Photodetection characterisation of SiPM technologies for their application in scintillator based neutron detectors

Small angle neutron scattering (SANS) experiments have become one of the mostly used techniques in the investigation of the properties of material on the atomic scale. Till 2001 3He based detectors were extensively used for neutron detection in these experiments, but due to the scarcity of 3He researchers started to look for alternatives [1]. Scintillation based solid state detectors appeared as a prominent alternative. A neutron scattered by a sample under test impinges a scintillating material and initiates a nuclear reaction that produces secondary particles which in a second scattering process generate a light emission that can be detected by an underlying photodetector. By determining the neutron scattering angle, the momentum transfer of the scattered neutron can be identified. Using this information the sample structure can be determined with very high spatial resolution [2].Silicon photomultipliers (SiPM), having lower bias voltages compared to photomultiplier tubes, insensitivity to a magnetic field, the possibility of modular design and higher readout rates, have the potential of becoming a photon detector of choice in these experiments. In a previous investigation, the increase of the dark signal due to irradiation of the SiPM with cold neutrons has been quantified and this technology proved feasible for this kind of applications [2]. Here, a detailed comparative analysis of photon detection efficiency (PDE) for three different SiPM technologies, before and after irradiation with cold neutrons, has been carried out. For this purpose, we irradiated one digital and two different analog SiPM arrays with cold neutrons having 5Å wavelength and up to a dose of 6E12 n/cm2 at the KWS-1 instrument of the Heinz Maier-Leibnitz Zentrum (MLZ) in Garching, Germany. As stated in [2] a dose for a 10-year equivalent non-stop operation of the detector at the KWS-1 instrument of the MLZ is expected to be around 8.2E9 n/cm2. We have used a customized measuring system for evaluating the PDE of the sensors. The used system consists of a monochromatic light source and a lock-in amplifier based readout for the wavelengths between UV and NIR parts of the spectra.The first result can be observed in Fig.1 where a relative change of 3.8% in the PDE has been measured on the digital SiPM array after being irradiated up to 1.85E12 n/cm2