Development of a low-energy high resolution X-ray camera for high-energy gamma photon background environments

Although a high-energy gamma camera can obtain images of ¹³⁷Cs distribution by detecting the 662-keV gamma photons, its spatial resolution is reduced because high-energy gamma photons penetrate the edge of the pinhole collimator. To solve this problem, we developed a low-energy X-ray camera that detects the characteristic X-ray photons (32–37 keV) that are emitted from ¹³⁷Cs to obtain high resolution images. We used a 45 × 45 × 1-mm-thick NaI(Tl) scintillator that was encapsulated in 0.1-mm-thick aluminum and optically coupled to a 2-inch square, position sensitive photomultiplier tube (Hamamatsu Photonics, PSPMT:H12700 MOD) as an imaging detector. The imaging detector was encased in a 2-cm-thick tungsten alloy container and a pinhole collimator was attached to its camera head. The spatial resolution and sensitivity were ～5 mm full-width at half-maximum and ～0.6 cps/MBq for the 1.5-mm pinhole collimator 10 cm from the collimator surface, respectively. We administered 5 MBq of ¹³⁷Cs to a soybean seedling, imaged the distribution of radionuclides for six hours, and successfully obtained a high resolution image of it with our developed X-ray camera. We believe our camera will be a powerful tool for such ¹³⁷Cs imaging in plants.     

Development of a Double Layered Scintillator for Separating and Detecting Low Energy Protons and Electrons

A scintillator system consisting of a thin (5,000 ? - 15,000 ?) CsI(Tl) layer evaporated onto a plastic scintillator (NE-102) has been developed for the purpose of distinguishing low energy protons from electrons and measuring the energy of each species. Evaporations in a high vacuum (10-8 Torr) produced layers of CsI(Tl) that scintillate with an efficiency comparable to optimally doped bulk material, If the CsI(Tl) layer thickness is 15,000 ?, it stops protons with energies below 170 keV and electrons with energies below 18 keV. Thus, protons with energies between about 25 and 250 keV can be distinguished from electrons with energies above 18 keV by examining the shape of the light pulse generated in the dual scintillator. Results obtained with protons and electron beams will be presented.     

A new scintillator structure for thermal neutron imaging

We are developing a new neutron sensitive scintillator structure designed for incorporation into digital imaging systems. Due to its internal structure and high scintillation efficiency, this new scintillator can simultaneously provide a high spatial resolution, high detection efficiency and excellent signal to noise performance. The scintillator screen consists of a microcolumnar CsI(Tl) film sandwiched between two neutron converter layers of transparent Gd-containing compounds. This paper discusses the scintillator design and characterization. Also, thermal neutron imaging performance evaluated by integrating prototype screens into a CCD camera will be outlined to demonstrate its applicability for such important applications as contraband and threat material detection.     

Hard X-Ray Images Created by a Series of Harmonically Related Modulation Collinators

A series of images of hard (30 - 300keV) X-ray sources have been created by use of a multi-pitch rotating modulation collimator (RMC) and a striped NaI(Tl)/CsI(Tl) scintillator detector arrangement. The performance of the system with respect to a series of point and extended objects has been analysed. The implications with respect to space instrumentation are discussed.     

Large-area APDs and monolithic APD arrays

In this paper, development of large-area planar avalanche photodiodes (APDs) and monolithic APD arrays for X-ray and scintillation detection is discussed. Single APDs with areas as large as 10 cm² have been fabricated and tested with a CsI(Tl) scintillator (3.8 cm diameter, 2.5 cm height). The resolution of the 662 keV photopeak has been measured to be 9% (FWHM). The X-ray detection performance, gain, and noise of these large APDs have been characterized. Multielement APD arrays have also been fabricated in various formats, such as 4×4 to 14×14 elements (2 mm pixels), and the uniformity of gain, noise, and sensitivity has been evaluated for 4×4 arrays using an ⁵⁵Fe source. Timing properties have been measured. Packaging issues related to the APD arrays are discussed     

