J. Glodo

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 a relatively new scintillator - LaBr/sub 3/ for gamma ray spectroscopy. Crystals of this scintillator have been grown using Bridgman process. This material when doped with cerium has high light output (/spl sim/60,000 photons/MeV) and fast principal decay constant (/spl les/25 ns). Furthermore, it shows excellent energy resolution for /spl gamma/-ray detection. Energy resolution of 3.2% (FWHM) has been achieved for 662 keV photons (/sup 137/Cs source) at room temperature. High timing resolution (260 ps FWHM) has been recorded with LaBr/sub 3/-PMT and BaF/sub 2/-PMT detectors operating in coincidence mode using 511 keV positron annihilation /spl gamma/-ray pairs. Details of its scintillation properties, and variation of these properties with changing cerium concentration are reported. Potential applications of this material are also addressed.

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

Homeland security applications often require detection of both neutron and gamma radiation. A combination of two detectors registering neutrons and gammas separately is typically used. Recently, a number of scintillators from the elpasolite crystal family were proposed, that provide detection of both types of radiation. The most promising are Cs2LiYCl6, Cs2LiLaCl6, and Cs2LiLaBr6. All are doped with Ce3+. They are capable of providing very high energy resolution. The best values achieved for each material are 3.9%, 3.4%, and 2.9% at 662 keV (FWHM), respectively. Since 6Li has an acceptable cross-section for thermal neutron capture, these materials also detect thermal neutrons. In the energy spectra, the full energy thermal neutron peak typically appears above 3 MeV gamma equivalent energy. Thus very effective pulse height discrimination can be implemented with these materials. The CLLC and CLYC emissions consist of two main components: Core-to-Valence Luminescence (CVL; 220 nm to 320 nm) and Ce emission (350 to 500 nm). The former is of particular interest since it appears only under gamma excitation. It is also very fast and decays with less than 2 ns time constant. The CVL provides a significant difference to temporal responses under gamma and neutron excitation thus it may be used for effective pulse shape discrimination.

LaBr/sub 3/:Ce scintillators for gamma ray spectroscopy

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+.

Selected Properties of Cs

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.

_{2}

In recent years, a number of materials from the elpasolite crystal family have been under development for either or both gamma ray and neutron detection. The scintillators show good energy resolution and thermal neutron detection efficiency. The latter is achieved due to the fact, that the selected compositions contain Li-6 ions. In order to effectively and reliably register both types of radiation, it is necessary to separate them through particle identification schemes. This can be accomplished using either pulse height or/and pulse shape discrimination, with the latter being more reliable. In this paper, we summarize our work and provide current status of pulse shape discrimination in the selected elpasolite scintillators. These include Cs2LiYCl6 (CLYC), Cs2LiLaCl6(CLLC), Cs2LiLaBr6(CLLB), and Cs2LiYBr6(CLYB).

LiYCl

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.

_{6}

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

, Cs

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).