Localised response retrieval from Hamamatsu H9500 for a coded aperture neutron-gamma imaging system based on an organic pixelated plastic scintillator (EJ-299-34)

Abstract

Localised response of a sensitive light detector, such as Hamamatsu H9500 multi anode photomultiplier tube (MAPMT), is of vital importance for coded-aperture imaging systems. When coupled with a suitable sensitive detector (e.g. EJ-299-34 plastic scintillator), output signals of the MAPMT can be retrieved to infer the interaction location in the pixelated scintillator. Given the number of anodes in H9500 (256), significant processing power would be necessary to handle each pulse individually. Therefore, a readout electronics system was developed, based on resistive network approach, which reduces the number of output signals to individual X and Y coordinates, and subsequently allows particle identification. Coordinates retrieved in this manner can be analysed in real time and used to infer the two-dimensional location. Particle type can be also exploited by pulse shape discrimination (PSD) application to the scintillator response. In this study, 169 anodes were used (due to coded-aperture design rules), and reduced to two X and Y output signals. These have been digitised using a bespoke FPGA based two channel 14-bit 150 MSPS digitiser. The digital data are transferred to a host application using UART to USB converter operating at 12 Mbits/s. Promising results have been observed when the scintillator's response was tested in single particle field of Cs-137. However, further tests performed in the mixed-field environment of Cf-252 suggest that faster digitiser may be required to obtain the required PSD performance.