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Silicon Photomultiplier (SiPM)

In modern material sciences and solid state physics, samples are irradiated with a neutron beam, and the way the neutrons are scattered by the sample reveals information about the inner structure or behaviour of the sample. In order to measure the response, position (and often time) sensitive neutron detectors are required. A certain kind of these detectors uses a scintillator screen to absorb neutrons and emit flashes of light, which can then be detected by a photodetector. Usually, Photo Multiplier Tubes (PMT) are used in neutron detectors.

A new alternative photodetector, the Silicon Photomultiplier (SiPM), has some advantages over PMTs: a lower operating voltage, insensitivity to magnetic fields, and higher countrate capabilities, to name a few. However, neutron radiation is very damaging to silicon. Due to dopant change and point
defects, the dark count rate of SiPMs is increased by neutron irradiation. Therefore, SiPMs have largely be ignored as a replacement for PMTs in neutron detection applications.

Our recent study shows, that this effect is not too devastating, and that we can expect a life time of around 10 years for such a detector. Therefore, we are currently developing a prototype for a detector using a scintillator screen and digital SiPMs as photodetectors. It has an active area of 13.6 x 13.6 cm² with a position resolution of 1 mm. The specified countrate is 40 kcps/cm² and the detection efficiency 90 % for cold neutrons with a wavelength of 5 Å.

The SiPM modules used have a pixel size of 4 mm, so in order to achieve a position resolution of 1 mm, we need to employ a technique called Anger camera principle. Here, the light of a single event is spread out over a region larger than a single pixel, and by comparing the relative signals of different
pixels, we can reconstruct the position of the neutron event with a better
resolution than the pixel size.