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This paper summarizes the initial investigation of large diameter (2-inch) [Cs.sub.2]LiY[Cl.sub.6] : 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. Index Terms--[Cs.sub.2]LiY[Cl.sub.6], elpasolites, gamma-ray spectroscopy, neutron detectors, radiation detectors, scintillators.

Phase-pure transparent LSO ceramic was obtained, with its scintillation properties rivaling those of single crystalline LSO. The transparent LSO ceramic was prepared by a nano-technology approach. The densities of the LSO ceramic increased with increasing sintering temperature. Further SEM examination confirmed the decrease of the porosity during densification of the ceramic. Pores were segregated at grain boundaries after sintering. Hot isostatic pressing (HIPing) was employed to further densify the ceramic and eliminate porosity after sintering. Transparent polycrystalline LSO ceramics were obtained after the final HIP. XRD examination confirms single monoclinic LSO phase. A light output as high as 30,100 ph/MeV was obtained using a [sup.22]Na excitation source. LSO ceramic showed an energy resolution of 15 % (FWHM) at 662 keV ([sup.137] Cs source) and a fast scintillation decay of 40 ns due to the 5d [right arrow] 4f transition of [Ce.sup.3+]. The excellent scintillation and optical properties make LSO ceramic a promising candidate for future gamma-ray spectroscopy as well as medical imaging applications. Index Terms--[Lu.sub.2]Si[O.sub.5] : Ce (LSO), ceramic, scintillator, transparent.

New scintillators for high-resolution gamma ray spectroscopy have been identified, grown and characterized. Our development efforts have focused on two classes of high-light-yield materials: europium-doped alkaline earth halides and cerium-doped garnets. Of the halide single crystals we have grown by the Bridgman method--Sr[I.sub.2], Ca[I.sub.2], Sr[Br.sub.2], Ba[I.sub.2] and Ba[Br.sub.2]--Sr[I.sub.2] is the most promising. Sr[I.sub.2] (Eu) emits into the [Eu.sup.2+] band, centered at 435 nm, with a decay time of 1.2 [micro]s and a light yield of up to 115,000 photons/MeV. It offers energy resolution better than 3% FWHM at 662 keV, and exhibits excellent light yield proportionality. Transparent ceramic fabrication allows the production of gadolinium- and terbium-based garnets which are not growable by melt techniques due to phase instabilities. The scintillation light yields of cerium-doped ceramic garnets are high, 20,000-100,000 photons/MeV. We are developing an understanding of the mechanisms underlying energy dependent scintillation light yield non-proportionality and how it affects energy resolution. We have also identified aspects of optical design that can be optimized to enhance the energy resolution. Index Terms--Ceramic scintillators, gamma ray spectrometers, scintillators, strontium iodide.

Single crystals of Sr[I.sub.2]:Eu and Sr[I.sub.2]:Ce/Na were grown from anhydrous iodides by the vertical Bridgman technique in evacuated silica ampoules. Growth rates were of the order of 5-30 mm/day. Radioluminescence spectra of Sr[I.sub.2]:Eu and Sr[I.sub.2]:Ce/Na exhibit a broad band due to [Eu.sup.2+] and [Ce.sup.3+] emission, respectively. The maximum in the luminescence spectrum of Sr[I.sub.2] :Eu is found at 435 nm. The spectrum of Sr[I.sub.2] :Ce/Na exhibits a doublet peaking at 404 and 435 nm attributed to [Ce.sup.3+] emission, while additional impurity--or defect--related emission is present at approximately 525 nm. The strontium iodide scintillators show very high light yields of up to 120 000 photons/MeV, have energy resolutions down to 3% at 662 keV (Full Width Half Maximum) and exhibit excellent light yield proportionality with a standard deviation of less than 5% between 6 and 460 keV. Index Terms--Alkaline-earth halides, crystal growth, energy resolution, scintillation detectors.

In this paper we present initial results on mixed Lu[I.sub.3]-Y[I.sub.3]-Gd[I.sub.3] 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 [Ce.sup.3+] 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 115,000 [+ or -] 13,000 ph/MeV for Lu[I.sub.3] to 68,000 [+ or -] 8,000 ph/MeV for LU[I.sub.3]-Y[I.sub.3] composition. The energy resolution at 662 keV was difficult to estimate since the samples lacked in crystal quality. Lu[I.sub.3]-Gd[I.sub.3] composition was also tested for the detection of thermal neutrons from a moderated [sup.252] Cf source. The neutron peak appears at 81 keV when compared to 60 keV peak in [sup.241] Am spectrum recorded under the same conditions. The light output per a single detected neutron is about ~ 6,350 photons. Index Terms--Gd[I.sub.3], Lu[I.sub.3], Y[I.sub.3], iodides, neutron detectors, radiation detectors, scintillator.

