Performance of different phoswich configurations in a balloon flight experiment

Four different phoswich units were included in a modular hard X-ray balloon experiment launched on August 10, 1982 in order to check the dependence of background level and rejection efficiency of NaI(Tl)/CsI(Na) phoswich detectors on scintillator thicknesses. The results concern the dependence of the background intensity on both the thickness of the active shield, for a fixed primary detector thickness, and on the thickness of the primary detector, given the active shield thickness. A direct comparison of phoswich detectors with passively shielded NaI(Tl) crystals is also given. As a consequence practical hints for designing new phoswich detectors are derived and the limiting sensitivity of these detectors for hard X-ray observations of celestial sources is inferred.     

富集10B的Ce:Li6Lu(10BO3)3晶体生长及其闪烁性能

为了提高中子探测效率, 以富集¹⁰B的H₃¹⁰BO₃为原料, 通过提拉法生长了富集¹⁰B的Ce:Li₆Lu(¹⁰BO₃)₃晶体。X射线激发发射光谱测试表明: 其发光峰位于360~480 nm, 属于Ce³⁺离子典型的5d - 4f跃迁发光, 其闪烁发光效率为BGO晶体的3.9倍。在350 nm紫外光和¹³⁷Cs所发出的662 keV的γ射线激发下测得的衰减时间分别为21.0 ns 和31.7 ns, 在¹³⁷Cs辐射源激发下所测得的相对光输出是CsI(Tl)晶体的20%, 能量分辨率为9.7%。在慢化²⁵²Cf中子源激发下可以观测到明显的中子全能峰, 其能量分辨率为33%。上述研究结果表明, Ce:Li₆Lu(¹⁰BO₃)₃晶体具有较高的闪烁效率、快的衰减时间和良好的中子探测效率, 是一种具有应用前景的中子探测用闪烁晶体。     

CsI(Tl)/plastic phoswich detector enhanced in low-energy gamma-ray detection

A phoswich detector composed of a thin plate CsI(Tl) scintillator and a plastic scintillator (BC-400) has been designed and evaluated in order to improve the sensitivity in the low-energy region of a large-area plastic scintillation detector. This newly designed phoswich detector can be applied to both gross gamma measurement and energy spectrometry for low-energy gamma-ray emitters. Judging by estimations of minimum detectable activity, the lower measurable energy of a large-area plastic scintillation detector can be expanded down to a few tens of keV by adding a thin plate CsI(Tl) scintillator.     

A new hybrid photomultiplier tube as detector for scintillating crystals

In this work, we have attentively studied the performance of a new hybrid photomultiplier tube (HPMT) as detector for photons from scintillating crystals. The HPMT is equipped with a YAP window in order to improve light collection and increase measured light response from scintillating crystals. Several measurements have been performed on BGO, LSO, CsI(Tl) and NaI(Tl) planar crystals having three different surface treatments as well as on YAP : Ce and CsI(Tl) matrices. Such crystals have been coupled to two HPMTs, one equipped with a YAP window (Y-HPMT) and the other with a conventional quartz window (Q-HPMT). Measurements on crystals coupled to the Y-HPMT have shown a consistent improvement of the light response, thanks to the presence of the YAP window. Indeed, the light response measured with the Y-HPMT was on average equal to 1.5, 2.1 and 2.6 times that obtained with the Q-HPMT for planar crystals with white painted (diffusive), fine ground and polished rear surfaces, respectively. With regards to crystal matrices, we measured a light response increase of about 1.2 times.     

Temperature dependence of BGO-CsI(Tl) phoswich detector properties

We have studied the variations of the performances, and particularly the discrimination efficiency, of a BGO-CsI(Tl) phoswich detector in the temperature range 2.5–40°C.The scintillation decay time evaluation has been carried out by means of a double constant fraction discriminator and a time to amplitude converter on 1 μs shaped pulses. Good discrimination between BGO and CsI(Tl) events has been obtained over the whole temperature range, even though small efficiency variations are present because of the relative displacement of the acceptance window of the discriminator due to the decay time dependence on temperature. A criterium to minimize this effect, useful in particular for the BGO-CsI(Tl) phoswich, has been developed.     

