Alkaline-earth halides can be made into bright scintillators if purity is maintained during synthesis and growth. In order
to investigate precursor purity, beaded halide precursors were heated under vacuum and evolved gas was assessed by
residual gas spectroscopy. These precursors included cesium chloride, lithium chloride, yttrium chloride, cerium
chloride, strontium iodide, europium iodide, barium bromide, and europium bromide. Water and CO<sub>2</sub> desorption, sulfur release, argon release, and halide dissociation was observed in samples. Triply-oxidized precursors showed multiple
paths to decomposition. The data inform approaches toward purification and growth.
Thallium bromide (TlBr) has been under development for room temperature gamma ray spectroscopy due to high density, high Z and wide bandgap of the material. Furthermore, its low melting point (460 °C), cubic crystal structure and congruent melting with no solid-solid phase transitions between the melting point and room temperature, TlBr can be grown by relatively simple melt based methods. As a result of improvements in material processing and detector fabrication over the last several years, TlBr with electron mobility-lifetime products (μ<sub>e</sub>τ<sub>e</sub>) in the mid 10<sup>-3</sup> cm<sup>2</sup>/V range has been obtained. In this paper we are going to report on our unipolar charging TlBr results for the application as a small animal imaging. For SPECT application, about 5 mm thick pixellated detectors were fabricated and tested. About 1 % FWHM at 662 keV energy resolution was estimated at room temperature. By applying the depth correction technique, less than 1 % energy resolution was estimated. We are going to report the results from orthogonal strip TlBr detector for PET application. In this paper we also present our latest detector highlights and recent progress made in long term stability of TlBr detectors at or near room temperature. This work is being supported by the Domestic Nuclear Detection Office (DNDO) and the Department of Energy (DOE).
Thermal neutron detectors in planar configuration were fabricated from LiInSe<sub>2</sub> and B<sub>2</sub>Se<sub>3</sub> crystals grown at RMD Inc.
All fabricated semiconductor devices were characterized for the current-voltage (I-V) characteristic and neutron
counting measurement. Pulse height spectra were collected from <sup>241</sup>AmBe (neutron source on all samples), as well as
<sup>137</sup>Cs and <sup>60</sup>Co gamma ray sources. In this study, the resistivity of all crystals is reported and the collected pulse height
spectra are presented for fabricated devices. Note that, the <sup>241</sup>AmBe neutron source was custom designed with
polyethylene around the source as the neutron moderator, mainly to thermalize the fast neutrons before reaching the
detectors. Both LiInSe<sub>2</sub> and B<sub>2</sub>Se<sub>3</sub> devices showed response to thermal neutrons of the <sup>241</sup>AmBe source.
Thallium bromide (TlBr) and related ternary compounds, TlBrI and TlBrCl, have been under development for room
temperature gamma ray spectroscopy due to several promising properties. Due to recent advances in material
processing, electron mobility-lifetime product of TlBr is close to Cd(Zn)Te's value which allowed us to fabricate large
working detectors. We were also able to fabricate and obtain spectroscopic results from TlBr Capacitive Frisch Grid
detector and orthogonal strip detectors. In this paper we report on our recent TlBr and related ternary detector results
and preliminary results from Cinnabar (HgS) detectors.
We are working to perfect the growth of divalent Eu-doped strontium iodide single crystals and to optimize the design of
SrI<sub>2</sub>(Eu)-based gamma ray spectrometers. SrI<sub>2</sub>(Eu) offers a light yield in excess of 100,000 photons/MeV and light yield
proportionality surpassing that of Ce-doped lanthanum bromide. Thermal and x-ray diffraction analyses of SrI<sub>2</sub> and EuI<sub>2</sub>
indicate an excellent match in melting and crystallographic parameters, and very modest thermal expansion anisotropy.
