W.M. Higgins

Scintillation Properties of 1 Inch

We have grown and investigated 1 inch diameter (CLYC) crystals for gamma and neutron detection. The samples provided excellent results. For example, 5.1plusmn0.1% energy resolution was obtained at 662 keV (5.5% at 511 keV). The light output of 4500plusmn350 photoelectrons/MeV (PMT, Hamamatsu R6233) was measured. The samples also showed excellent plusmn1.2% non-proportionality in the 14.4 to 1274 keV range. This suggests a possibility for even better energy resolution with a superior photodetector. The intrinsic energy resolution of investigated crystals was estimated to be 2.1%. The neutron detection was also confirmed. The neutron peak was observed at about 3.2 MeV (gamma equivalent) and its resolution was 2.9plusmn0.1%. Gamma-neutron pulse shape discrimination was also achieved.

{\rm Cs}_{2}{\rm LiYCl}_{6}{:}{\rm Ce}

In this communication we report on our investigation of scintillation properties of LaBr/sub 3/:Ce as a function of Ce concentration. We have studied crystals nominally doped with 0.5, 5, 10, 20, and 30% Ce (by mole). Reports published so far suggested that as the Ce content increases, there is a decrease in light output and little or no change in decay time constants. Our results show that the light output does not change with Ce concentration up to 30% and depends mostly on the crystal and measurement quality. On the other hand we have found timing properties to be a strong function of concentration. As the Ce content increases the principal decay time constant of scintillation decreases from /spl sim/26 ns for 0.5% Ce to /spl sim/17 ns for crystals with >5% Ce. Moreover, there is also a significant change in rise time constants. The rise time measured for a sample doped with 0.5% Ce is up to 9 ns, whereas for samples doped with >10% Ce it reduces to less than 0.5 ns. The change of rise time has a major effect on the timing properties of this scintillator, with timing resolution improving from 390 ps to less than 200 ps (FWHM).

Crystals

In this paper we report on the scintillation properties of cerium doped strontium - and barium hafnate. Radioluminescence, pulse height, scintillation decay and timing spectra are presented. Radioluminescence spectra of SrHfO3:Ce3+ and BaHfO3:Ce3+ consist of a broad band due to Ce3+ emission peaking at 410 nm and 400 nm, respectively. The light yield of BaHfO3:Ce3+ and SrHfO3:Ce3+ is approximately 40 000 photons/MeV when compared to a crystal of BGO. The principal decay time constant for SrHfO3:Ce3+ and BaHfO3:Ce3+ is 42 and 25 ns, respectively. A timing resolution of 276 ps (FWHM) was obtained with transparent optical ceramic of SrHfO3:Ce3+.

Effects of Ce concentration on scintillation properties of LaBr/sub 3/:Ce

This paper summarizes the initial investigation of large diameter (2-inch) Cs2LiYCl6:Ce (CLYC) crystals grown at RMD. Although the crystals had some cracks, the tested sample provided adequate results: It produced a clear thermal neutron peak, a difference in time profiles under gamma and neutron excitation was observed, and pulse shape discrimination (PSD) based on two integration windows and their ratios was achieved. The PSD provided an excellent separation between gamma and neutron events. The discrimination ratio was better than 1:1000 (based on 15,000 events). We have also tested a relatively smaller sample with excellent crystal quality for the energy resolution. The results surpassed those previously obtained. The recorded energy resolution at 662 keV was 4.7% (FWHM) using a standard bialkali PMT. This is better than the previously measured 5.1% value. A 4.3% energy resolution was obtained with a super bialkali PMT.