Simulation of phoswich detectors using MCNPX and EGSNRC

The in vivo monitoring Lab at KIT uses two phoswich detectors for routine lung counting. A simplified model of one of them has been implemented in the two Monte Carlo codes EGSnrc and MCNPX. The active part of the detector consists of a crystal of NaI(Tl) and one of CsI(Tl): the energy deposited in both the crystals have been studied to consider the effect of the anticoincidence logic, present in the read-out electronics of the detectors and not yet studied with Monte Carlo simulations. Only the NaI(Tl) crystal has then been used to study the escape peaks at several energies, which are more prominent at low energies. The results from the two codes have been compared. The comparison of the codes predictions for the escape peaks has been then extended using the model of a germanium detector.     

A simple phoswich system

Normal phoswich detector systems use a combination of NaI(Tl) and CsI(Na) scintillators and require the application of careful pulse-shape discriminator techniques to resolve the two components in the scintillation light output which have decay constants of 250 and 630 ns respectively. These techniques provide a good anticoincidence veto efficiency for a relatively narrow range in the ratio of energy deposits in the two crytals and for a detector system whose temperature is carefully controlled. This paper describes the performance of a simple phoswich which makes use of the fast UV signal from a BaF₂ crystal to provide a prompt veto signal. The performance to be expected from various combinations of a BaF₂ anticoincidence crystal with other primary detectors is presented. These simulations have been verified by simple experimental tests.     

Study of radiation damage to long BGO crystals

Results are presented on the decrease of the scintillation light output of long bismuth germanate (BGO) crystals at room temperature due to irradiation by ⁶⁰Co photons and 25 MeV electrons with doses from 50 to 5000 rad. After irradiation the light output recovers spontaneously. This recovery can be described by the sum of a minimum of three exponential functions. The shortest time constant is of the order of several hours, while the longest time constant is several hundred hours. The initial decrease in light output depends non-linearly on the dose applied and saturation effects have been observed. The damage induced by irradiation with 25 meV electrons is similar to the damage caused by ⁶⁰Co photons. The data are consistent with the view that only the absorption properties of BGO are changed by irradiation. For comparison, a NaI(Tl) crystal and a CsI(Tl) crystal have also been irradiated. Also, the effects on light transmission due to radiation damage to BGO are reported.     

Evaluation of the GSO:Ce scintillator in the X-ray energy range from 40 to 140 kV for possible applications in medical X-ray imaging

The purpose of the present study was to evaluate, under X-ray medical imaging conditions, the X-ray luminescence efficiency (XLE) and the optical quantum gain (OQG) of the Gd₂SiO₅:Ce scintillator in single crystal form, suitable for tomographic applications. Intrinsic physical properties and light emission characteristics of the Gd₂SiO₅:Ce scintillator, were also studied. Both experimental and Monte Carlo techniques were used. Various X-ray tube voltages (40–140 kV), currently employed in X-ray imaging applications, were used. XLE was found to vary slowly with X-ray tube voltage from (0.021±0.003) to (0.017±0.003). OQG varied from (317±18) to (466±23) light photons per incident X-ray. These values were adequately high for imaging applications using the particular energy range. Additionally, it was found by Monte Carlo simulations that for crystal thicknesses higher than 0.5 cm both XLE and OQG reached saturation levels, indicating that higher thickness crystals are of no practical use in X-ray medical imaging.     

Bismuth germanate scintillators: Applications in nuclear safeguards and health physics

Bismuth germanate (BGO) scintillators are preferable to NaI(Tl) scintillators or germanium detectors for some applications. We describe two systems based on BGO scintillators for applications in nuclear safeguards and health physics. The first system, which consists of eight scintillators and a computer-based data acquisition system, is very efficient. The second, which consists of one scintillator and a small analyzer, is less efficient but portable. A computer code that uses measured response functions and photopeak efficiencies, unfolds the BGO distributions measured with these systems to determine gamma-ray flux spectra and dose rates. One application of these systems is the accurate determination of flux spectra and dose rates from containers of uranium or plutonium. A second application determined these quantities from a replica of Little Boy, the device exploded over Hiroshima.     

