Characterization of a tunable quasi-monoenergetic neutron beam from deuteron breakup

Abstract

A neutron irradiation facility is being developed at the 88-inch cyclotron at Lawrence Berkeley National Laboratory for the purposes of measuring neutron reaction cross sections on radioactive targets and for radiation effects testing. Applications are of benefit to stockpile stewardship, nuclear astrophysics, next generation advanced fuel reactors and cosmic radiation biology and electronics in space. The facility will supply a tunable, quasi-monoenergetic neutron beam in the range of 10–30MeV or a white neutron source, produced by deuteron breakup reactions on thin and thick targets, respectively. Because the deuteron breakup reaction has not been well studied at intermediate incident deuteron energies, above the target Coulomb barrier and below 56MeV, a detailed characterization was necessary of the neutron spectra produced by thin targets.Neutron time-of-flight (TOF) methods have been used to measure the neutron spectra produced on thin targets of low-Z (titanium) and high-Z (tantalum) materials at incident deuteron energies of 20MeV and 29MeV at 0°. Breakup neutrons at both energies from low-Z targets appear to peak at roughly half of the available kinetic energy, while neutrons from high-Z interactions peak somewhat lower in energy, owing to the increased proton energy due to breakup within the Coulomb field. Furthermore, neutron spectra appear narrower for high-Z targets. These centroids are consistent with recent preliminary proton energy measurements using silicon telescope detectors conducted at LBNL, though there is a notable discrepancy with spectral widths. Prospects for producing a tunable, quasi-monoenergetic neutron facility of 106–108n/cm2/s at LBNL are promising.