新型无机闪烁体的能量分辨率

1、成都理工大学学生毕业设计（论文）外文译文学生姓名：刘勇学号：2006xxxxxxxx专业名称：核工程与核技术译文标题（中英文）：新型无机闪烁体的能量分辨率（New inorganic scintillatorsaspects ofenergy resolution）译文出处：荷兰代尔夫特理工大学，国际共和研究所，核辐射集团指导教师审阅签名：曾国强译文正文：新型无机闪烁体的能量分辨率摘要：通过对Y射线探测器的能量分辨率的讨论。实验表明，该能量分辨率是可以显著改善的，对于新的闪烁体LaCl3（Ce），在能量为662Kev是的能量分辨率为3%。科学家B.V.保留所有 权。PACS 系统：07.85.

2、Nc; 78.55.Hx; 78.90 +t关键词：无机闪烁；LaC（Ce），能量分辨率，伽玛射线探测器介绍：3无机闪烁体被广泛应用于伽玛射线检测。探测器的选择主要根据有关探测要求的基础 而定。例如：效率，精力和时间分辨率，死时间，位置分辨，增长的可能性较大的晶体，品体 质量（辐射硬度，力学性能等）和成本。例如见文件在1-4。在许多情况下，能量分辨率是最重要的。然而，通常的一个半导体探测器，例如：Ge， 只是基于对无机闪烁探测器的应用。我们处理这个问题，是否可以提高无机闪烁探测器的探测 的能量分辨率，因此，在闪烁体的适用性可以拓展，我们将讨论研究一种新的闪烁体。无机闪烁体的基础要素：在制定有效

3、的伽玛射线检测新闪烁体，我们选择的材料一般都具有较高的密度R和高原 子序数。此外，该材料闪烁光传输率该较高，因此，我们依靠离子晶体或者某种共价品体。但 与导带和化合价之间的禁止能量，E大到足够可以传输。另一方面，良好的能量，时间和位置分辨率，我们需要大量的闪烁光Nph （相对变异数81/Nph），禁差距竟可能的小。、=（E/EQ SQ（1）在右边第项代表的生成电子空穴对的数目Neh吸收伽玛射线所产生的能量。平均生成个电 子空穴对所需要的能量为：Eeh2.5Egapo S是闪烁体中心的能量发光转化效率。Q是闪烁体受 激发后的光子发光效率。在这三个阶段S是至少可以预测的。目前这在很大程度依赖于闪烁

4、体 的缺陷，不同于闪烁体中心，可以同时俘获电子或者电子空穴。这些缺陷可能来自晶体本身结 品时侯的，也可能来自某些杂质的原因。接下来我们考虑发光中心我们将只讨论了镧系金属离子Ce3+，该离子在第四层有一个壳 层电子，受激发吼跃迁到底五层。随后的退激将发生在第4层和第5层之间的电偶极子，伴随的 衰减时间tN30ns。作为前提说明和必要条件的描述，该Ce3+必须是一种合乎规格的材料来融入闪烁体中心。在大多数情况下，发射光谱与光传感器的灵敏度曲线拟合当好。我们观察到每 Mev能量的伽马射线的光子产量2000。现在我们来讨论能量分辨率。分辨率是：R= AE (FWHM) /E在一个光电峰能量为E的伽马脉

5、冲幅度谱可表示为：R2=Rsci2+Riid2+Rnise2( 2 )Rsci表示由于光源的光检测器不是理想的原因，即不服从泊松分布的统计数据时对光的贡献。 由于材料的不均匀性，在伽马射线的吸收和光子数量的收集依赖于伽马射线的入射位置和闪烁 光检测器耦合的不完善，所以并没有跟入射伽马射线的能量成正比的响应(不相称的响应)。 Rlid表示在理想的光源检测机制和光源检测器下的结果，后者的理想偏差也包括在Rsic里面， 这个我们以后再讨论。Rnise表示电子噪声。理想的闪烁体通过理想的光电倍增管(PMT)可以完全的传输光子。所以Rsci=Rnise=0 因此R2=Rlid2假设经Y-射线吸收(a)