Scintillation properties of RbGd2Br7:Ce.Advantages and limitations

The scintillation properties of RbGd₂Br₇ crystals, doped with Ce³⁺ concentrations of 0.02, 0.11, 0.88, 2.05, 4.1, and 9.8%, are studied under X-ray and γ-quanta excitations. For the RbGd₂Br₇ sample doped with 9.8% Ce, the authors measured a light yield of 56000±6000 photons per MeV of absorbed γ-ray energy with a main decay time of 43±1 ns, using a Hamamatsu R1791 photomultiplier (PMT), a ¹³⁷Cs radioactive source, and a shaping time of 10 μs. A time resolution of 790±10 ps was measured for the RbGd₂Br₇:9.8% Ce compound, using BaF₂ as second scintillator, two XP2020Q PMTs, a ²²Na source, and an energy threshold set at E⩾511 keV. With the R1791 PMT, an energy resolution of 4.1% (FWHM over peak position) for the 662-keV full absorption peak has been observed for two crystals of 7×4×2 mm³ and 15×5×1 mm³ with 4.1 and 9.8% Ce content, respectively. Moreover, the nonproportional responses of three RbGd₂Br₇:Ce compounds with different concentrations (0.11, 2.05, and 9.8%) were studied revealing an almost-constant light output response from 17.4 keV to 1 MeV. These properties are compared to three other well-known scintillators: NaI:Tl, CsI:Tl, and Lu₂SiO₅:Ce     

Well-type phoswich counter for low-flux X-ray/γ-ray detection

Phoswich counters that can detect low-flux hard-X-ray/γ-ray from localized sources are described. The counter consists of a small inorganic scintillator with a fast decay time (the detection part) glued to the interior bottom surface of a well-shaped block of another inorganic scintillator with a slow decay time (the shielding part). The first model counter was built by using a new scintillator, gadolinium silicate (GSO), in the decision part and CsI(Tl) in the shielding part. A detector system consisting of 64 such phoswich counters (total area ~740 cm²) was flown onboard a balloon, setting a limit to the ⁵⁷Co line flux from SN 1987A, at ~10^-4 cm^-2 s The sensitivity for continuum flux was around a few ×10 ^-6 cm^-2 s^-1 keV^-1 between 100 and 200 keV. In the second model, CsI(Tl) was replaced by bismuth germanate (BGO), and radioactive contaminations in GSO were reduced. Improvements are planned to reach a sensitivity (3σ) around 5×10^-7 cm^-1 s^-1 keV^-1 for the continuum     

A high position resolution X-ray detector: an "Edge on" illuminated capillary plate combined with a gas amplification structure

We have developed and successfully tested a prototype of a new high position resolution hybrid X-ray detector. It contains a thin-wall lead glass capillary plate converter of X-rays combined with a microgap parallel-plate avalanche chamber operating in various gas mixtures at 1 atm. The operation of these converters was studied in a wide range of X-ray energies (from 6 to 60 keV) at incident angles varying from 0-90/spl deg/. The detection efficiency, depending on the geometry, photon's energy, incident angle and the mode of operation was between 5-30% in single step mode and up to 50% in a multi-layered combination. Depending on the capillary's geometry, the position resolution achieved was between 50-250 /spl mu/m in digital form and was practically independent of the photon's energy or gas mixture. The usual lead glass capillary plates operated without noticeable charging up effects at counting rates of 50 Hz/mm/sup 2/ and hydrogen treated capillaries-up to 10/sup 5/ Hz/mm/sup 2/. The developed detector has several important potential advantages over the exciting X-ray detectors and may open new possibilities for medical imaging, for example in mammography, portal imaging, radiography (including security devices), as well as many other applications.     

一种新型溴化铈闪烁体探测器性能研究

闪烁探测器在射线能量与时间的探测中发挥着重要作用。针对国产新型溴化铈（CeBr₃）闪烁晶体，本文建立实验系统和方法对其关键性能指标进行了测试，具体包括：建立了快符合时间分辨系统测量装置，测试得到新型闪烁探测器对于⁶⁰Co源的时间分辨率为142 ps；建立了单光子计数法测量装置，测试得到溴化铈晶体的衰减时间为16.8 ns，上升时间为0.18 ns；建立了相对光输出测量装置，测试得到其相对于同规格NaI(Tl)闪烁体探测器对于661.66 keV射线的光输出为161%；同时针对其能量响应性能进行了测试，测得对于661.66 keV射线其能量分辨率为4.7%。将国产新型溴化铈闪烁晶体的性能测试结果与国际上同类晶体以及氟化钡、溴化镧、碘化钠等常用无机闪烁体的性能进行了比较，对其应用前景进行了探讨。     

Phantom experiments on a PSAPD-based compact gamma camera with submillimeter spatial resolution for small animal SPECT