Charged particles identification with a CsI(Tl) scintillator

A CsI(Tl) scintillator with two light decay components is used to detect and identify p, d, t, ³He and α particles with a low energy threshold. Besides, the addition of a thin plastic scintillator in front of the CsI(Tl) crystal allows charge identification for ions with Z up to 19.     

Beam test of the CsI(Tl) calorimeter for the BELLE detector at the KEK-B factory

We studied the performance of a prototype electromagnetic calorimeter for the BELLE detector at the KEK proton synchrotron for an energy range of 0.25–3.5 GeV. The prototype consisted of an array of 6 × 5 CsI(Tl) crystals with 30 cm length (16.2 radiation lengths) and about 6 cm × 6 cm cross section. The scintillation light of each CsI(Tl) crystal was read out by two large-area PIN photodiodes and charge-sensitive preamplifiers attached at the rear face of the crystal. We measured the energy and position resolution for electrons and the e/π separation for two sets of matrix configurations: one corresponded to the center and the other to the edge of the barrel calorimeter. The overall performance measured by the test proves that the prototype calorimeter is satisfactory for the use in the BELLE detector.     

Comparison of the BGO and CsI(Tl) energy resolutions for p and α

Energy resolution and linearity of a BGO-photodiode have been measured for 20–40 MeV p and 14–25 MeV α. Comparisons are made to BGO and CsI(Tl) read by photomultipliers.     

Cs2LiYCl6:Ce闪烁晶体的光学及闪烁性能

用坩埚下降法生长得到Cs₂LiY _0.95Cl₆: 5%Ce(CYLC)闪烁晶体, 通过X射线衍射分析证明Cs₂LiYCl₆:Ce的晶体结构属于钾冰晶石结构, 并与理论计算结果基本吻合。在吸收光谱中观测到源于Ce³⁺离子从4f向5d_1~5电子跃迁的吸收峰和自陷激子吸收峰。X射线和紫外激发和发射光谱测试表明, 位于300 nm的发光属于Cs₂LiYCl₆:Ce晶体的本征芯价发光, 321 nm的发光归因于自陷激子发光, 350~450 nm范围的发光属于Ce³⁺离子5d-4f 跃迁发光。在³⁷Cs源伽马射线激发下, CYLC晶体的能量分辨率达到8.1%, 衰减时间分别为58 ns和580 ns。综上所述可知, Cs₂LiYCl₆:Ce晶体将是一种在中子和伽马射线分辨领域具有广泛应用前景的闪烁晶体。     

Energy non-linearity effects in the response of ionic crystal scintillators to X-rays with energy in the region of the K-absorptions edges: experimental results

The response of a YAP, NaI(Tl) and BaF₂ scintillators to X-rays with energies around the Y, I, and Ba K-absorption edges, respectively, was investigated. For all the scintillators, the amplitude response follows different linear trends for X-ray energies below and above the respective K-edges, presenting a discontinuity at these energies. An abrupt decrease of about 3%, 5% and 2% were observed in the detector amplitude at the K-edges, for the YAP, the NaI(Tl) and the BaF₂ scintillator, respectively, corresponding to a decrease of 0.5±0.1, 1.7±0.3 and 0.8±0.2 keV in the energy calibration line. These discontinuities result in a region within 0.5±0.1, 1.6±0.3 and 0.9±0.2 keV where the X-ray energy cannot be obtained unambiguously. The scintillation yields for X-rays present abrupt decreases of about 3%, 4% and 2%, respectively, at the K-edges. The measured non-linearity effects are significantly larger than those obtained for gaseous and semiconductor detectors. The higher amplitude non–linearity observed in NaI(Tl) is attributed to the larger light yield non-linearity in the electron response of this crystal.     