6、Nph的闪烁光子生产和到达的光电倍增管阴极，(b)光 电子是后来n Nph，(c)这些=n Nph电子在第一倍增极和到达(d)倍增极的k (k = 1, 2.) 放大后为。k并且我们假设 j 2= 3= k=的，并且 /产1的。我们可以 得出：R2=Rlid2=5.56 /n ( -1) 5.56/、(3)Nel表示第一次到达光电倍增管的数目。在试验中， 110 2= 3= k，因此，在实际情 况下，我们可以通过(3)看出R2的值比实际测得大。请注意，对于一个半导体二极管(不倍增极结构)(3)也适用。那么Nel就是是在二极管产 生电子空穴对的数目。在物质不均匀，光收集不完整，不相称和偏差的影响

7、从光电子生产过程中的二项式分布及电子收集在第一倍增极不理想的情况下，例如由于阴极不均匀性和不 完善的重点，我们有：|R2=Rsci2+Riid2a5.56(v N-1/Nei)+1/Nei(4)v N光子的产生包括所有非理想情况下的收集和1/Nel的理想情况。为了说明，我们在图上显示，如图1所示。 E/E的作为伽玛射线能量E的函数，为碘 化钠：铊闪烁耦合到光电倍增管1。20 4U 1002(K)400 tOOOE （keV）图。1。对AE/E的示意图（全曲线）作为伽玛射线能量E功能的碘化钠：铊晶体耦合到光电倍增管。虚 线/虚线代表了主要贡献。例如见9,10。对于Rsci除了 1/（Nel）i/

8、2的组成部分，我们看到有两个组成部分，代表在0-4% 的不均匀性，不完整的光收集水平线，等等，并与在0-400代表非相称keV的最大曲线。表 1给出了 E=662Kev时的数值（137Cs） 在传统的闪烁体资料可见。从图一我们可以清楚的看到在低能量EV100Kev，如果Nel，也就是N h增大的话，是 可以提高能量分辨率的。这是很难达到的，因为光额产量已经很高了（见表1）在能量E 300Kev时，Rsci主要由能量支配其能量分辨率，这是没办法减小Rsci的。然而，在下一节我 们将会讲到，可以用闪烁体在高能量一样有高的分辨率。3。新的闪烁和能量分辨率在表1中显示的是能量为662Kev是的光电峰的

9、分辨率，在代尔夫特理工大学和伯尔尼大学的 合作下开发的传统闪烁探测器探测应用新的闪烁体记录下来的数据，在第1列表示Ce掺杂浓 度为mol%。第二列给出N，即每兆电子伏特，产生的光子数。第三列给出Nel，电子的或电 子空穴在每吸收662 keV的伽马射线探测器产生的光量子对数。从对时间的积分看所示的数据， 第四列给出了 662Kev照片峰实验R值。Rlid的计算是通过Nel，包括一个忽略倍增极统计5% 的盘整（第五列）。为准则和APD的它代表了探测器（过量）和电子噪音，光电倍增管的Rnoise 被认为是可以忽略的。从4-6列的值Rsci计算公式是（2）。Table 1EriGTgy lesulu

10、tiun data at 662keV foi sume old and new scintillatuis; fui d-sfinitiurLS s睥 te?:tCrystalN 103/MeV网直蜘keVJi%玲孩Size (mm )Light d-stectuiRef.NaI:Tl4060006.73.209PMTtypicalCsI:Tl6560006.63工05.SdisLini x 7.5PMT XP2254BPhilips17CsI :T16526,0004.31.53.SdiairL2.& x 5SDD风YAlQj: Ce21iyoo4.32.32.62.53 x-3 x 10