Scintillation Properties of SrHfO

Thallium bromide (TlBr) is a high atomic number (81, 35), dense (7.56 g/cm3) wide band gap (2.68 eV) semiconductor. In addition, TlBr has a cubic crystal structure and melts congruently at a relatively low temperature (~460 C). Recently, mobility-lifetime product of electrons in TlBr has been reported to be greater than 0.001 cm2/V. These properties make TlBr a promising material for room temperature gamma radiation detection. Employing device designs such as small pixel arrays that depend primarily on the motion of a single carrier type allows fabrication of thicker devices with better energy resolution than planar devices of the same thickness. We report on our recent progress in developing larger TlBr detectors. Over the past several months we have increased the electron mobility-lifetime product of our TlBr by more than one order of magnitude. Electron mobility-lifetime values as high as 3.0 times 10-3 cm2/V have been measured. Devices with small pixel design have been built with 3, 5, and 10 mm thickness and pixel pitch of 1 mm, 1.5, and 2.0 mm respectively. Pulse height spectra have been recorded over a range of energies from 60 keV to 662 keV. Energy resolution (FWHM) as high as approximately 5% at 122 keV and 1.7% at 662 keV has been obtained without any 3-D corrections. Such arrays are well suited for 3-D correction techniques similar to those applied to CZT devices, indicating that further improvement in energy resolution should be achievable. These latest results demonstrate promise for TlBr as a room temperature semiconductor gamma ray detector.

_{3}

Thallium bromide (TlBr) is attractive for high energy radiation detection, given its large molecular weight and wide energy bandgap. However, TlBr exhibits levels of ionic conductivity that can lead to an undesirable leakage, or dark current, thereby reducing sensor performance. To investigate the role of dopants in controlling the ionic conductivity, single crystals of TlBr were grown using zone refining and/or vertical Bridgman methods with controlled levels of donor (Pb) dopants. Their electrical properties were examined as a function of temperature (20-300°C) with frequency dependent impedance spectroscopy. A Schottky-based defect equilibria model was fitted to the resulting conductivity data, and enthalpies of Schottky defect formation (0.91 ± 0.03 eV), cation migration (0.51 ± 0.03 eV), and anion migration (0.28 ± 0.05 eV) were extracted. Br vacancies were found to posses about 5 orders of magnitude higher mobility than that of T1 vacancies at 20°C.

:Ce

In this paper we present initial results on mixed LuI3-YI3-GdI3 scintillators for gamma and neutron detection. The scintillation properties were investigated and compared to the results obtained for the binary compositions. Small samples, few millimeters in size, were tested. Under X-ray excitation each crystal exhibits bright green emission in the 425 to 750 nm spectral range. The exact peak position depends slightly on the material composition. The emission is due to d-f transitions on the Ce3+ ion. The scintillation under gamma excitation is fast and decays with about 30 ns time constant. The rise time varies slightly and it can be as fast as 0.5 ns. The light output of the investigated samples varied from 115,000plusmn13,000 ph/MeV for LuI3 to 68,000plusmn8,000 ph/MeV for LuI3-YI3 composition. The energy resolution at 662 keV was difficult to estimate since the samples lacked in crystal quality. LuI3-GdI3 composition was also tested for the detection of thermal neutrons from a moderated 252Cf source. The neutron peak appears at 81 keV when compared to 60 keV peak in 241Am spectrum recorded under the same conditions. The light output per a single detected neutron is about ~6,350 photons.

^{3+}

In this paper, we report on a new scintillator, cerium bromide (CeBr3), for gamma-ray spectroscopy. Crystals of this scintillator have been grown using Bridgman process. In CeBr3, Ce3+ is an intrinsic constituent as well as a luminescence center for the scintillation process, has high light output (~68,000 photons/MeV) and fast decay constant (~17 ns). Furthermore, it shows excellent energy resolution for gamma-ray detection. For example, energy resolution of <4% (FWHM) has been achieved using this scintillator for 662 keV photons (137Cs source) at room temperature. High timing resolution (>200 ps - FWHM) has been recorded with CeBr3-PMT (photomultiplier tubes) and BaF2-PMT detectors operating in coincidence using 511 keV positron annihilation gamma-ray pairs. Potential applications of this material are addressed

and BaHfO

We are working to perfect the growth of divalent Eu-doped strontium iodide single crystals and to optimize the design of SrI2(Eu)-based gamma ray spectrometers. SrI2(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 SrI2 and EuI2 indicate an excellent match in melting and crystallographic parameters, and very modest thermal expansion anisotropy. We have demonstrated energy resolution with SrI2(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 Eu2+ -doped crystals. The light yield proportionality of SrI2(Eu) is found to be superior to that of other known scintillator materials, such as LaBr3(Ce) and NaI(Tl).

_{3}

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 3x10-3 cm2/V has been produced. The material analysis and detector characterization results are included.