We demonstrate a position sensitive avalanche photodiode (PSAPD) based compact gamma camera for the application of small animal single photon emission computed tomography (SPECT). The silicon PSAPD with a two-dimensional resistive layer and four readout channels is implemented as a gamma ray detector to record the energy and position of radiation events from a radionuclide source. A 2 mm thick monolithic CsI:Tl scintillator is optically coupled to a PSAPD with a 8mm×8mm active area, providing submillimeter intrinsic spatial resolution, high energy resolution (16% full-width half maximum at 140 keV) and high gain. A mouse heart phantom filled with an aqueous solution of 370 MBq (99m)Tc-pertechnetate (140 keV) was imaged using the PSAPD detector module and a tungsten knife-edge pinhole collimator with a 0.5 mm diameter aperture. The PSAPD detector module was cooled with cold nitrogen gas to suppress dark current shot noise. For each projection image of the mouse heart phantom, a rotated diagonal readout algorithm was used to calculate the position of radiation events and correct for pincushion distortion. The reconstructed image of the mouse heart phantom demonstrated reproducible image quality with submillimeter spatial resolution (0.7 mm), showing the feasibility of using the compact PSAPD-based gamma camera for a small animal SPECT system.     

用于~(16)C核集团结构实验研究的望远镜探测器阵列性能研究

对16C核集团结构实验研究中使用的望远镜探测器阵列的性能进行了研究,此望远镜阵列主要用于靶后反应产物的探测及粒子鉴别。探测器阵列由9组望远镜组成,其中包括了硅微条、位置灵敏硅探测器(PSD)、大面积硅探测器(SSD)和CsI探测器单元在内的多种类型的探测器。用241Amα源对组成望远镜的各探测器进行了测试和优化,且在中国科学院近代物理研究所的重离子加速器放射性次级束流线(RIBLL)上进行了在束测试。结果表明,研制的望远镜探测器阵列具有较好的能量分辨、高的角分辨及粒子鉴别能力,可满足通过碎裂反应来研究16C等原子核的集团结构的需要。     

X-ray CCD相机在杆箍缩二极管X射线焦斑诊断中的应用

针孔成像法是诊断杆箍缩二极管X射线焦斑的常用方法。本文建立基于增感屏、光锥耦合、CCD相机的X-ray CCD相机系统,取代针孔成像法中基于闪烁体、物镜、CCD相机的图像获取系统,提高了诊断系统的紧凑性。对所建立的X-ray CCD相机系统的空间分辨能力进行了测试,系统的空间分辨能力受增感屏限制,使用铅制分辨卡测得系统的空间分辨率为5lp/mm,使用刀口法测得调制传递函数为0.5时的频率为1.5lp/mm。测试结果表明,在针孔成像倍率为0.5时,可满足1.5 mm左右的X射线焦斑诊断的需要。并开展了杆箍缩二极管侧面焦斑诊断实验,获得了侧面焦斑图像,且进行了图像复原处理。     

Readout of the Optical PET (OPET) Detector

The design of an imaging system capable of detecting both high-energy γ-rays and optical wavelength photons is underway at the Crump Institute for Molecular Imaging. This system will noninvasively image small animal models in vivo for the presence of positron emission tomographic (PET) and optical signals. The detector will consist of modules of multichannel photomultiplier tubes (MC-PMT) coupled to arrays of scintillator crystals. The MC-PMT will detect both the photons produced due to bioluminescence and the photons generated by the interaction of γ-rays within the crystals. The long wavelength photons produced through bioluminescence are only slightly attenuated by these crystals and are detected directly at the photocathode of the MC-PMT, resulting in signals of small (5-10 mV) short (~15 ns) pulses. In contrast, annihilation (511 keV) γ-rays interacting in the scintillator crystal send large bursts of photons to the PMT, and result in pulses that can be as large as 500 mV and > 200 ns duration. The processing of pulses with such different characteristics in a single circuit requires significant alteration of the standard pulse processing circuitry used in PET scanners. In this paper, we discuss the requirements of such a circuit and show the results of implementation of one design using single and multiple channel PMTs.     

Scintillation Characteristics of Thin NaI(Tl) and CsI (Tl) Layers Fabricated by Vacuum Deposition

The scintillation characteristics of thin vacuum-deposited layers of NaI (Tl) and CsI (Tl) were investigated. Scintillation layers with thicknesses ranging from 0.2 to 20 mg/cm2 were evaluated as detectors with 6- and 22-keV x rays. In this energy region they were found to be comparable in performance to the thicker, commercially available NaI (Tl) and CsI (Tl) scintillation crystals. Fabrication methods, scintillator evaluation techniques, and applications in selective low-energy x-ray detection will be reported.     