11、APD 6307073500 AdvPhutlTLG/RbGd3Bi7:56SSOO4.12.603.2PMT R1791Hamamatsu1gLad3：0.57%.Ce40600073.206.2PMT R1791HamamatsuLaa3: 10%Ce4973003.12.S01.4disiinE x 5PMT R1791HamamatsuIW6LaQ3: LOCe493.651.7L.85.2.64diam8 x 5APD 6307073510UAdvPhotlrLC表1表示的是在能量为662Kev的一些传统闪烁体的能量分辨率的数据，其定义见文中。正如第二节提及到的NaI（Tl）的结果符

12、合图1。这是他们这种材料的特点。Cs（Tl）也有 类似的特征，例如见17。使用硅探测器（SDD）探测出了一个很好的能量分辨率R=4.3%， SDD有比较高的光电倍增管其中从0到8%-16%的种类而定，所以从0到60%的闪烁体发光 （探测效率最高为565nm）。然而，这并不能解释R值变小，显然对于使用Cs（Tl）探测时 Rsci=3.8%，即远小于上述的晶体的值。另外一个好的结果是，最近公布了 YALO3（Tl）。采 用雪崩二极管（APD），R=4.3%。而且，R的值不能通过高量子效率来解释，在这种情况下， Rs技 2.5%。energy keV图。2。LaCl3 （Tl） 662千电子伏的脉冲

13、y射线光谱测定高度在（直径85平方毫米）耦合到光电倍增管（R1791,形成时 间为10毫秒）。SAUnoo在代尔夫特，伯尔尼的方案中，我们选择Ce掺杂闪烁的要求，并在第1和第2条所述原 则的基础材料。我们专注于卤化物，特别是漠化物和氯化物，目标针对探测效率高于或等于 NaI（Tl）的，至少相当于光子产量产量，更快速的反应和更好的能量分辨率。我们最新介绍 的闪烁体RbGd2Br7（Ce）12.13。我们得到的R=4.1%。通过式2我们计算得出Rsci=3.2% 这个R和R .的值明显比NaI（Tl）的值小。RbGd2Br7（Ce）有了一个小部分的改善，使得 Rd有较高的光子产量。相对于NaI（T

14、l）和Cs（Tl）而言与光电倍增管的灵敏度曲线和闪烁发 光光谱更好的匹配，CsI的情况一样。另外一种新的闪烁体是LaCl3（Ce）。起初这种材料的性能并不乐观，与0.57%的Ce混 合后，具有较高的光子产额，使用光电倍增管可以读出，但是分辨率只有7%。但是，提高Ce 的掺杂度是可以得到更好的分辨率，如Ce10%时，R=3.1%。看表格2La（+Ce）KX射线的逃逸封以及脱离光峰。在这种情况下，Rsci=1.4%，也就是说这项闪烁体的贡献非常小。只要不 改变的Rsci价值与APD的读出应有的更大的Nel，并考虑到电子噪音，可以预算的分辨率V 2.9%。正如文章的第一节R=3.65%，表明，Rsc

15、i从1.4%增长到了 2.64%。这可能是由于APD 的入口处窗口不均匀响应。对于能读出YAlO3（Ce）较高的值的APD已经非常好了。4.结论：在上一节，我们了解到，由于LaCl3 （Ce）的良好分辨率我们可以观察到较小的Rsci （见表1）这部分可能是由非相称的影响小的贡献Rsci解释。这是RbGd2Br7 （Ce）在此情 况下观测到的迹象，在662Kev的光子产额（光子/兆电子伏特）是在0到5%和0，能量 50-1400Kev之间的幅度变化。同样对于YA1O3 (Ce)所占的比例还是比较小为0到7%。 对于NaI(T1)和NaI(T1)，却相反它的范围却是0到15%。对于LaC13(Ce

