Experiment proposal for the determination of neutron spectra from targeted electron beams

There is a dearth of experimental data on the production and yields of neutrons from targeted electron beams; yet, for accelerator radiation protection these data are of the greatest importance in setting up methods of shielding and other means for protecting people against ionizing radiation. Although adequate for simple cases and lateral production angles, empirical analytical methods are not suitable for the more complicated geometries or source configurations often met with in practice. Monte Carlo (MC) methods that model the transport of neutrons provide far better results in many cases but rely on the random generation of the energy of a source particle selected for any beam condition, production angle and target configuration. A number of theoretical approaches to the derivation of a model for the production of particle events at energies greater than the giant resonance region have been made. Many of these are based on the quasi deuteron model of the nucleus and operate over photon energies in the range 30 MeV to 400 MeV. A method is also available, based on the vector meson dominance model which is designed to work above the photopion resonance region where the cross section levels off at a few GeV (Ranft 1987). Both of these models are limited in utility to a certain energy range and both show some discrepancies with existing empirical methods. More recently a new fragmentation model was developed, which could be used over a large energy range and modeled all production processes. This new method also showed differences from the traditional approaches and a thorough comparison indicated that the event generator in conjunction with conventional MC transport codes produced results a factor two to three higher than the results using the empirical methods. This unsatisfactory situation can only be resolved by making measurements of proper physical quantities in the radiation field. Measurements of the angular fluence spectrum will permit a satisfactory resolution of the uncertainties surrounding the event generators and the transport codes. The conversion of the known energy fluence to the dose equivalent can then be dealt with by using an appropriate set of conversion coefficients. The measurement of the angular fluence spectrum is, however, not easy to do and this is particularly the case where interference from background radiation is high. The purpose of this paper is to describe a suitable spectrometer and discuss a suitable experiment which will answer many of the questions touched on above