An Energy and Direction Independent Fast Neutron Dosemeter Based on Electrochemically Etched CR-39 Nuclear Track Detectors Part I: Theoretical Studies

A computer-based model is presented, which simulates the dose equivalent response of electrochemically etched CR-39 to fast neutrons of various energies and angles of incidence. Most previous calculations of the response of CR-39 have neglected the production of recoiling oxygen and carbon nuclei as well as a particles in CR-39, and have considered that only recoil protons resulting from (n,p) elastic scattering in the etch-removed layers contribute to the reponse. We calculate that these 'heavy recoils' and a particles are the major source of electrochemically etchable tracks in bare CR-39 at neutron energies above ~2 MeV under typical etching conditions. Our calculations have been extended to predict the response of CR-39 used in conjunction with various combinations of polymeric front radiators and, in view of the potential use of CR-39 for fast neutron dosimetry, we have determined the radiator stack configuration which produces the most energy independent response (less than ±30% variation over the energy range 70 keV to 19 MeV for normally incident neutrons). Again, the heavy recoils and a particles cannot be neglected and, for energies above ~2 MeV, these produce typically about 20% of the total response of our optimum stack. This type of fast neutron dosemeter is, however, strongly direction dependent. We have integrated the response over all appropriate angles to predict the dose equivalent response for two representative neutron fields, and we suggest a method for minimising the angular dependence.