16、)不相称的是， 目前还没有测量，是无法预测的晶体类型显示的最小的非均衡影响。目前还不清楚什么影响晶体的光收集和不均匀的。LaC13容易潮解，我们是通过一个石 英晶体与光电倍增管耦合后才能用LaC13的测量，将这个晶体和光电被怎管一起耦合在一个防 潮的盒子里面。硅润滑脂(通用电气公司，粘度60,000cst)负责耦合和样品所用的聚四氟乙 烯覆盖。观察到的光子的产量和能量分辨率是一样的。Rsci值较小的原因，集中在第一倍增 极的影响是相当重要的，如阴极不均匀性和不完善。在LaC13(Ce)的情况下对APD的应用 也同样说明了。其结果比预期的结果要差。下一个步骤将是越来越多的大晶体。那是光电效应的不

17、匀称的影响将会更大。显然，我们需要更多关注的是闪烁探测器的优化和研 究。按照闪烁体对温度的依赖性研究可能要好一点，冷却的准则可能需要更好的结果，但前提 是闪烁的响应没有变得更糟。我们都应该牢记，相对较小的R值只能在较高的能量实现。(见图1)在能量V100Kev 他的本质是光产额的原因，至于产量方面见图5。更加值得注意的是在能量为662Kev时的能 量分辨率是3.1%，这相当于市场上可见的CdZnTe闪烁体。至于其他的性能，在LaCl3 (Tl) Ce含量为10%的情况下，在26nm内衰减发射出20000个光子，其中90%只在1ms内发射。 LaCl3 (Tl)的检测效率和NaI (Tl)差不多

18、，想了解更多信息(见16)。最后，通过上述的结论我们可以明显知道，在探测效率和能量分辨率的提高，我们可以 通过新的闪烁体来实现，这将比传统的闪烁体更好。特别是对于LaCl3 (Tl)而言，其他的材 料将继续发展下去。5.参考文献：南韦布，医疗影像，亚当希尔杰，布里斯托尔，1990年物理。页Dorenbos，C.W.E.Eijk，关于无机闪烁体及其应用，SCINT95，代尔夫特大学出版社国际 会议论文集，代尔夫特，荷兰，1996年。殷之文，FengXiqi，黎呸军，薛志林法律程序无机闪烁的国际会议及其应用，SCINT97，中 科院上海分行出版社，上海，中国，1997年。巴顿伯克斯的理论和实践的闪

19、烁计数，帕加马，纽约，1967年。页 Dorenbos，等。，IEEE 期刊。Nucl。科技。42 (6)(1995) 2190。JD 瓦伦丁节，等。，IEEE 期刊。Nucl。科技。45 (3)(1998) 512。卜蜂阿列等。，读出了镧(III)闪烁晶体的一大面积雪崩光电二极管，发表于2000年电 机及电子学工程师联合会高中，麦克风，里昂，法国，16-202000年10月。页 Dorenbos，等。，Nucl。Instr。和冰毒。乙 132 (1997) 728。澳吉洛-No.el，等。，IEEE 期刊。Nucl。科技。46 (5)(1999) 1274。澳吉洛-No.el，等。，JD 鲁

20、闽。85 (1999) 21。16 16 E.V.D.van Loef，等。，闪烁性能镧：Ce3 +的品体：快速，高效和高能量分辨率闪烁体，在2000年电机及电子学工程师联合会NSSMIC提出，法国里昂，10月16-20日2000。卜蜂阿列等。，IEEE 期刊。Nucl。科技。45 (3)(1998) 576。长菲奥里尼，楼佩蒂，Nucl。Instr。和冰毒。401 (1997) 104。米 Moszynski，等。，Nucl。Instr。和冰毒。一个 442 (2000) 230。电动汽车产品，产品目录。外文正文：.NewinorganicscintillatorsFaspectsofene

21、rgyresolutionCarel W.E. van Eijk* Radiation Technology Group, IRI, Delft University of Technology, Mekelweg 15, 2629 JB Delft, The NetherlandsAbstractEnergy-resolution of inorganic-scintillator gamma-ray detectors is discussed. Experiment shows that the resolution can be significantly improved. For