We have demonstrated energy resolution with SrI<sub>2</sub>(4-6%Eu) of 2.6% at 662 keV and 7.6% at 60 keV with small crystals,
while the resolution degrades somewhat for larger sizes. Our experiments suggest that digital techniques may be useful
in improving the energy resolution in large crystals impaired by light-trapping, in which scintillation light is re-absorbed
and re-emitted in large and/or highly Eu<sup>2+</sup> -doped crystals. The light yield proportionality of SrI<sub>2</sub>(Eu) is found to be
superior to that of other known scintillator materials, such as LaBr<sub>3</sub>(Ce) and NaI(Tl).
Many materials used in radiation detectors are environmentally unstable and/or fragile. These properties are frustrating
to researchers and add significantly to the time and cost of developing new detectors as well as to the cost of
manufacturing products. The work presented here investigates the properties of HgS. This material was selected for
study based partly on its inherent stability and ruggedness, high density, high atomic number, and bandgap. HgS is found
in nature as the mineral cinnabar. A discussion of the physical properties of HgS, experimental characterization of
natural cinnabar, and initial radiation detection results are presented along with a discussion of potential crystal growth
techniques for producing crystals of HgS in the laboratory.
Some applications, particularly in homeland security, require detection of both neutron and gamma radiation. Typically,
this is accomplished with a combination of two detectors registering neutrons and gammas separately. Recently, a new
scintillator, Ce doped Cs<sub>2</sub>LiLaCl<sub>6</sub> (CLLC) that can provide detection of both has been investigated for gamma and
neutron detection. This material is capable of providing very high energy resolution, as good as 3.4% at 662 keV
(FWHM), which is better than that of NaI(Tl). Since it contains <sup>6</sup>Li, it can also detect thermal neutrons. In the energy
spectra, the full energy thermal neutron peak appears near 3 GEE MeV. Thus very effective pulse height discrimination
can be achieved with this material. The CLLC emission consists of two main components: Core-to-Valence
Luminescence (CVL) spanning from 220 nm to 320 nm and Ce emission found in the range of 350 to 500 nm. The
former emission is of particular interest since it appears only under gamma excitation. It is also very fast, decaying with
a 2 ns time constant. This provides CLLC with different temporal responses under gamma and neutron excitation and it
can be used for effective pulse shape discrimination.
TlBr is a promising semiconductor for gamma-ray detection at room temperature, but it has to be extremely pure to
become useful. We investigated the purification and crystal growth of TlBr to improve the mobility and lifetime of
charge carriers, and produce TlBr detectors for radioisotopic detection. Custom equipment was built for purification and
crystal growth of TlBr. The zone refining and crystal growth were done in a horizontal configuration. The process
parameters were optimized and detector grade material with an electron mobility-lifetime product of up to 3x10<sup>-3</sup> cm<sup>2</sup>/V
has been produced. The material analysis and detector characterization results are included.
Single crystals of LaBr<sub>3</sub>:1% Pr and CeBr<sub>3</sub>:1% Pr have been grown by the vertical Bridgman technique. Crystals of
these scintillators can be used in the fabrication of gamma-ray spectrometers. The LaBr<sub>3</sub>:1% Pr and CeBr<sub>3</sub>:1% Pr
crystals we have grown had light outputs of ~73,000 and ~50,000 photons/MeV, respectively, and principal decay
constants of 11μs and 26 ns, respectively. There were a number of emission peaks observed for these compounds. The
emission wavelength range for the LaBr<sub>3</sub>:1% Pr and CeBr<sub>3</sub>:1% Pr scintillators were from about 400 to 800 nm. The
CeBr<sub>3</sub>:1% Pr scintillator had a dominating emission peak due to CeBr<sub>3</sub> at 390 nm. These two materials had energy
resolutions of 9 and 7% FWHM, respectively, for 662 keV photons at room temperature. In this paper, we will report on
our results to date for vertical Bridgman crystal growth and characterization of Pr-doped LaBr<sub>3</sub> and Pr-doped CeBr<sub>3</sub>
crystals. We will also describe the special handling and processing procedures developed for these scintillator