Development and characterization of a modular acquisition system for a 4D PET block detector

Next generation PET scanners should fulfill very high requirements in terms of spatial, energy and timing resolution. Modern scanner performances are inherently limited by the use of standard photomultiplier tubes. The use of Silicon Photomultiplier (SiPM) matrices is proposed for the construction of a 4D PET module based on LSO continuous crystals, which is envisaged to replace the standard PET block detector. The expected spatial resolution of the module for the photon hit position is below 1 mm, and it will perform at the same time, the Depth Of Interaction (DOI) calculation and the Time Of Flight (TOF) measurement. The use of large area multi-pixel Silicon Photomultiplier (SiPM) detectors requires the development of a multichannel Digital Acquisition system (DAQ) as well as of a dedicated front-end in order not to degrade the intrinsic detector performances. We have developed a flexible and modular DAQ system for the read-out of two modules in time coincidence for Positron Emission Tomography (PET) applications. The DAQ system is based on a previously developed custom front-end ASIC chip (BASIC) which allows to read-out SiPM matrices preserving their spectroscopy and timing capabilities. Here we describe the acquisition system architecture and its characterization measurements.     

A prototype of very high resolution small animal PET scanner using silicon pad detectors

A very high-resolution small animal positron emission tomograph (PET), which can achieve sub-millimeter spatial resolution, is being developed using silicon pad detectors. The prototype PET for a single slice instrument consists of two 1 mm thick silicon pad detectors, each containing a 32×16 array of 1.4×1.4 mm pads readout with four VATAGP3 chips which have 128 channels low-noise self-triggering ASIC in each chip, coincidence units, a source turntable and tungsten slice collimator. The silicon detectors were located edgewise on opposite sides of a 4 cm field-of-view to maximize efficiency. Energy resolution is dominated by electronic noise, which is 0.98% (1.38 keV) FWHM at 140.5 keV. Coincidence timing resolution is 82.1 ns FWHM and coincidence efficiency was measured to be 1.04×10⁻³% from two silicon detectors with annihilation photons of ¹⁸F source. Image data were acquired and reconstructed using conventional 2-D filtered-back projection (FBP) and a maximum likelihood expectation maximization (ML-EM) method. Image resolution of approximately 1.45 mm FWHM is obtained from 1-D profile of 1.1 mm diameter ¹⁸F line source image. Even better resolution can be obtained with smaller detector element sizes. While many challenges remain in scaling up the instrument to useful efficiency including densely packed detectors and significantly improved timing resolution, performance of the test setup in terms of easily achieving sub-millimeter resolution is compelling.     

Coincidence timing of Schottky CdTe detectors for tomographic imaging

Schottky CdTe detectors are good candidates for large pixel array imagers. For use of such arrays in Positron Emission Tomography (PET) the coincidence timing of the detectors is very important, since it reduces the background from random coincidences. The coincidence timing of a small 2×2×1 mm³ detector has been measured as 11.6 ns, and thus is not much worse than the timing of BGO scintillators, commonly used in PET.     

近程光电视觉传感器感知物体空间信息的研究

本文论述了二种新型光电近程视觉传感器的工作原理、计算方法和系统构成。一种是由多个点光源和位置敏感器(PSD)构成的传感器,文中从误差分析、统计运算的角度出发推导出确定被测对象空间信息的算法;另一种是由多个点光源和单个探测元件构成的传感器,这种传感器通过对位相信息的测定来感知物体的三维信息。两种传感器都能得到被测点到传感器之间的距离信息及被测表面空间取向的信息。所论及的传感器可用于机器人对三维物表面轮廓的追踪运动以及提供物体形状的三维描述信息等方面。     

Thermal detection of particles using bolometric devices

Low temperature calorimeters can be used to measure the energy released by a single particle with very high resolution.We have developed a system for studying the behaviour of various kinds of calorimeters from the point of view of their thermoelectrical properties and also their characteristics as particle detectors. Preliminary experimental results of a composite detector are shown (the resolution is 3% for the 8.78 MeV α-line from ²²⁸Ra). Future developments both for optimizing the resolution and improving the system performances are briefly discussed.     

