J. Huizenga

Experimental characterization of monolithic-crystal small animal PET detectors read out by APD arrays

Minimizing dead space is one way to increase the detection efficiency of small-animal PET scanners. By using monolithic scintillator crystals (e.g., 20 mm/spl times/10 mm/spl times/10 mm LSO), loss of efficiency due to inter-crystal reflective material is minimized. Readout of such crystals can be performed by means of one or more avalanche photo-diode (APD) arrays optically coupled to the crystal. The entry point of a gamma photon on the crystal surface can be estimated from the measured distribution of the scintillation light over the APD array(s). By estimating the entry point, correction for the depth-of-interaction (DOI) is automatically provided. We are studying the feasibility of such detector modules. To this end, a 64-channel test setup has been developed. Experiments to determine the effect on the spatial resolution of crystal surface finish and detector geometry have been carried out. The first results of these experiments are presented and compared to simulation results. The crystal surface finish has only a small influence on the spatial resolution. The spatial resolution of 20 mm/spl times/10 mm/spl times/10 mm detectors is significantly better when read out on the front side than when read out on the back side. With a 20 mm/spl times/10 mm/spl times/20 mm crystal coupled to two APD arrays, a very small resolution degradation of only /spl sim/0.2 mm is observed for an incidence angle of 30/spl deg/ compared to normal incidence.

Comparative study of silicon detectors

We studied three different types of silicon sensors: PIN diodes, circular drift detectors, both made at the Delft University of Technology (DUT), and Hamamatsu S5345 avalanche photodiodes. Measurements have been carried out in the same optimized experimental setup, both at room temperature and at low temperatures. Comparison is made for direct X-ray detection and CsI(Tl) scintillation light readout.

Signal to Noise Ratio of APD-Based Monolithic Scintillator Detectors for High Resolution PET

Monolithic scintillator detectors, consisting of several cm3 of scintillating material coupled to one or more Hamamatsu S8550 avalanche photodiode (APD) arrays, are proposed as detectors for high resolution positron emission tomography (PET). In this work, the factors contributing to the variance on the signals are investigated, and their effects on the energy, time and spatial resolutions are analyzed. Good agreement was found between a model of the energy resolution and experiments with a 20 x 10 x 10 mm3 LYSO:Ce crystal coupled to a single channel large-area APD (LAAPD). With the same crystal coupled to an APD array, differences between model and experiment were observed at high APD gain. The measured energy resolution of ~11% FWHM was dominated by scintillation photon statistics, with less important roles for the APD excess noise factor and electronic noise. On the other hand, electronic noise was an important factor both for the time and the spatial resolutions. The time resolution was found to depend strongly on the APD bias voltage, and was best at the highest bias. A time resolution of 1.6 ns full width at half maximum (FWHM) was measured against a BaF2 -PMT detector. The best spatial resolution measured was 1.64 mm FWHM, without correction for the ~0.9 mm FWHM measurement beam. It is estimated that an intrinsic spatial resolution of 1.26 mm FWHM can be achieved at the center of the detector with an infinitely narrow test beam.

Design of a new tissue-equivalent proportional counter based on a gas electron multiplier

By employing a Gas Electron Multiplier a new type of mini multi-element Tissue-Equivalent Proportional Counter (TEPC) has been designed and constructed. In this paper, we describe the design of this novel counter. The first pulse height measurements with this counter for both methane- and propane-based Tissue Equivalent gases are presented. These results show promising properties for application of this novel type TEPC in microdosimetric measurements.

2D dosimetry in a proton beam with a scintillating GEM detector

A two-dimensional position-sensitive dosimetry system based on a scintillating gas detector is being developed for pre-treatment verification of dose distributions in particle therapy. The dosimetry system consists of a chamber filled with an Ar/CF(4) scintillating gas mixture, inside which two gas electron multiplier (GEM) structures are mounted (Seravalli et al 2008b Med. Phys. Biol. 53 4651-65). Photons emitted by the excited Ar/CF(4) gas molecules during the gas multiplication in the GEM holes are detected by a mirror-lens-CCD camera system. The intensity distribution of the measured light spot is proportional to the 2D dose distribution. In this work, we report on the characterization of the scintillating GEM detector in terms of those properties that are of particular importance in relative dose measurements, e.g. response reproducibility, dose dependence, dose rate dependence, spatial and time response, field size dependence, response uniformity. The experiments were performed in a 150 MeV proton beam. We found that the detector response is very stable for measurements performed in succession (sigma = 0.6%) and its response reproducibility over 2 days is about 5%. The detector response was found to be linear with the dose in the range 0.05-19 Gy. No dose rate effects were observed between 1 and 16 Gy min(-1) at the shallow depth of a water phantom and 2 and 38 Gy min(-1) at the Bragg peak depth. No field size effects were observed in the range 120-3850 mm(2). A signal rise and fall time of 2 micros was recorded and a spatial response of <or=1 mm was measured.

