Results are presented on the development of a portable detector suitable for detection of individual thermal neutrons. The
device is based on direct absorption of neutrons in an absorber film containing 10B. The resultant charge arising from the
capture products is detected by a p-n junction partly formed from this absorber and internal to the device. When a small
bias voltage is applied (typically a few volts) a current pulse is observed due to the movement of this charge in the
electric field of the p-n junction. For each detected neutron the charge pulse height, rise time and time of detection are
recorded. Device performance, in terms of efficiency and spectral response, is explored as a function of neutron
absorber thickness, geometry and overall diode electrical characteristics and validated against neutron source
measurements at the UK National Physical Laboratory (NPL). The diodes have a natural background suppression
capability through traditional pulse height and pulse rise time discrimination. The manufacturing process permits
fabrication of arrays of diodes, with typical areas of ~15 mm2, thus increasing the collecting area and the signal to noise
ratio, albeit with increased readout complexity. The associated multi-channel readout electronics is standard, however,
and commonly used in existing X-ray sensors. Simple portable sensors based on these heterodiodes are expected to have
applications in the detection of nuclear materials in a variety of security related situations.
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