The recent development of fast scintillating crystals with high light output (such as La[Br3.sub.3], LYSO, and LSO) has made it possible to incorporate time-of-flight information into whole-body positron emission tomography scanners (TOF-PET). The development of fast scintillators has continued, and scintillators such as Ce[Br.sub.3] and the six crystals we report on here are candidates for future TOF-PET scanners. We have performed preliminary tests on six prototype scintillating crystals composed of [YI.sub.3] (2% Ce), (Lu,Gd)[I.sub.3] (2% Ce), Lu[I.sub.3] (10% Ce), Lu[I.sub.3] (5% Ce), [LuI.sub.3] (2% Ce), and Ce[(Br, Cl).sub.3]. The crystals were coupled to a PMT to observe 511 keV photons produced by positron captures in a [sup.22]Na point source. A La[Br3.sub.3] coincidence detector was used in order to make timing resolution measurements. The prototype samples had a range of timing resolutions from 160 ps to 250 ps (FWHM). Ce[(Br,Cl).sub.3] was found to have the best timing resolution, with 160 ps, followed by Lu[I.sub.3] (2% Ce) with 180 ps. These results are competitive with the 160 ps we observed for a pair of La[Br3.sub.3] crystals in coincidence with each other. The 2% Ce doped Lu[I.sub.3] sample was found to have the best overall energy response, with an energy resolution (FWHM) of 6% at 511 keV. Energy resolution and light yield measurements are presented for each of the crystals. Index Terms--Halides, Lu[I.sub.3], positron emission tomography, scintillation, time-of-flight.

We have grown and investigated 1 inch diameter [Cs.sub.2]LiY[Cl.sub.6]:Ce (CLYC) crystals for gamma and neutron detection. The samples provided excellent results. For example, 5.1 [+ or -] 0.1% energy resolution was obtained at 662 keV (5.5% @511 keV). The light output of 4500 [+ or -] 350 photoelectrons/MeV (PMT, Hamamatsu R6233) was measured. The samples also showed excellent [+ or -] 1.2% non-proportionality in the 14A 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 2.9 [+ or -] 0.1%. Gamma-neutron pulse shape discrimination was also achieved. Index Terms--[Cs.sub.2]LiY[Cl.sub.6], energy resolution, neutron detectors, radiation detectors, scintillator.

In this paper we report on the thermoluminescence of [Cs.sub.2]LiY[Cl.sub.6], [Cs.sub.2]LiY[Cl.sub.6]:[Ce.sup.3+] and [Cs.sub.2]LiY[Cl.sub.6]:[Pr.sup.3+] crystals. Low-temperature glow curves and isothermal decay spectra are presented. All three crystals exhibit prominent glow peaks below 300 K: The [Cs.sub.2]LiY[Cl.sub.6] sample shows a major peak at 58 K in addition to a broad band between 75 K and 150 K. The glow curve of [Cs.sub.2]LiY[Cl.sub.6]:[Pr.sup.3+] is similar to that of [Cs.sub.2]LiY[Cl.sub.6] but shows additional peaks at 222 K and 277 K. The glow curve of [Cs.sub.2]LiY[Cl.sub.6]:[Ce.sup.3+] is dominated by a major glow peak at 214 K with minor peaks at 50 K and 88 K. Lifetimes of traps derived from isothermal decay spectra measured at the temperature maximum of selected glow peaks are of the order of tens of seconds. Index Terms--[Cs.sub.2]LiY[Cl.sub.6], elpasolites, scintillation detectors, thermoluminescence.

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 SrHf[O.sub.3]:[Ce.sup.3+] and BaHf[O.sub.3]:[Ce.sup.3+] consist of a broad band due to [Ce.sup.3+] emission peaking at 410 nm and 400 nm, respectively. The light yield of BaHf[O.sub.3]:[Ce.sup.3+] and SrHf[O.sub.3:][Ce.sup.3+] is approximately 40 000 photons/MeV when compared to a crystal of BGO. The principal decay time constant for SrHf[O.sub.3]:[Ce.sup.3+] and BaHf[O.sub.3]:[Ce.sup.3+] is 42 and 25 ns, respectively. A timing resolution of 276 ps (FWHM) was obtained with transparent optical ceramic of SrHf[O.sub.3]:[Ce.sup.3+]. Index Terms--Ceramics, cerium compounds, hafnium compounds, scintillation detectors.

In this paper, we report on a new scintillator, cerium bromide (Ce[Br.sub.3]), for gamma-ray spectroscopy. Crystals of this scintillator have been grown using Bridgman process. In this material [Ce.sup.3+] is an intrinsic constituent as well as a luminescence center for the scintillation process. Samples of Ce[Br.sub.3] showed high light output (~ 68000 photons/MeV) and fast decay constant (~ 17 ns). Furthermore, they exhibited excellent energy resolution for gamma-ray detection. For example, energy resolution of < 4% full width at half maximum (FWHM) has been achieved using this scintillator for 662 keV photons ([sup.137]Cs source) at room temperature. High timing resolution (< 200 ps--FWHM) has been recorded with Ce[Br.sub.3]-photomultiplier (PMT) and Ba[F.sub.2]-PMT detectors operating in coincidence using 511 keV positron annihilation gamma-ray pairs. Index Terms--[Ce.sup.3+], gamma-ray spectroscopy, radiation detectors, scintillators.