Introduction of a thermal response to the DSTL PADC personal neutron dosemeter

The response of the Defence Science and Technology Laboratory (DSTL) PADC personal neutron dosemeter is strongly dependent upon neutron energy, with a range of 300-500 tracks per cm2 per mSv for energies between 1 and 5 MeV. Below 1 MeV the response drops off sharply. This lack of sensitivity is undesirable when the dosemeter is employed with the softened fission spectra encountered in the workplace. In order to incorporate a thermal response, a polypropylene converter doped with LiF has been placed directly in front of the PADC elements. Tritons produced in the thermal neutron reaction 6Li (n,t)alpha at 2.7 MeV will then penetrate the PADC, leaving a trail of damage. The reaction rate within the converter has been calculated using MCNP for thermal neutrons and a range of higher energies, while transport of the tritons is modelled using the SRIM/TRIM package to determine the resultant track density and depth distribution. The modelling and experimental work have demonstrated that a concentration of 0.2% natural lithium by weight results in a track density in a thermal field comparable with that produced per unit personal dose equivalent by neutrons greater than 1 MeV in the standard dosemeter. Additional MCNP modelling has demonstrated that the dosemeters' albedo response to intermediate energy neutrons can be enhanced considerably by placing a boron-doped shield in front of the converter and increasing its lithium concentration.