Large Area Wavelength Shifting Fibre Thermal Neutron Detectors Using 64 Channel Flat Panel Pmts

Large area thermal neutron detectors are needed in fields such as nuclear safeguarding and neutron scattering science. In neutron scattering, position sensitive 3He detectors have historically been employed on instruments requiring large area detectors. Cost-effective alternatives to large area 3-He detectors are currently being sought due to 3He becoming rare and expensive. ZnS: Ag/(LiF)-Li-6 scintillator detectors are currently in use at ISIS, the SNS and J-PARC and are being investigated as the next generation of large area detectors at ISIS. Wavelength shifting fibre (WLSF) technology has resulted in a cost effective light transport option. WLSF have previously been coupled to single anode PMTs, 16 channel multi-anode PMTs, standard 64 channel MAPMTs and Si-PMs. Each of these have their own advantages and disadvantages with the main purpose being to economically readout scintillation events from the ZnS: Ag/(LiF)-Li-6 without sacrificing detector performance. Recent developments flat panel PMT (FP-PMT) technology have made single photon detection feasible allowing a new, cost-effective, solution for WLSF detectors. Large, 6mm x 6mm, pixel size combined with 64 channels allows for a high degree of fibre coding while drastically reducing the high voltage, electronics and cabling requirements. The performance of 64 channel FP-PMTs in large area WLSF coupled ZnS: Ag/(LiF)-Li-6 detectors is presented here. Current ISIS WLSF detector technology was used to evaluate the performance of the FP-PMT. Performance of the 64 channel FP-PMT is discussed in terms of readout method, pixel to pixel cross-talk pixel to pixel uniformity. A new position determination algorithm is shown to eliminate the effect of pixel to pixel cross-talk. A factor of up to 2.5 in gain variation from pixel to pixel is shown to have less than 10% effect on the uniformity of the neutron detector response. The 64 channel FP-PMT is shown to be a viable option for large area wavelength shifting fibre detectors.