A moderator type dose equivalent monitor for environmental neutron dosimetry

The neutron monitors used daily in working areas around particle accelerators must be practical, economical, easy to read and to install and sensitive to low neutron dose equivalents. An environmental neutron detector, which responds satisfactorily to these requirements, was developed at LNF (Laboratori Nazionali di Frascati, Roma, Italy), for neutron monitoring in the area of the storage ring ADONE. It consists of a cylindrical moderator of polyethylene with five TLD-600, sensitive to n+γ radiation and five TLD-700, sensitive to γ radiation, placed in the centre. A simulation of the instrument was performed using the 3-D Monte Carlo computer code MCNP (Monte Carlo Neutron and Photon transport), in order to obtain the theoretical fluence and dose equivalent response as a function of neutron energy. An accurate calibration of the instrument was carried out at the PTB (Physikalisch-Technische Bundesanstalt, Braunschweig, FRG) exposing it to monoenergetic neutron beams 14.2, 5, 0.57, 0.144 MeV and thermal neutrons. A further calibration has been carried out at the CRE ENEA (Frascati, Roma, Italy), with a continuous spectrum from a 252Cf fission neutron source (E-bar = 2.3 MeV). From the results of the calibration, an average instrument response of 1.3-1.5 nC.mSv-1 seems to represent a sufficiently conservative choice in the evaluation of dose equivalent for high energy neutrons (over 5 MeV); however, for neutrons highly degraded in energy, such a factor could give an overestimation of the dose equivalent of more than a factor two. (author)