Scintillation detectors
Scintillation detectors
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DOI：
10.1080/10448632.2012.725331
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发表时间：
2012-11
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通讯作者：
M. Dalton
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作者：
M. Dalton
第23卷·第4期· 2012中子新闻26引言闪烁探测器在许多弹性中子散射仪器中得到了非常有效的应用。然而，用于非弹性中子散射（INS）的大面积探测器阵列几乎完全基于3 He比例探测器阵列。这些探测器具有非常低的伽马灵敏度和固有的探测器背景，确保良好的信号/噪声;因此，它们非常适合INS实验，其中非弹性散射信号比弹性散射信号弱得多。在缺乏价格实惠的3 He的情况下，闪烁探测器被认为是开发用于INS的大面积探测器的候选者。（中子探测器开发国际合作组织，[1]）闪烁探测器的波长漂移（WLS）与ZnS/6LiF或ZnS/B2 O3闪烁探测器一致与光电倍增管耦合的光纤是目前最有前途的发展选择。选择ZnS闪烁体是因为它是一种非常明亮的闪烁体，每个吸收的中子发射约150，000个光子，同时脉冲形状辨别可用于区分中子和伽马事件。选择明亮的闪烁体是重要的，因为使用任何形式的光纤耦合导致从闪烁体发射的光的小于1%被传输到PMT。良好的脉冲波形鉴别能力使探测器能够实现极低的伽马灵敏度。ZnS：Ag/6LiF闪烁体是可商购的，但具有若干缺点。它对自己的光线是不透明的，这限制了可以有效使用的屏幕厚度。此外，存在与中子刺激相关的显著余辉，其可持续超过100 μs，这限制了探测器的速率能力。在开发基于这种闪烁体的任何新探测器时，需要仔细考虑这些特性。特别是，全面优化的电子硬件和信号处理对于获得最佳性能至关重要。光纤耦合已被选为INS探测器的耦合介质，因为考虑到所需的大面积探测器，该技术具有更高的成本效益。透明纤维技术在实现各种探测器和像素形状和尺寸的设计方面非常成功。然而，读出给定探测器区域所需的WLS光纤的数量远小于清晰光纤读出所需的数量。例如，对于SNS橡树岭的POWGEN探测器（参见第28页），读取该探测器所需的WLS光纤数量比覆盖相同区域所需的相应透明光纤数量少约60倍。这意味着组装成本的显著降低。因此，WLS光纤已被选定为读出方法。真空光电倍增管是一种成熟的技术，用于将光子转换为具有高增益和极低噪声的电子，[2，3]。它们有各种形状和尺寸可供选择，包括单阳极PMT和多阳极（MA）PMT。在这些MA PMT中，许多小PMT像素被封装在相同的包络中，并且共享相同的HT和分压器网络。像素的数量通常包括每PMT 4、16、64或256个像素。MA PMT在需要大量PMT通道并且每个PMT的光阴极面积相对小~4-36 mm 2的情况下是特别有利的。单阴极光电倍增管对于较大的光电阴极面积变得有用。在500-1000 mm 2范围内的面积是容易获得的。SiPMT非常紧凑，与真空PMT不同，它可以在没有额外磁屏蔽的情况下在强磁场中工作。在中子散射探测器的要求方面，SiPMT遭受大的单光子噪声和小的光阴极面积，这排除了它们直接用于大面积探测器，其中光阴极尺寸和低固有噪声是关键的。如果来自闪烁体的光输出足够高并且可以在设备中收集七个或更多个光电子，则可以实际上消除光子噪声。随着工业上对这些器件的不断开发和特性的不断改进，SiPMT在中子散射探测器应用中的应用可能会越来越多。在ICND合作[1]中，五个设施正在开发用于闪烁探测器的ZnS闪烁体WLS光纤探测器
Volume 23 • Number 4 • 2012 Neutron News 26 Introduction Scintillation detectors have been used very effectively in many elastic neutron scattering instruments. However, the large area detector arrays for inelastic neutron scattering (INS) have almost exclusively been based on arrays of 3He proportional detectors. These detectors have very low gamma-sensitivity and intrinsic detector background ensuring excellent signal/noise; therefore they are well suited to INS experiments where the inelastic scattering signals are much weaker than those arising from elastic scattering. In the absence of readily available 3He at an affordable price, scintillation detectors are regarded as candidates for the development of large area detectors for INS. Among the possible scintillation detectors, the ICND (International Collaboration for the Development of Neutron Detectors, [1]) scintillation detector workgroup agreed that ZnS/6LiF or ZnS/B2O3 scintillators read out with wavelength shifting (WLS) fi bre coupled to photomultiplier tubes (PMTs) was the most promising option for development at the present time. The ZnS scintillator was selected since it is a very bright scintillator with an emission of ~150,000 photons per absorbed neutron, whilst pulse shape discrimination can be used to distinguish neutron and gamma events. It is important that a bright scintillator is selected since the use of any form of fi bre coupling results in less than 1% of the light emitted from the scintillator being transmitted to the PMTs. Good pulse shape discrimination enables detectors with very low gamma-sensitivity to be realised. ZnS:Ag/6LiF scintillator is commercially available, but has several disadvantages. It is opaque to its own light which limits the thickness of screen that can be used effectively. In addition, there is a signifi cant afterglow associated with neutron stimulation which can last for more than 100 μs and this limits the rate capability of the detector. These features need to be carefully considered when developing any new detector based on this scintillator. In particular, fully optimised electronics hardware and signal processing are crucial in extracting best performance. Fibre coupling has been selected as the coupling medium for detectors for INS since this technology is signifi cantly more cost effective in view of the large area detectors required. Clear fi bre technology is very successful in enabling the design of a variety of detector and pixel shapes and sizes to be realised. However, the number of WLS fi bres required to readout a given detector area is much less than that for clear fi bre readout. For example, for the POWGEN detector at SNS Oak Ridge, see later on page 28, the number of WLS fi bres required to read out this detector is ~60 times less than the corresponding number of clear fi bres that would be required to cover the same area. This represents a signifi cant reduction in the cost of assembly. Thus WLS fi bre has been selected as the readout method. Vacuum PMTs are a mature technology for converting photons to electrons with high gain and very low noise, [2, 3]. They are available in a wide variety of shapes and sizes and include single anode PMTs and multi anode (MA) PMTs. In these MA PMTs a number of small PMT pixels are enclosed in the same envelope and share the same HT and voltage divider network. The number of pixels typically includes 4, 16, 64 or 256 pixels per PMT. MA PMTs are particularly advantageous where a high number of PMT channels is required and the photocathode area for each PMT is relatively small ~4–36 mm2. Single cathode PMTs become useful for larger photocathode areas. Areas in the region of 500–1000 mm2 are readily available. SiPMTs are very compact and unlike their vacuum PMT counterparts, can operate in strong magnetic fi elds without additional magnetic shielding. In terms of neutron scattering detector requirements, SiPMTs suffer from a large single photon noise and a small photocathode area and this precludes their immediate use for large area detectors where photocathode size and low intrinsic noise are critical. If the light output from the scintillator is suffi ciently high and seven or more photoelectrons can be collected in the devices, photon noise can be virtually eliminated. As industry continues to develop these devices and the characteristics continue to improve, it is likely that SiPMTs will fi nd increasing applications in neutron scattering detector applications. Within the ICND collaboration [1], fi ve facilities are developing ZnS scintillator WLS fi bre detectors for Scintillation detectors