22、the new scintillator LaCl3 : Ce, an energy resolution of 3.1% was observed at 662 keV. r 2001 Elsevier Science B.V. All rights reserved. PACS: 07.85.Nc; 78.55.Hx; 78.90+tKeywords: Inorganic scintillators; LaCl3:Ce; Energy resolution; Gamma-ray detectorsIntroductionInorganic scintillators are widely

23、applied for gamma-ray detection. Detector selection occurs on the basis of requirements concerninge.g . efficiency,energy and time resolution, dead time, position resolution, the possibility to grow large crystals, crystal quality (radiation hardness, mechanical properties, etc.) and cost. E.g. see

24、papersin 1-4. In a number of cases energy resolution is allimportant. Then, in general a semi-conductor detector, e.g. Ge, is applied instead of a detector based on an inorganic scintillator. We address the question whether the energy resolution of an inorganic scintillator can be improved and, cons

25、equently, the scintillator applicability can be extended. Amongothers a new scintillator will be discussed.Inorganic-scintillator basicsIn developingnew scintillators for efficient gamma-ray detection, we select in general a material with a relatively high density r, and high atomic number Z. Furthe

26、rmore, the material should transmit scintillation light efficiently. Consequently we rely on ionic crystals or crystals with some degree of covalency, but with a forbiddengap energy between valence and conduction band, Egap, large enough to transmit the light. On the other hand, for good energy, tim

27、e and position resolution we need a large number of scintillation photons, Nph, (relative variancep1=Nph), and consequently the forbidden gap should be as small as possible as 5Nph= (E/Eeh)SQ(1)The first term on the right side represents the number of thermalized electron-hole pairs Ne2h produced in

28、 absorbing gamma-ray energy E. The average energy required to produce one thermalized electron-hole pair: Ee2hE2:5Egap. S is the transport/transfer efficiency of the e2h pair/ energy to the luminescence centre (LC) of the scintillator, and Q is the efficiency for photon emission once the LC is excit

29、ed (quantum efficiency). Of the three stages S is the least predictable. It depends very much on defects present in the scintillator, other than the LC, that may capture electrons or holes or both. These defects can arise from the interaction itself, from crystal growing, or be due to impurities 3,4

30、. Next we consider the luminescence centre. We will confine ourselves to the Ce3+ lanthanide ion 6,7. This ion has one electron in the 4f state which is lifted to the empty 5d shell upon excitation. Subsequent de-excitation will occur by an allowed 5d-4f electric dipole transition with a decay time

31、tX30 ns. The Ce3+ ion has to be incorporated as LC in a material with specifications as described in the previous paragraph and the requirement that it can act as a host for the 3+ ion. In most cases the emission spectrum matches the light sensor sensitivity curve rather well. Light yields 20,000 ph

32、otons per MeV of absorbed gamma-ray energy are observed 3,4,6. We now turn to energy resolution. The resolution, R DEeFWHMT=E, of a photopeak atenergy E in a gamma-ray pulse height spectrum can be expressed as, e.g. see 8:R2=R 2+R 2+R . 2(2)sci lid niseR2 sci represents the contributions from the sc

33、intillator due to the fact that it is not a perfect light source, i.e. at the light detector it does not deliver a number of photons obeyingPoi sson statistics due to material inhomogeneities, light collection dependence on the gamma-ray-absorption position and imperfect scintillator-light detector

34、coupling, and it does not have a response proportional to the gamma-ray energy (non-proportionality effect).R2 lid represents the light detection mechanism for a perfect light source and an ideal light detector. Deviations from ideality of the latter are usually also included in Rsci. We will come b

35、ack to this.R2 noise represents the electronic noise. For an ideal scintillator read out by an ideal photomultiplier tube (PMT) Rsci Rnoise 0 and (2) becomes R2 R2 lid. Assumingth at upon gamma-ray absorption (a) Nph scintillation photons are produced and arrive at the photocathode of the PMT, (b) s

