Naoko Inadama

A depth of interaction detector for PET with GSO crystals doped with different amounts of Ce

A new method for a four-stage depth of interaction (DOI) detector is proposed. The four-stage DOI detector is constructed with two kinds of Gd/sub 2/SiO/sub 5/:Ce (GSO) crystals doped with different amounts of Ce, 0.5 mol% and 1.5 mol%. The amount of Ce in GSO determines the scintillation decay time constant, and it is 60 ns for the 0.5 mol% GSO and 35 ns for 1.5 mol% GSO. This difference led to the introduction of pulse-shape discrimination, which would distinguish between two kinds of event data from respective GSOs and sort them into two groups. By independently applying Anger-type position arithmetic to the data of each group, two two-dimensional (2-D) histograms are obtained. The crystal of interaction can be identified on these histograms in which only 0.5 mol% or 1.5 mol% GSO crystal elements are expressed. To evaluate this method, we constructed the four-stage DOI detector by alternately stacking 1.5 mol% GSO crystal stages and 0.5 mol% stages. The result of a scanning measurement with a /sup 137/Cs gamma-ray beam proved that the DOI detector had enough accuracy in crystal identification.

Measurement of 32/spl times/8/spl times/4 LYSO crystal responses of DOI detector for jPET-RD

jPET-RD is designed to achieve high sensitivity as well as high spatial resolution by the use of four-layer depth of interaction (DOI) information of the detector. We have previously proposed the DOI encoding method that enables four layers DOI identification using only single kind crystal elements. The basic idea was tested by using Gd2SiO5, and the first prototype detector was developed using Lu2(1-x)Y2xSiO5 (LYSO). In this work, we prepared a pair of jPET-RD prototype detectors composed of four layers of a 32 (transaxial)times8 (axial) LYSO (Lu: 98%, Y: 2%) crystal block and a 256-channel flat panel position sensitive photomultiplier tube (256ch FP-PMT). The size of each crystal element is 1.46 mmtimes1.46 mmtimes4.5 mm. The crystal block (46.5 mmtimes11.6 mmtimes18.0 mm) is placed on the central area of a 256ch FP-PMT (49 mmtimes49 mm useful area) and coupled with silicone rubber. First, we evaluated performance of the prototype DOI detector by uniform gamma ray irradiation. Then response functions of the prototype DOI detector were measured with collimated single gamma rays and finally coincidence responses are estimated with a pair of prototype DOI detectors in the experimental setup which simulates jPET-RD system. In the performance evaluation, the energy resolution of all events was 14.7% and the time resolution was found to be 0.66 ns. The response functions were 1.56 mm FWHM and 4.51 mm FWHM in average in transaxial and depth direction, respectively. The FWHMs of coincidence responses were 5.4 mm (non-DOI) and 3.7 mm (averaged DOI). It is confirmed that the spatial resolution is improved by using DOI information

Quantum confinement effects in semiconducting scintillators

A physical method to develop a scintillator with both a large light output and a quick response is proposed and demonstrated. A direct-gap semiconductor often exhibits ultrafast scintillation components with subnanosecond decay time constants at very low temperatures. However, the exciton luminescence is significantly quenched at room temperature to lose the practical merits. The authors found that the thermal quenching was effectively prevented by constructing a low-dimensional quantum confinement system and a practical amount of light output was obtained at room temperature without losing the quickness of the radiative recombination of the exciton. Crystals of (C6H13NH3)2 PbI4 having a multiple quantum well structure exhibited three decay components of 390 ps, 3.8 ns and 16 ns with the ratios of 28%, 29% and 43%, respectively. The total light output at 300 K was 11% of that of NaI:Tl