We describe an attempt at the utilisation of two low level light sensors to improve on the design of a dose monitoring system, specifically for underwater applications with consideration for the effects of water attenuation. The gamma radiation ‘RadLine®’ detector consists of an inorganic scintillating crystal coupled to a fibre optic cable which transports scintillation photons, up to hundreds of metres, to an optical sensor. Analysed here are two contemporary technologies; SensL׳s MiniSL a silicon photomultiplier (SiPM) and a Sens-Tech photon counting photomultiplier tube (PMT).

A clinical radiotherapy linear accelerator (linac) is implemented as test beam, subjecting the RadLine® to a highly controlled dose rate (ranging from 0 Sv h−1 to 320 Sv h−1), averaging at 2 MeV in energy. The RadLine׳s underwater dose monitoring capabilities are tested with the aid of epoxy resin ‘solid water’ phantom blocks, used as a substitute for water.

Our results show that the MiniSL SiPM is unsuitable for this application due to extremely high background noise levels, however the Sens-Tech PMT performs satisfactorily and the detected dose rate due to the effects of water attenuation compares strongly with MCNP simulation data and NIST database values. We conclude that the PMT shows promise for its ultimate use in the First Generation Magnox Storage Pond (FGMSP) on the Sellafield site.