Euromedim 2006 :1st European Conference on Molecular Imaging Technology

SiAPD Specifications for Multi-face Scintillation Readout

10 May 2006, 14:00

1h

Palais du Pharo, Marseille

poster • Conversion materials and photodetectors Poster session : detection modules and electronics

Speaker

Dr Raffaele Scafè (Casaccia Research Center, ENEA, Rome, Italy)

Description

This work is aimed to design a gamma-ray scintillation detector, suitable for radionuclide imaging and able to measure event-by-event the energy and the point-of- interaction of primary radiation. In principle, to obtain these quantities, multi- face readout of a parallelepiped crystal is necessary. Light intensity values from each crystal face are used for centroiding the event through an appropriate algorithm, while the sum of responses is proportional to the energy of the detected gamma-ray. The efficiency of light collection is affected by some geometrical factors concerning the crystal-APD assembly. In particular the width of APD peripheral dead zones and the thickness of the APD entrance window affect the portion of scintillation light detected by the photosensors. To evaluate the influence of these factors on the assembly response, in terms of total light collection and position evaluation, a simplified model has been developed. The model calculates luminance values at photosensor’s active areas as a function of the point of interaction. It only considers geometric effects, under the hypothesis of isotropic scintillation emission, and negligible light reflection at crystal walls. This first step hypothesizes the same interaction probability in every point of the crystal while a further study takes into account, using a finite-element model, the probability of photoelectric interaction as a function of photon energy and crystal material. A detector setup has been considered including a crystal sized 11.4x11.4x12.4mm3 and 6 photosensors, each coupled to a face of the scintillator. A number of cases with the APD overall dimensions fitting the crystal face size, but having different active areas, have been considered. Further cases have been considered, with increasing optical window thicknesses. In particular, simulations concerned photosensors with rectangular active areas surrounded by dead zones in the range from 0mm to 1mm and optical windows in the range from 0 to 0.5mm. The highest considered values for both parameters are considered as easily obtainable using the technology available for detector assembling. Energy and position responses vs. dead zones width and optical window thickness are presented. The first study shows the strong influence of dead areas on both analyzed quantities, mainly at crystal borders. A dead zone width of 0.5mm and an optical window 0.3mm thick seem realistic values to obtain a response with acceptable linearity inside an inner crystal volume approximately correspondent to active zones. The second study points out how the light collection is affected by photon energy and crystal material, which determine the depth of interaction. According to this schematization the detector response can be represented, voxel-by- voxel, as the product of two parameters obtained from the studies above described. This study helps to design the detector assembly, in terms of crystal material, size and light readout geometry, to optimize its response in the fixed energy range.