Speakers
Description
Self-Powered Neutron Detectors for ISIS
$^1$Katherina Gelborova, $^1$Steven Lilley
$^{1}$ ISIS Neutron and Muon Source, Science and Technology Facilities Council, Rutherford Appleton Laboratory, Didcot OX11 0QX, United Kingdom
Self-powered neutron detectors (SPNDs) have been a reliable choice for the mapping and monitoring of neutron flux in fission reactors for their compactness, simplicity and robustness compared to other in-core detectors. They do not require a power supply, as the signal current arises from the movement of electrons produced in the SPND by incident neutrons and gamma rays. There is a prompt part of the signal, which is mainly due to either neutron-capture gamma rays or gamma radiation from the environment causing the emission of photoelectrons, Compton electrons and pair-production electrons. There is also a delayed component of the signal that saturates with time, mainly from the beta-decay of neutron-activated products in the SPND. The detector materials are often selected to optimise one of these signal-producing processes.
Recent research in this area has looked at evaluating the suitability of commercial SPNDs to the neutron and gamma spectra of fusion and accelerator neutron sources. As these sources have higher energy neutron spectra than fission reactors, the proportions of the different signal components may be very different, or a sufficiently high sensitivity may not be achievable at all. At the ISIS Neutron and Muon Source, installing SPNDs in the target, reflector, and moderator assemblies (TRaMs) could provide more direct monitoring of the proton beam flux, pulse shape and position. The neutron spectrum in the ISIS TRaM goes up to around 800 MeV, with a peak flux for neutrons with energy on the order of 100 keV, and a total flux on the order of 10$^{13}$ n/cm$^2$/s. A neutronics investigation of the expected detector response is required to assess whether implementing SPNDs at ISIS would be a useful project.
Goldstein developed a method for Monte Carlo SPND analysis using the MCNP neutronics code, which has since been used and developed extensively. In this work, the transport simulation steps are modified to use the CERN FLUKA code to model SPNDs in the ISIS TRaM, but the subsequent steps for calculating the detector sensitivity from the simulation results are kept the same. By evaluating the sensitivity with several common SPND material combinations, and varying geometric parameters, the suitability of such a detector to the ISIS radiation field can be understood.