Energy and Timing Measurement with Time-Based Detector Readout for PET Applications: Principle and Validation with Discrete Circuit Components

A new signal processing method for PET application has been developed, with discrete circuit components to measure energy and timing of a gamma interaction based solely on digital timing processing without using an amplitude-to-digital convertor (ADC) or a constant fraction discriminator (CFD). A single channel discrete component time-based readout (TBR) circuit was implemented in a PC board. Initial circuit functionality and performance evaluations have been conducted. Accuracy and linearity of signal amplitude measurement were excellent, as measured with test pulses. The measured timing accuracy from test pulses reached to less than 300 ps, a value limited mainly by the timing jitter of the prototype electronics circuit. Both suitable energy and coincidence timing resolutions (~18% and ~1.0 ns) have been achieved with 3 × 3 × 20 mm(3) LYSO scintillator and photomultiplier tube-based detectors. With its relatively simple circuit and low cost, TBR is expected to be a suitable front-end signal readout electronics for compact PET or other radiation detectors requiring the reading of a large number of detector channels and demanding high performance for energy and timing measurement.     

红外探测器光谱响应率定标方法

采用新的方法对波长范围1-3μm的红外探测器绝对光谱响应率进行定标.红外探测器的光谱响应率定标是在两套定标系统上利用两种参考探测器实现的.首先在红外光谱比较系统上利用一个平响应的腔式热电堆探测器作为参考探测器,测量待测红外探测器相对于标准探测器的连续光谱响应率;然后在可见一近红外定标系统上,利用低温辐射计和激光器在几个单立波长上进行绝对光谱响应率测量.这样,通过计算就能得出待测红外探测器在每个波长上的绝对光谱响应率.采用上述方法对TS-76探测器进行光谱响应率定标,联合不确定度小于0.95%.     

发动机喘振故障检测的神经网络免疫识别模型

提出了一种航空发动机喘振故障检测的神经网络免疫识别模型。该模型利用人工免疫系统的反面选择原理来构建神经网络检测器,通过训练将失速压力信号的模式特征存储在分布的检测器中。检测器用于捕获信号的失速模式特征,当检测器与特征样本匹配时则激活该检测器,根据检测器的激活情况来发现失速点。对某型涡喷发动机压力测量信号的分析结果表明,该方法对由失速气团造成的压力信号突变具有较强的分辨力,可以用于发动机喘振的早期检测。     

High-performance photo detector for correlative feeble lighting using pixel-parallel sensing

We have developed a high-performance photo detector, which can detect a feeble projected light with pulse modulation in strong ambient light from other light sources. It is useful to expand the application field of the three-dimensional measurement system using a light-section method. A correlation circuit and a current-mode suppression circuit of constant illumination allow high sensitivity, high selectivity, and adaptive suppression of background illumination. A logarithmic-response circuit is employed to avoid saturation for wide dynamic ranges. The photo detector can quickly detect the modulated light by pixel-parallel sensing. It has advantages for some applications which require availability in various background illuminations and safe light projection for human eyes. The photo detectors have been developed and successfully tested. The high sensitivity under -18-dB signal-to-background ratio (SBR) is realized over the 47.2-dB dynamic range. The minimum SBR is -22.8 dB and the potential frame rate is 2000 fps. In addition, the photo detector shows high selectivity in a multiple-lighting system due to the suppression of orthogonally modulated light.     

陷阱探测器面响应均匀性的测量

对陷阱探测器的面响应均匀性进行了测量。在主要灵敏面内 ,面响应均匀性测量结果的重复性达到 4×10 -5。给出了对 S1337、S12 2 7等 4种不同硅光电二极管构成的陷阱探测器的面响应均匀性的测量结果。测量结果表明 ,不同类型探测器之间面响应不均匀性存在显著差异 ,其中 S1337构成的陷阱探测器的面响应均匀性最好。正确选择合适类型探测器 ,对保持和传递低温辐射计达到的很低的不确定度非常重要。     

局部高空间分辨率的应用适应性PET系统设计初探

研制成本可负担的高空间分辨率正电子发射断层成像(positron emission tomography,PET)系统在PET成像应用中具有决定性的意义,也是PET成像面临的关键性挑战之一。设想一种PET系统,能够根据应用的需求对不同性能的探测器进行布局,在一个具体时刻,对部分成像区域获得很高的性能,而在其他的区域获得普通性能。提出了一种应用适应性PET系统原型,由大部分普通固有空间分辨率的探测模块和少量更高甚至极端高固有空间分辨率的探测模块组成。研究了在含有连续放置的一段高固有空间分辨率探测模块的PET系统中,高性能模块的布局位置和数量对视场内不同位置点的空间分辨率的影响。初步研究结果表明,所提出的系统能够为局部区域带来高空间分辨率,其空间分辨率提升程度与高性能模块布局位置有关,在同一高性能模块布局位置下,视场中不同位置点的提升程度不同。该研究结果也显示,根据应用需求规划探测模块的构成和布局以在感兴趣区域获得局部高空间分辨率是可能的。     