36、ubsequently ZNph photoelectrons are produced in the photocathode, (c) of these aZNph electrons arrive at the first dynode and (d) at dynode k (k 1; 2;y) the amplification is dk, and furthermore assuming d1 d2 d3 dk d and d=d21E1, one can derive 8,9R2=Rlid2=5.568/8nNph (8-1) 5.56/Nel(3)with Nel the n

37、umber of photoelectrons arrivingat the first dynode. In practice d1E10 d2 d3 dk. Consequently under practical circumstances R2 is B10% larger than the value obtained from (3). Note that for a semiconductor diode (no dynode structure) (3) is also applicable. Then Nel is the number of electron-hole pa

38、irs produced in the diode.In the non-ideal case of material inhomogeneities, incomplete light collection, non-proportionality effects and deviations from the binomial distributions of the photoelectron-production process and the electron collection at the first dynode,e.g. due to photocathode inhomo

39、geneity and imperfect focusing, we have R2=Rsci2+Rlid25.56(vN-1/Nel)+1/Nel(4)with vN the variance of the light production includingthe effect of all the non-ideal processes and 1=Nph the variance in the ideal case. To illustrate the above we show in Fig. 1 schematically DE=E as a function of the1004

40、0AE/Eao(%)1041=020401(H)200400E (keV)Fig. 1. Schematic of DE=E (full curve) as a function of gammarayenergy E for a NaI :Tl crystal coupled to a PMT. The dotted/dashed lines represent the main contributions. gamma-ray energy E, for a NaI : Tl scintillator coupled to a PMT, e.g. see 9,10. In addition

41、 to the 1=ONel component we observe two componentsof Rsci, the horizontal line at B4% representing inhomogeneity, incomplete light collection, etc., and the curve with a maximum at B400 keV representingnon- proportionality. In Table 1 numerical values are presented at E 662 keV (137Cs). For informat

42、ion on energy resolution of more traditional scintillators e.g. see. It is clear from Fig. 1 that at low energies, Eo100 keV, significant energy-resolution improvement can only be obtained if Nel, i.e. Nph, is increased. This is not easy as light yields are already high (Table 1). At E 300 keV the R

43、sci components are dominatingthe energy resolution. There is no recipe to decrease Rsci. Yet, in the next section we show that scintillators can be found with a better energy resolution at the higher energies.New scintillators and energy resolutionIn Table 1 we show data relevant for the energy reso

44、lution of the 662 keV photopeak, recorded by means of a few traditional scintillators and some new scintillators developed in a collaboration of Delft University of Technology and University ofBern 11-16. In column 1 Ce-dopingcon centrations reindicated in mol%. The second column gives N, the light

45、yield in photons per MeV, the third column gives Nel, the number of electrons or electron-hole pairs produced in the light detectorper absorbed 662 keV gamma quantum. For integration times see the mentioned papers. The fourth column gives the experimental R values of the 662 keV photopeaks. Rlid is

46、calculated from Nel using(3), includinga 5% correction for neglecting dynode statistics (column 5). For PMTs Rnoise is considered to be negligible, for SDDs and APDs it represents the detector (excess) and electronic noise. From the values of columns 4-6 Rsci is calculated using(2).As already mentio

47、ned in Section 2, the NaI : Tl results correspond with Fig. 1. They are characteristic of this material. CsI : Tl, has a similar characteristic R-value. E.g. see 17. A very good energy resolution of R 4:3%, obtained usinga silicon drift detector (SDD), was reported in18. The SDD has an efficiency of

48、 B60% fordetection of the scintillation light (maximum at 565 nm) which is high compared to that of a PMT, B8-18%dependingon the type. Yet, this does not explain the small R-value. Apparently Rsci 3:8% for the used CsI : Tl crystal, i.e. much smaller thanfor the crystal of the row above. Another ver

49、y good result was recently reported for the scintillator YAlO3 : Ce. Usingan avalanche photodiodeTable LEmTgy lesulutiun data at 662 keV feu sume old and new scintillatuis; fui d-sfinitiurLS s睥 textCrystalN 103/MeV虬的皿胡职玲孩Size (mm )Light d-stectuiRef.NaI:Tl4060006.73.209PMTtypicalCsI:Tl6560006.63工05.

