Response of a CR-39 fast neutron dosemeter with a polyethylene converter improved with Makrofol

A personal fast neutron dosemeter is being developed using CR-39 as detector and several hydrogenated materials as proton converters. Configurations with polyethylene plates of different thicknesses placed before the CR-39 detector had been used to date. In the present work a Makrofol plate (300 mu m thick) was placed between the polyethylene (3 mm thick) converter and the CR-39 (500 mu m thick) detector, to a flatter energy response to neutrons arriving at various incidence angles. Dosemeters having this configuration have been exposed to monoenergetic neutron beams with energies ranging from 144 keV to 15.1 MeV, as well as to neutrons coming from a Cf-252 source, and for incidence angles between 0 degrees (normal incidence) and 85 degrees, at several known dose equivalent values. The CR-39 was electrochemically etched. Recorded tracks were counted using a semi-automatic system, based on an image analyser. The directional dose equivalent response in terms of H'(10,alpha) was evaluated from the net measured track densities. This response was also simulated using a Monte Carlo method taking into account the dosemeter configuration and etching conditions. Simulation of the response to normally incident neutrons allows a fit of the parameters related to the CR-39 proton registration efficiency. Excellent agreement was found between results estimated from the Monte Carlo method and experimental ones. The calibration factor obtained for estimating the directional dose equivalent with this dosemeter is 5.6 x 10(-3) mSv.cm(2) and the lower detection limit is 60 +/- 20 mu Sv.