be primarily Eu.sup.2+.
IN USE
[0063] Embodiments of the present invention may be used in a wide
variety of applications, and potentially any application in which
high light yield or high resolution is useful.
[0064] Illustrative uses of various embodiments of the present
invention include, but are not limited to, applications requiring
radiation detection. Search, surveillance and monitoring of
radioactive materials are a few such examples. Various embodiments
can also be used in the nuclear fuel cycle, homeland security
applications, nuclear non-proliferation, medical imaging, etc.
[0065] Yet other uses include detectors for use in treaty
inspections that can monitor the location of nuclear missile
warheads in a nonintrusive manner. Further uses include
implementation in detectors on buoys for customs agents at U.S.
maritime ports, cargo interrogation systems, and instruments that
emergency response personnel can use to detect or search for a
clandestine nuclear device.
[0066] While various embodiments have been described above, it
should be understood that they have been presented by way of
example only, and not imitation. Thus, the breadth and scope of a
preferred embodiment should not be limited by any of the
above-described exemplary embodiments, but should be defined only
in accordance with the following claims and their equivalents.
* * * * *