Optoacoustic methods for frequency calibration of ultrasonic sensors

The frequency response of ultrasonic detectors is commonly calibrated by finding their sensitivity to incident plane waves at discrete frequencies. For certain applications, such as the emerging field of optoacoustic tomography, it is the response to point sources emitting broadband spectra that needs to be found instead. Although these two distinct sensitivity characteristics are interchangeable in the case of a flat detector and a point source at infinity, it is not the case for detectors with size considerably larger than the acoustic wavelength of interest or those having a focused aperture. Such geometries, which are common in optoacoustics, require direct calibration of the acoustic detector using a point source placed in the relevant position. In this paper, we report on novel cross-validating optoacoustic methods for measuring the frequency response of wideband acoustic sensors. The approach developed does not require pre-calibrated hydrophones and therefore can be readily adopted in any existing optoacoustic measurement configuration. The methods are successfully confirmed experimentally by measuring the frequency response of a common piezoelectric detector having a cylindrically focused shape.     

CHICSi—a compact ultra-high vacuum compatible detector system for nuclear reaction experiments at storage rings. II. Detectors

We describe the detectors for identification of charged particles and fragments in CHICSi, a large solid angle multi-telescope system mounted inside an ultra-high vacuum (UHV), cluster-jet target chamber. CHICSi performs nuclear reaction experiments at storage rings. The telescopes consist of a first very thin, 10–14 μm Si detector, a second 300 μm (or possibly 500 μm) ion implanted Si detector supplemented by a 6 mm GSO(Ce) scintillator read out by a photodiode (PD) or by a third 300 μm Si detector. The telescopes provide full charge separation up to Z=17 and mass resolution up to A=9 in the energy range 0.7–60A MeV. The thin p-i-n diode detector, etched out from a 280 μm Si wafer, and the GSO/PD detector, both exclusively developed for CHICSi, provide an energy resolution 8%, while the standard 300 μm detectors have 2% energy resolution. Radiation stability of the Si detectors is confirmed up to an integrated flux of 10¹⁰ alpha particles. The GSO detector has 70% light collection efficiency with the optical coupling to the PD a simple open, 0.2 mm, gap. A new method, developed to perform absolute energy calibration for the GSO/PD detector is presented.     

Development of low-noise high value chromium silicide resistors for cryogenic detector applications

Extremely high sensitivity detectors, such as silicon bolometers are required in many NASA missions for detection of photons from the X-ray to the far infrared regions. Typically, these detectors are cooled to well below the liquid helium (LHe) temperature of 4.2 K to achieve the maximum detection performance. As photodetectors, they are generally operated with a load resistor and a pre-set bias voltage, which is coupled to the input gate of a source-follower field effect transistor (FET) circuit. It is imperative that the detector system signal-to-noise performance be limited by the noise of the detector and not by the noise of the external components. The load resistor value is selected to optimize the detector performance. These two criteria tend to be contradictory in that these detectors require load resistors in the hundreds of megaohms, which leads to a higher Johnson noise. Additionally, the physical size of the resistor must be small for device integration as required by such missions as the NASA High Resolution Airborne Wide-Band Camera instrument and the Submillimeter High-Angular Resolution Camera for the Caltech Submillimeter Observatory, both of which employ 384 detectors and resistors. We have designed, fabricated and characterized thin film resistors using a CrSi/TiW/Al metal system on optical quality quartz substrates. The resistor values range from 75 MΩ to over 650 MΩ and are Johnson noise limited to below LHe temperatures. The resistor film is sputtered with a sheet resistance ranging from 300 to 1600 Ω/□ and the processing sequence developed for these devices allows for chemically fine-tuning the sheet resistance in situ. The wafer fabrication process was of sufficiently high yield (>80%) providing clusters of good resistors for integrated multiple detector channels, a very important feature in the assembly of these two instruments.     

High-numerical-aperture-based virtual point detectors for photoacoustic tomography

The focal point of a high-numerical-aperture (NA) ultrasonic transducer can be used as a virtual point detector. This virtual point detector detects omnidirectionally over a wide acceptance angle. It also combines a large active transducer surface and a small effective virtual detector size. Thus the sensitivity is high compared with that of a real point detector, and the aperture effect is small compared with that of a finite size transducer. We present two kinds of high-NA-based virtual point detectors and their successful application in photoacoustic tomography. They can also be applied in other ultrasound-related fields.     

