Sub-aquatic Response of a Scintillator, Fibre Optic and Silicon Photomultiplier Based Radiation Sensor

We describe here, the sub-aquatic response of the RadLine (R) detector (a small, novel, remotely operated radiation detection instrument) when irradiated with gamma doses between 6 and 400 Svhr(-1). The National Nuclear Laboratory's (NNL, UK) RadLine (R) consists of an inorganic scintillating crystal coupled to a fibre optic cable which transports scintillation photons to a detector at the other end. A CCD camera is normally used for photon collection, however in this paper we trial a newer technology; the silicon photomultiplier (SiPM), namely SensL's MiniSL. SiPMs have performance characteristics similar to photomultiplier tubes (PMT), whilst benefiting from the practical advantages of solid-state technology which include; low operating voltage, robustness, compactness, insensitivity to magnetic fields and over-exposure to light. The MiniSL was chosen as its peak photon wavelength is well matched to the output from the scintillation crystal, as well as its fast recovery time (within the nano-second range). We use a clinical radiotherapy linear accelerator (linac) machine which produces x-rays by accelerating elections onto a target which then emits x-rays by Bremsstrahlung. The linac is rated at 6MeV in energy, with a peak of approximately 2MeV. The machine is capable of generating a highly precise dose at known distances between treatment head and scintillation crystal. Analysing the data gathered we were also able consider how the RadLine (R) might perform in larger aquatic environments for example First Generation Magnox Storage Ponds (FGMSP). Built in the 1950's they were originally intended to hold spent nuclear fuel for reprocessing, however now parts of the spent fuel have corroded; some of which are buried under a layer of sediment. Removal is not a trivial task due to elevated radiation levels, and the complexity of the environment. RadLine (R) has the potential to be of significant use for this and in other similar situations.