Detecting neutrons. Cosmically.
Who doesn’t want to detect high enery neutrons? They can damage avionic electronics and potentially damage cells. A clever neutron detector was constructed by a team led by Dr Steve Monk at the Engineering Department, Lancaster University. The detector works when characteristic charges are created in it – from high energy neutrons interacting with a special boron coating. The signal produced in the photodetector is fed to a high frequency ADC. Software, written by Steve, then decides if the pulse shape is characteristic of the neutron having caused the flash, or just some spurious noise. That’s the clever bit. The instrumentation was designed by Hybrid Instruments who are based in the Engineering Department. I got to do the soldering. The picture shows one of the detectors, there were 12 in all encased in the instrument. The funky looking component at the front is one of the photodetectors. The completed instrument looks like a white cannonball, as the detectors are all plugged into a football sized sphere of polyethylene. The polyethylene acts to slow down the neutrons to aid in their detection.
This work was published in ‘Review of Scientific Instruments’ volume 79 (2008). Here’s the title and abstract:
A portable energy-sensitive cosmic neutron detection instrument
S. D. Monk and M. J. Joyce
Department of Engineering, Lancaster University, Lancaster LA1 4YR, United Kingdom
Hybrid Instruments Ltd., Priory Close, St. Mary’s Gate, Lancaster LA1 4WA, United Kingdom
BAE SYSTEMS (Military Air Solutions), Warton, Preston PR4 1AX, United Kingdom
Computing Department, InfoLab21, South Drive, Lancaster University, Lancaster LA1 4WA, United Kingdom
The construction and testing of a portable energy-sensitive neutron instrument are described. This
instrument has been designed and constructed for the primary purpose of characterizing cosmic-ray
neutron fields in the upper atmosphere and in cosmic reference field facilities. The instrument
comprises a helium-3 proportional counter surrounded by 15 mm of lead and 140 mm of
polyethylene creating a spherical structure with a diameter of 34 cm. The instrument also
incorporates 12 boron-coated diodes, six on the outside of the polyethylene layer with six placed
within the structure. The dimensions, materials, and arrangement of these in the instrument have
previously been optimized with the MCNPX Monte Carlo simulation software to provide a
compromise between the requirements of portability and spectral response. Testing took place at
several locations and experimental data from the instrument’s operation at the high-altitude
Jungfraujoch laboratory in the Swiss alps are presented. © 2008 American Institute of Physics.