50、SdisLini x 7.5PMT XP2254BPhilips17CsI Tl6526,0004.31.53.SdiairL2.& x 5SDD风YAlQj: Ce21iyoo4.32.32.62.53 x-3 x 10APD 6307073500 AdvPhutlTLG/RbGd3Bi7: 9用.Ce56ssoo4.12.603.2PMT R1791Hamamatsu1gLad3：0.57%.Ce40600073.206.2PMT R1791HamamatsuLaa3: 10%Ce4973003.12.S01.4disiinE x 5PMT R1791HamamatsuLaQ3: LOCe

51、493.651.7L倍.2.64diam8 x 5APD 6307073510AdvPhotlTLCIHTable 1Energy resolution data at 662 keV for some old and new scintillators; for definitions see text (APD), R 4:3% 19. Again this R-value cannot be explained by the high quantum efficiency of B70%. In this case RsciE2:5%. In the Delft-Bern program

52、me we selected Cedoped scintillator materials based on requirements and principles mentioned in Sections 1 and 2. We focused on halides, in particular bromides and chlorides, aimingat detection efficiencies equal to or better than that of NaI : Tl, at least equal light yield, a faster response and a

53、 better energy resolution. Recently we introduced the new scintillator RbGd2Br7 :Ce 12,13. We obtained R 4:1%. Using(2) we calculate Rsci 3:2%. These values of R and Rsci are significantly smaller than the correspondingvalues obtained with a PMT for NaI : Tl and Cs : Tl. Part of the improvement is a

54、 consequence of the smaller Rlid due the high light yield of RbGd2Br7 :Ce compared to that of NaI : Tl and the better matchingof the scintillation-emission spectrum with the PMT sensitivity curve in comparison with the CsI : Tl case. Another new scintillator is LaCl3 : Ce. At first this material did

55、 not appear to be very promising.Doped with 0.57% Ce it has a high light yield butthe energy resolution is 7% using PMT readout 14. However, at a higher doping concentration the resolution improves dramatically, e.g. at 10%Ce R 3:1% 15,16. See Fig. 2. The La(+Ce) K X-ray escape peak is well separate

56、d from the photopeak. In this case Rsci 1:4%, i.e. the scintillator contribution is very small. Provided that the value of Rsci does not change, one would expect a resolution of o2.9% with APD readout due to the much larger Nel and takinginto account the electronic noise. As indicated in Table 1, R

57、3:65% 11. It appears that Rsci increased from 1.4% to 2.64%. This may be due to inhomogeneous response of the APD entrance window. For APD readout of YAlO3 :Ce a higher Rsci value was reported as well 19.SAUnooi 200180。-160014001200-LaCI :10%Ceo o o o o 8- . - 皿Do 4J2600800energy keVFig. 2. Pulse he

58、ight spectrum of 662 keV gamma rays detected in a LaCl3 : 10%Ce crystal (diam 8_5mm2) coupled to a PMT (R1791, shapingtime 10 ms).DiscussionIn the previous section we learned that the very good energy resolution observed for LaCl3 : Ce is due to the small Rsci value (see Table 1). Part of this may b

59、e explained by a small contribution of the non-proportionality effect to Rsci. An indication of this was observed in the case of RbGd2Br7 :Ce 13. The relative light yield (photons/MeV), normalized to that at 662 keV, is constant within B5% in the range B50 -400 keV. Also for YAlO3 :Ce a relatively s

60、mall nonproportionalitywas measured of B7% for the same energy range 9. For NaI : Tl and Cs : Tl, on the contrary, the spread is B15%. For LaCl3 : Ce the non-proportionality has yet to be measured. At present it is not possible to predict which type of crystals shows the smallest non-proportionality