A Compton Gamma Rays Detector for Small-Animals Imaging

The bio-distribution and targeting capability of pharmaceuticals may be assessed in small animals by imaging gamma-rays emitted from radio-isotope markers. Detectors based on the Compton concept allow higher gamma-ray efficiency compared to conventional gamma cameras which employ collimators, and feature millimeter spatial resolution and compact geometry. We are developing a Compton camera that has to address some requirements: the high rates typical of the Compton concept; detection of gamma-rays of different energies that may range from 140kev ( 99m Tc) to 511 kev (+ emitters); presence of gamma and beta radiation with energies up to 2 Mev in case of 188 Re. The camera consists of a thin position-sensitive Tracker that scatters the gamma ray (based on the Silicon Drift Detector (SDD) developed for the LHC-Alice exp.), and a second position-sensitive detection system, to completely absorb the energy of the scattered photons (Calorimeter). Application to higher energy gamma radiation (usually up to 511 kev gammas coming from + emitters) has to be taken into account as well, and calorimeter thickness has to be sized accordingly. This paper is mainly concerned with the design and performance tests of the calorimetric tract of a CC prototype (a module 50x50 mm^2) being developed by this collaboration. Tests have been carried out at the Laboratory for Radiopharmaceuticals and Molecular Imaging (LRIM) situated by the LNL site, which is authorized to deal with radio-isotopes in liquid solution and small animals testing.