Novel Al2O3:C,Mg fluorescent nuclear track detectors for passive neutron dosimetry

The latest advances in the development of a fluorescent nuclear track detector (FNTD) for neutron and heavy charged particle dosimetry are described and compared with CR-39 plastic nuclear etched track detectors (PNTDs). The technique combines a new luminescent aluminium oxide single crystal detector (Al(2)O(3):C,Mg) with an imaging technique based on laser scanning and confocal fluorescence detection. Detection efficiency was obtained after irradiations with monoenergetic neutron and proton beams. Dose dependences were measured for different configurations of the detectors exposed in fast- and thermal-neutron fields. A specially developed image processing technique allows for fast fluorescent track identification and counting. The readout method is non-destructive, and detectors can be reused after thermal annealing.     

Nonlinearity Measurements of High-Power Laser Detectors at NIST

We briefly explain the fundamentals of detector nonlinearity applicable to both electrical and optical nonlinearity measurements. We specifically discuss the attenuation method for optical nonlinearity measurement that the NIST system is based upon, and we review the possible sources of nonlinearity inherent to thermal detectors used with high-power lasers. We also describe, in detail, the NIST nonlinearity measurement system, in which detector responsivity can be measured at wavelengths of 1.06 µm and 10.6 µm, over a power range from 1 W to 1000 W. We present the data processing method used and show measurement results depicting both positive and negative nonlinear behavior. The expanded uncertainty of a typical NIST high-power laser detector calibration including nonlinearity characterization is about 1.3 %.     

氮化镓肖特基结紫外探测器的异常特性测量

测量了CaN肖特基结紫外探测器在有、无光照下的I-V异常特性。分别用362nm和368nm光束对有源区进行横向扫描,得到了光照不同部位时探测器在无偏压、2V反向偏压下的电流。紫外光照到肖特基结压焊电极附近及透明电极边沿附近区域时,探测器在反向偏压下有较大增益,空间响应均匀性变差,在禁带内有两个增益响应峰波长——364nm和368nm。探测器在810nm光照射下,反向偏压下的光响应增益、持续光电导存在光淬灭现象。探测器紫外光照完后,俘获中心及表面陷阱所俘获的部分电荷在高反向偏置电压下老化可以通过隧穿或发射效应释放出来,经过高反向偏置电压老化完后的探测器在同一低反向偏置电压下暗电流比老化前的要小。测量结果为GaN器件的研制提供了参考数据。     

Advances in silicon carbide X-ray detectors

The latest advances in SiC X-ray detectors are presented: a pixel detector coupled to a custom ultra low noise CMOS preamplifier has been characterized at room and high temperature. An equivalent noise energy (ENE) of 113 eV FWHM, corresponding to 6.1 electrons r.m.s., has been achieved with the detector/front-end system operating at +30 °C. A Fano factor of F=0.10 has been estimated from the ⁵⁵Fe spectrum. When the system is heated up to +100 °C, the measured ENE is 163 eV FWHM (8.9 electrons r.m.s.). It is determined that both at room and at high temperature the performance are fully limited by the noise of the front-end electronics. It is also presented the capability of SiC detectors to operate in environments under unstable temperature conditions without any apparatus for temperature stabilization; it has been proved that a SiC detector can acquire high resolution X-ray spectra without spectral line degradation while the system temperature changes between +30 and +75 °C.     

Application of a Ge semi-conductor detector to whole-body counter

To calibrate a whole-body counter, it is necessary to find a determination method for peak efficiencies of detectors used in the whole-body counter. For this purpose, peak efficiencies of a Ge semiconductor detector for point and volume sources were evaluated in the photon energy range 60-1836 keV by Monte Carlo simulation and experiment. It was found that the calculated peak efficiency curves as a function of energy without modelling of the actual sensitive region of the detector are similar in shape to those measured. The calculated peak efficiencies of the detector having an apparent dead layer (1 mm) were also found to agree with the experimental values (deviations from -10-24%). Consequently, the simulation method for peak efficiencies was validated. In addition, an optimum design for a whole-body counter with Ge semiconductor detectors was examined by simulation. This simulation provides a method to determine an optimum arrangement of detectors in a whole-body counter, offering a uniform response to various 137Cs distributions in a human body.