Experimental characterization of novel small animal PET detector modules based on scintillation crystal blocks read out by APD arrays

Minimizing dead space is one way to increase the detection efficiency of small-animal PET scanners. By using monolithic blocks of scintillating material (e.g. 20 mmtimes10 mmtimes10 mm LSO), loss of efficiency due to inter-crystal reflective material is minimized. Readout of such blocks can be performed by means of one or more avalanche photo-diode (APD) arrays optically coupled to the block. The primary event position and depth of interaction (DOI) information are derived from the measured distribution of the scintillation light over the APD array(s). We are studying the feasibility of such detector modules, both by simulation and by experiment. A 64-channel setup for testing the above type of detector modules has been developed. Experiments to verify the effect of crystal surface finish, detector geometry and reconstruction algorithm parameters on the spatial resolution have been carried out. The first results of these experiments are presented in this paper, and compared to simulation results. This research is conducted in collaboration with the Crystal Clear Collaboration (CCC)

First Results of a Scintillating GEM Detector for 2-D Dosimetry in an Alpha Beam

The characterization of a scintillating GEM based gas detector for quality control of clinical radio-therapeutic beams is presented. Photons emitted by the Ar/CF4 gas mixture are detected by means of a CCD camera; in addition, the charge is measured. The detector response has been studied as a function of alpha particle energy and dose rate. The measured signal underestimation, at the Bragg peak depth, is only few percent with respect to an air filled ionization chamber.

Low noise p-channel JFETs for X-ray spectroscopy with silicon drift detectors

We present noise performance of different p-channel JFETs fabricated by silicon-detector compatible technology. This JFET can be used as a front-end transistor of a resistorless charge preamplifier for low energy X-ray detection. Our concern is the read-out of silicon drift detectors. Firstly, a fully implanted p-channel JFET was fabricated by an optimized DIMES-03 process (temperature budget of 950/spl deg/C). The JFET is top-gate driven with W/L=100/1. We have measured the white series noise of 2.9 nV//spl radic/Hz and the corner frequency of the 1/f noise of /spl sim/500 Hz. This JFET is therefore a very good alternative for an external front-end transistor for low noise charge preamplifiers. To achieve further improvement, an integration of this JFET into the read-out anode of the drift detector is necessary. We present the modified DIMES-03 process which was used to fabricate p-JFETs on low- and high-ohmic wafers. The noise parameters measured on these JFETs will be discussed.

Characterization of a scintillating GEM detector with low energy x-rays.

A two-dimensional position-sensitive dosimetry system based on a scintillating gas detector is being developed with the aim of using it for pre-treatment verification of dose distributions in charged particle therapy. The dosimetry system consists of a chamber filled with an Ar/CF(4) scintillating gas mixture, inside which two cascaded gas electron multipliers (GEMs) are mounted. A GEM is a thin kapton foil with copper cladding structured with a regular pattern of sub-mm holes. In such a system, light quanta are emitted by the scintillating gas mixture during the electron avalanches in the GEM holes when radiation traverses the detector. The light intensity distribution is proportional to the energy deposited in the detectors sensitive volume by the beam. In the present work, we investigated the optimization of the scintillating GEM detector light yield. The light quanta are detected by means of a CCD camera or a photomultiplier tube coupled to a monochromator. The GEM charge signal is measured simultaneously. We have found that with 60 microm diameter double conical GEM holes, a brighter light signal and a higher electric signal are obtained than with 80 microm diameter holes. With an Ar + 8% CF(4) volume concentration, the highest voltage across the GEMs and the largest light and electric signals were reached. Moreover, we have found that the emission spectrum of Ar/CF(4) is independent of (1) the voltages applied across the GEMs, (2) the x-ray beam intensity and (3) the GEM hole diameter. On the other hand, the ratio of Ar to CF(4) peaks in the spectrum changes when the concentration of the latter gas is varied.

X-ray detection with multi-anode sawtooth silicon drift detectors

We present results of room temperature and low-temperature X-ray measurements with 500 /spl mu/m anode pitch multi-anode linear and sawtooth silicon drift detectors. An analysis of the influence of split events due to the lateral spread of the drifting electron cloud, electronic noise and the general spectroscopic performance of the detectors is given. An energy resolution of 450 eV FWHM was determined for the 5.89 keV line of /sup 55/Fe at 233 K. Split events are completely eliminated.