Response of TlCl(I,Be) Crystals to Energetic Ionizing Radiation

Studies of scintillation TlCl(I,Be) crystals have been conducted using 8-GeV negative pions and 50- to 130-MeV positrons. Pulse shapes and resolutions of these crystals have been measured. Pulses from TlCl(I,Be) are observed to have a fast rise time (?2 nsec) and a complicated decay scheme consisting of at least three separate components. Attempts were also made to observe the Cerenkov radiation from these crystals due to highly relativistic particles. The resolution of the pulse height spectrum for a 2.9-cm diameter by 3.8-cm length TlCl(I,Be) scintillator was measured for 8-GeV negative pions. For comparison, the resolution of the same size CsI(Tl) scintillator was measured using the same experimental configuration. The resolution of CsI(Tl) is only slightly greater than theoretical prediction. The resolution of TlCl(I,Be) is greater, being limited by photon statistics even for 35 MeV of deposited energy. The resolution of a 7.6-cm diameter by 7.6-cm length TlCl(I,Be) crystal used as a Total Absorption Shower Counter for 50-130 MeV positrons has also been measured. This resolution is in reasonable agreement with previously reported data.     

CSR外靶实验终端中CsI（Tl）晶体的测试

为测量重离子加速器冷却储存环（HIRFL-CSR）的外靶实验终端上不同能量的γ射线,一种用于探测γ射线的高能量分辨的探测装置正在中国科学院近代物理研究所建设,该探测器由中国科学院近代物理研究所自行生长的铊激活的碘化铯CsI（Tl）晶体组成。与日本Hamamatsu公司生产的S8664-1010型雪崩光二极管（APD）耦合,测试其光输出的非均匀性和能量分辨,从测试结果给出了所需CsI（Tl）晶体合格的标准。目前已完成该γ探测球计划的六分之一,所提供的晶体合格率达94%以上。     

惰性气体β-γ符合测量系统探测器能量及分辨率刻度

β-γ符合法是全面禁止核试验条约(CTBT)放射性核素核查中惰性气体氙测量的一种重要方法,探测器能量及分辨率刻度是其首要解决的关键技术。本工作详细介绍了β-γ符合测量系统NaI(Tl)闪烁体和塑料闪烁体探测器能量及分辨率刻度的方法和结果,采用γ放射性核素点源刻度NaI(Tl)γ射线能量及分辨率,利用137Cs661.66keVγ射线康普顿散射电子刻度塑料闪烁体β射线能量及分辨率,并与131Xem内转换电子刻度的β射线能量分辨率结果进行了比较。结果表明:用137Cs康普顿散射电子刻度塑料闪烁体β射线能量是一种简便可行的方法,但用其刻度的β射线分辨率比实际的大。     

球载大面积Phoswich能量分辨率研究

对混合型球载硬Ｘ射线天文望远镜（ＨＡＰＩ－４）ＰｈｏｓｗｉｃｈＮａＩ（Ｔｌ）／ＣｓＩ（Ｎａ）探测器的能量分辨率进行了研究。实测表明,光电倍增管光阴极面响应的非均匀性与入射光谱有关,用蓝色滤光片所测非均匀性小于２０％,采用对晶体窗玻璃进行罩膜处理,改变荧光收集条件,以牺牲幅度换取均匀性能量分辨率的改善效果是十分明显的,可将Ｐｈｏｓｗｉｃｈ探测器能量分辨从４５％（６０ｋｅＶ）提高到２４％,此外,对几种     

Soft X-ray Imaging using a Neon Filled Plasma Focus X-ray Source

A 3 kJ Mather-type UNU/ICTP plasma focus device with neon filling is used, for the first time, as a soft X-ray source for imaging of thin biological samples including insects. A charge-coupled-device (CCD) based pinhole projection system, placed in a differentially pumped chamber, is used for radiography using neon soft X-rays. The image brightness, contrast and resolution have been optimized by varying soft X-ray yield, pinhole size, camera chamber length and X-ray filters. The system can simply be modified for table-top soft X-ray microscopy of thin biological samples.     

NaI:TI与LaBr_3:Ce~(3+)闪烁体探测器能量分辨的GEANT4模拟

闪烁体探测器γ能谱测量中,闪烁光子的产生、传输及光电器件收集、倍增均对能谱的分辨有巨大影响。为了精确模拟闪烁体探测器的能量分辨,采用蒙特卡洛模拟软件GEANT4建立了一套模型,分别对Na I(TI),LaBr_3:0.5%Ce~(3+)闪烁体探测器测量能量为662 ke V的γ射线能谱进行模拟,获得的结果与实验测试结果能够很好的符合,验证了建立模型合理性与可靠性,为闪烁体的设计提供了一套更精确的开发工具。