Improved modelling of the neutron spectrum for the ASP accelerator

As part of CCFEʹs nuclear data and technology programmes a series of material irradiations have been performed at the ASP accelerator to determine integral reaction cross-sections for fusion relevant materials. The integral reaction cross section can be used as part of the validation of the cross section data. The ASP machine accelerates deuterons onto a tritiated target to produce approximately 14 MeV neutrons via the D–T fusion reaction. These neutrons interact with the material creating radioactive isotopes. The gamma emissions from the activated products are then measured using a high resolution gamma spectroscopy system. An important part of evaluating the results of these and future experiments lies in an accurate determination of the neutron energy spectrum. Initially a neutron spectrum determined by MCNP modelling was used based on a source term calculated using relativistic kinematics. The work reported here improves the understanding of the neutron spectrum using a combination of enhanced modelling and experimental data as input information to be used in spectrum unfolding. Recent advances in simulation techniques allow us to use deuteron cross sections for low energy deuterons and hence model the production of neutrons by the deuterons explicitly. This means that it is possible to model the effect of changing various deuteron beam parameters such as radius, energy and position on the target to understand what influence these have on the spectrum seen at the material of interest. The spectrum unfolding, based on threshold reactions, has not previously been performed for the ASP accelerator and provides an experimental method to improve the understanding of the neutron spectrum in the irradiation position. The combination of both modelling and experimental work to improve the understanding of the neutron spectrum has led to better understanding of the facility and the influences on the neutron spectrum. The combination of modelling and experimental work performed in this work to characterise the ASP neutron spectrum may also be applicable to other current and future neutron irradiation facilities such as IFMIF where an accurate knowledge of the neutron spectrum in each irradiation location will be required.