Fast neutron detector based on TimePix pixel device with micrometer spatial resolution

Fast neutrons are increasingly used in many fields. Fast neutrons are conventionally detected by scintillators with relatively large volume and low spatial resolution. In this paper we present a novel detection technique based on tracking of protons recoiled by fast neutrons. The tracking is performed by the silicon pixelated detector Timepix (300 um thick silicon sensor, 256 × 256 square pixels with 55 um pitch) covered by a hydrogen rich converter (e.g. plastic material). The performance of the TimePix device for detection of highly ionizing particles such as protons was already published. The technique utilizes the charge sharing effect and it is based on proper analysis of individual recorded tracks (clusters). The range of protons recoiled by fast neutrons is often greater than the pixel size allowing to determine not only energy and position but also the impact angle (precision is better than 2 degrees for 5 MeV protons). Protons lose part of their energy in the plastic before reaching the Si sensor. This energy can be measured if a plastic scintillator with an attached silicon photomultiplier (SiPM) is used. The two devices (SiPM and Timepix) can be operated in coincidence reducing significantly the undesired background radiation. Having all information about the recoiled proton and knowing the original direction of the neutron it is possible to reconstruct the exact position of the neutron-proton collision in the converter and the original neutron energy. The final spatial resolution of the neutron position determination reaches the subpixel level (about 20 ¿m). The expected energy resolution is about 0.5 MeV for 10 MeV neutrons. Preliminary experimental results are presented.