Phoswich detectors combining doubly or triply ZnS(Ag), NE102A, BGO and/or NaI(Tl) scintillators for simultaneous counting of α, β and γ rays

Phoswich detectors for simultaneous counting of α, β and γ rays have been developed: ZnS(Ag)/Au Mylar/NE102A, ZnS(Ag)/Au Mylar/BGO and ZnS(Ag)/NaI(Tl) for α and β(γ) rays and ZnS(Ag)/Au Mylar/NE102A/BGO and ZnS(Ag)/NE102A/NaI(Tl) for α, β and γ rays. They were prepared by coupling a ZnS(Ag) film scintillator for α counting with a scintillator(s) for β and γ counting having different rise time. In order to adjust each component of pulse height within a given dynamic range, a sheet of Au-coated Mylar (Au Mylar) was used, if necessary, as an optical ND filter for lowering transmittance of scintillation of the ZnS(Ag). Characteristics of these phoswiches were examined by a technique of pulse-shape discrimination. Excellent discrimination among the radiations was attained and small tailings from each other peak were obtained for the prepared phoswiches.     

Ultralow-concentration Sm codoping in CsI:Tl scintillator: A case of little things can make a big difference

CsI:Tl scintillators were hindered from computer tomography and high-speed imaging applications by a serious afterglow problem. In this study, the effects of ultralow-concentration Sm codoping on the scintillation characteristics of CsI:Tl were investigated. Pulsed X-ray excited afterglow after 50 ms in 0.005 mol% Sm-codoped CsI:Tl was lowered by over one order of magnitude in comparison with Sm-free one. The beneficial effects of ultralow-concentration Sm codoping also appeared to be maintaining the light yield and energy resolution. The light yield and energy resolution after 0.005 mol% Sm codoping were 71,700 ± 6000 photons/MeV and 6.9% at 662 keV, respectively.     

Gd3(Al,Ga)5O12:Ce闪烁晶体扭曲生长、组分偏析与性能研究

新型闪烁晶体Gd3(Al,Ga)5O12:Ce(简写为GAGG:Ce)在制备过程中易出现多晶扭曲生长、组分偏析等问题,严重影响晶体的性能.为了得到大尺寸高质量的GAGG:Ce晶体,采用X射线衍射(XRD)、电感耦合等离子体发射光谱(ICP-OES)和X射线激发发射谱(XEL)等手段,结合熔体特性分析了GAGG:Ce晶体...     

Depth-of-interaction determination in NaI(Tl) and BGO scintillation crystals using a temperature gradient

A method for determining the depth of interaction in both NaI(Tl) and BGO crystals is investigated. This technique takes advantage of the relationship between temperature and decay time in a scintillation crystal. The depth of interaction is ascertained by measuring the decay time in a crystal with a uniform temperature gradient applied along its depth. The application of determining the depth of interaction in positron emission tomography scanners is discussed.     

MgWO4–A new crystal scintillator

Magnesium tungstate (MgWO₄) crystals of ~1 cm³ volume were obtained for the first time using a flux growth technique. The crystal was subjected to comprehensive characterisation that included room-temperature measurements of the transmittance, X-ray luminescence spectra, afterglow under X-ray excitation, relative photoelectron output, energy resolution, non-proportionality of scintillation response to γ-quanta, response to α-particles, and pulse shape for γ-quanta and α-particles. The light output and decay kinetics of MgWO₄ were studied over the temperature range 7–305 K. Under X-ray excitation the crystal exhibits an intense luminescence band peaking at a wavelength of 470 nm; the intensity of afterglow after 20 ms is 0.035%. An energy resolution of 9.1% for 662 keV γ-quanta of ¹³⁷Cs was measured with a small (≈0.9 g) sample of the MgWO₄ crystal. The photoelectron output of the MgWO₄ crystal scintillator is 35% that of CdWO₄ and 27% that of NaI(Tl). The detector showed pulse-shape discrimination ability in measurements with α-particles and γ-quanta, which enabled us to assess the radioactive contamination of the scintillator. The results of these studies demonstrate the prospect of this material for a variety of scintillation applications, including rare event searches.