A fast neutron irradiation station has been designed at the University of Massachusetts -Lowell. The fluence delivered by this station is less than that of a nuclear reactor but it is essentially free of slow neutrons and the admixture of gamma rays is small. We describe the station, dosimetry methods, a computer code, verification techniques and applications. The information we provide should enable others to construct a similar station on their type CN or tandem accelerators.
I. INTF~ODUC~ONNeutron scattering studies in the MeV-region have been performed at Lowell in the past 25 years. In much of this work neutrons were generated by irradiating a thin metallic lithium target with protons produced by the Lowell 5.5 MV, type CN Van de Graaff accelerator. For the purpose of obtaining a neutron spectrum, which covers in a continuous form fast neutron energies from zero to 3 MeV, we started using thick metallic lithium targets in 1983. These thick targets have not yet been used in our neutron scattering work, but they have found application in a variety of studies. We estimate that, to this date, about 100 thick lithium targets have been used with a total run time in excess of lo00 hours. It seems, at this writing, that the technique and associated procedures have matured and that our experience should be reported to a wider audience.Applications of our neutron source cover a broad spectrum. Most work involved radiation damage studies in electronic devices, but it also included investigations of optical devices, irradiation of living biota, such as bacteria and seeds, and of dead specimens, e.g. bone. Our fast neutron source does not have the same fluence as a nuclear reactor, it generates about 6.2~lO~~ neutrons/s or about as much as 1 W (thermal) reactor or as much as a 25 Ci californium source. This source would cost close to %2x106, not including shielding and handling equipment. Some users prefer our fast neutron station to a reactor because they see one or the other of the following advantages: (i) The small number of gamma rays accompanying the neutron flux, typically one 422-keV gamma per U) fast neutrons or about 3~1 0~' neutrons/an2 rad(Si), (ii) the open geometry, which allows the placement of large pieces of auxilary equipment such as ovens, dewars, pressure or vacuum chambers etc. right up to the fast neutron station, (i) the simple neutron dosimetry based on 7Be assays, (iv) the point-like nature of the source which generates a fluence with a l/r2 dependence, and (v) the possibility of using the source with a pulsed beam with a 0.25 ns burst duration. Our station shares with fast burst reactors the advantage of the virtual absence of slow neutrons (E< 10 keV), with a slow-to-fast ratio under 0.1%.
SOURCE DESCRIPTION
A. The nuclear reactionsThe cycle of isoto es involved is shown in Fig. 1 [l]. Protons incident upon L i transform it into *Be, which decays to 7Be, via neutron emission. The decay may proceed to the ground state of 'Be, involving no, the first neutron group, or it may lead to the first exc...