Accelerator-based neutron fields for the mock up of workplace spectra – first experimental results and comparison with calculations

The energy dependence of the fluence response of a neutron radiation protection monitor generally deviates considerably from that of the fluence-to-dose equivalent conversion function. For the dose determination in a given neutron field such a monitor must therefore be calibrated by determining its yield and the dose equivalent in a neutron field similar to the given field. A wide class of neutron fields encountered at workplaces can be characterized by a superposition of thermal neutrons, a 1/E part in the region of intermediate neutron energies and, occasionally, some high-energy neutrons, e.g. remains of down-scattered fission neutrons. Our approach is to mock up appropriate neutron calibration fields by the production of neutrons in an Li(p,n) reaction (thick target) and the modification of these initial spectra with a moderator assembly consisting of two spherical shells of polyethylene and graphite, respectively. We optimized this set-up by numerical simulation of the neutron spectra with the MCNP Monte Carlo code. Spectra were calculated for the primary and for the moderated source under different angles ϑ, referring to the direction of the incoming protons. The achieved values of the mean fluence-to-ambient dose equivalent conversion coefficients, hΦ∗(10), range from 276 down to 16 pSv cm2. We present results of first measurements with Bonner spheres in the proposed neutron fields. We compare calculated count rates – i.e. folding the simulated neutron spectra with the Bonner sphere response functions – and the measured count rates.