Low Energy Dosimetry With Photon Counting Pixel Detectors Such as Medipix

Hybrid semiconductor photon counting pixel detectors like the Medipix detector have several advantages for an use in X-ray dosimetry. The noiseless photon counting principle allows to monitor low photon energies down to 3.5 keV. Due to the small pixel size (55 mum in case of Medipix2) dosimetry at very high dose rates is possible still processing each photon individually. The large amount of pixels in combination with the possible thickness of the sensor layer enables dosimetry at very low dose rates. A method has been developed to determine personal dose equivalents from the number of counts in energy deposition intervals measured with a semiconductor photon counting pixel detector, despite the strong influence of charge sharing effects among pixels. We tested the method experimentally by reconstructing the air kerma free in air for different qualities of X-radiation in the energy range below 150 keV with an accuracy better than 4%. We show that the response of a dosimeter based on a hybrid photon counting pixel detector can fulfill the IEC type testing requirements. The statistical precision is high due to the thickness and the large area of the sensor layer. We estimate that a dosimeter based on the Medipix detector will be able to cope with dose rates of more than approximately 57 Sv/h for mathdot Hp (0.07) or 19 Sv/h for mathdot Hp(10) . We outline the advantages and perspectives of using this kind of detector in a dosimeter in comparison to standard active personal dosimeters.     

High Speed HPD for Photon Counting

We have succeeded in the development of a high-speed Hybrid Photodetector (HPD), by using a reduced electron lens and a newly developed avalanche diode (AD) with very low capacitance. The HPD shows fast time response of less than 500 ps in both rise and fall times, and good timing resolution of 26 ps (one sigma) for single photons at full illumination on a photocathode 8 mm in diameter. Limiting factors of the timing resolution were further investigated, and were found to be the transit time difference of electrons in vacuum along the photocathode and the jitter of the electronics. The timing resolution is 15 ps for illumination of less than 5 mm on the central part of the photocathode, which includes the jitter of 13 ps from the electronics. This resolution is the world's highest for HPDs, and matches the resolution of MCP-PMTs. Both a GaAsP photocathode having close to 50% quantum efficiency in the visible and a bialkali photocathode having 34% QE in the UV were fabricated in view of making this device available for various applications. In this paper, we report the results of our evaluation, including discussion about the limiting factors of the timing resolution for the new HPD.     

High Efficiency Photon Counting Detectors for the FAUST Spacelab Far Ultraviolet Astronomy Payload

We compare the performance of sealed tube microchannel-plate position sensitive detectors having transmission CsI photocathodes or opaque CsI photocathodes. These devices were developed for the FAUST Spacelab payload to accomplish imaging surveys in the band between 1300? and 1800?. We demonstrate that photocathode quantum efficiencies in excess of 40% at 1216? have been achieved with the transmission and the opaque CsI photocathodes. The effect of the photoelectron trajectory on the spatial resolution is assessed. Spatial resolution of <70?m FWHM has been obtained and is maintained up to event rates of 5×104 sec-1. Background rates of 0.55 events cm-2 sec-1 have been achieved and low distortion (<1%) imaging has been demonstrated.     

时间相关单光子计数荧光寿命测量中数据获取和处理

本文介绍时间相关单光子计数荧光寿命测量中的数据获取系统和数据分析方法。     

RG64—High Count Rate Low Noise Multichannel ASIC With Energy Window Selection and Continuous Readout Mode

We report on the performance of a low noise and high count rate readout ASIC with binary architecture and energy window selection for X-ray imaging applications using semiconductor detectors. The ASIC called RG64 is designed in 0.35 mum CMOS process and its total area is 3900times5000 mum². The core of RG64 consists of 64 readout channels. Each channel is built of a charge sensitive amplifier with a second order shaper of peaking time 75 ns, two independent discriminators with an 8-bit offset correction circuit and two independent 20-bit counters with RAM memory buffers. The ENC of the circuit reaches the value of about 126 el. rms with 1 pF input load and 5 mW power consumption per single channel. The mean gain in the multichannel ASIC is about 50 muV/el., with the dispersion from channel to channel of 0.9% (on one sigma level). The deviation of the effective threshold voltage spread for given energy can be reduced to less than 7 el. rms (calculated to the charge sensitive amplifier input). High count rate measurements have been performed up to 2 Mcps of average rate of input pulses, both for AC and DC coupled silicon strip detectors with X-ray photons of energy 8.04 keV. The RG64 can operate both in the continuous readout mode and in the readout mode separate from exposure.     

低能V+注入花生种子的深度研究

用扫描电子显微镜和X射线能谱仪测定了低能V 注入干花生种子的穿透深度 浓度分布。结果表明 :这种分布是有长拖尾的高斯分布 ;2 0 0keV的V 注入干花生种子的最大穿透深度为 1 3 6 μm。比较了实验结果与TRIM95的计算值。     

Scintillating Glass, Fiber-Optic Plate Detectors for Active Target and Tracking Applications in High Energy Physics Experiments

We have studied the performance of scintillating glass fiber-optic plates using 10 GeV/c particles at the SLAC test beam. The plates were composed of: (1) Terbium activated cladded-glass cores in a matrix of 15 ?m spacing; and (2) Cerium activated cladded-glass cores in a matrix of variable spacing 6-10 ?m. The target plates were viewed with a three-stage, gatable image intensifier. Particle tracks and nuclear interactions were recorded on film for both materials. We observe 5 detected hits/mm for minimum ionizing particles for the Tb glass, and 1-2 hits/mm for the Ce glass. The test results indicate that scintillating glass fiber-optic plates can be used as high spatial resolution tracking detectors for both fixed target and colliding beam experiments.     

A Scintillating Glass, Fiber-Optic Plate Imaging System for Active Target and Tracking Applications in High Energy Physics Experiments

We have studied the performance of a scintillating glass fiber-optic plate with minimum ionizing particles. The target is composed of terbium activated cladded-glass cores in a matrix of 25 ?m spacing. The plate was viewed by a three-stage, electrostatically focussed image intensifier. Particle tracks were observed with a width consistent with the resolution of the device. Details of the detection system and future developments are discussed.