Accelerator Stray-Neutron Dosimetry: Spectra Oo Low- and Intermediate-Energy Neutrons

The control of stray radiations from the shields of large accelerators depends on good dosimetry and, in turn, on an understanding of the energy spectra and fluxes. For this reason, and because neutrons are generally considered the particle of greatest biological interest, the Health and Safety Laboratory has investigated stray neutron spectrometry for several years. The important requirement that the measurement devices be transportable, and the difficulties often encountered in the use of counters and similar detectors at accelerators, led first to the use of nuclear emulsions for the region between 0.5 and about 20 MeV. Supplementary neutron-flux measurements were obtained with cobalt-paraffin neutron-flux integrators. The emulsion processing and scanning techniques, and the iterative unfolding computations, have been discussed elsewhere. To extend the spectrometric range above 20 and below 0.5 MeV, we used variations of the multisphere Bonner spectrometer. For these systems, calculated response functions were incorporated into the unfolding computations, because these functions contain no interpolated maxima, as do those developed originally from measurements. Within the past year, measurements have been obtained at selected locations near a number of accelerators in the United States. Measurements with the two spectrometers are compared to evaluate the effect of the different energy ranges and resolutions. To expand further the measurable neutron energy range, two higher-energy spectrometers are in development. One is a Bonner spectrometer, with an 18-in.-diam. polyethylene sphere, to improve the resolution at high energies, near 100 MeV. The other is a neutron spectrometer with a small proton spark chamber designed for the 10 to 200 MeV region. From information on energy spectra and flux, we can obtain estimates of the important derivative quantities of average absorbed dose or dose-equivalent. We routinely infer from the spectral results the specific dose in rads/n cm2. The present results permit direct comparisons among various accelerators and indicate they are an aid in examining shield adequacy and selecting survey instruments. The two higher-energy spectrometers, together with the emulsion and standard Bonner spectrometer, will permit more extensive investigations of stray neutron spectra from thermal energies to about 200 MeV. (author)