Speaker
Description
The development of accelerator driven systems, spallation neutron sources and other nuclear technologies require cross section data of neutron-induced reactions above 20 MeV. But the cross section data for neutron energies above 30 MeV are not so abundant in EXFOR, mainly due to the difficulties in obtaining monoenergetic neutron sources. Therefore, the activation cross sections have been measured by using quasi-monoenergetic neutrons. Quasi-monoenergetic or continuous-energy neutrons above 20 MeV have been produced by inducing reactions such as $^7$Li(p,n)$^7$Be, $^9$Be(p,n)$^9$B, $^6$Li(d,n)$^7$Be and $^9$Be(d,n)$^1$$^0$B. The work reported in the literature show that the beryllium target has so far been used for producing only such quasi-monoenergetic neutrons with a finite width in energy, and has not been used to produce monoenergetic neutron beams. The PHITS code which combines incorporating the intra-nuclear cascade (INC) model with the Distorted Wave Born Approximation (DWBA) method, was developed for the simulation of the neutron yields of the $^7$Li(p, n)$^7$Be and $^9$Be(p, n)$^9$B reactions over the energy range of 10 to 50 MeV.
In the present work, it is observed that by bombarding 0.25 mm thick beryllium with protons, produce two monoenergetic neutron peaks are produced due to the $^9$Be(p,n)$^9$B reaction. For 35, 40 and 45 MeV protons, respectively, two peaks of neutrons for each proton energy appear at the neutron energies of (i) 29.4 and 31.8 MeV, (ii) 34.5 and 36.8 MeV, and (iii) 39.7 and 42.0 MeV. For validating the simulated monoenergetic neutron peaks, two experimental approaches were employed: one by inducing nuclear reactions with ‘effective’ threshold energies in-between the two peaks of monoenergetic neutrons, and other was to measure the 209Bi(n,4n)206Bi reaction cross section. In validating the monoenergetic neutron peak due to the ground state of $^9$B, samples of $^9$$^3$Nb, $^6$$^3$Cu and $^2$$^0$$^9$Bi were irradiated with neutrons from the $^9$Be(p,n)$^9$B reaction with 35, 40 and 45 MeV protons. The induced activity of $^9$$^0$Nb, $^6$$^0$Cu and $^2$$^0$$^4$Bi radioisotopes observed in gamma ray spectrum provided evidence that $^9$$^3$Nb(n,4n)$^9$$^0$Nb, $^6$$^3$Cu(n,4n)$^6$$^0$Cu and $^2$$^0$$^9$Bi(n,6n) $^2$$^0$$^4$Bi reactions were induced, respectively, by the neutrons of higher energies. These neutron energies are just above the ‘effective’ threshold energies of the respective (n, xn) reactions. In addition, the cross sections of the $^2$$^0$$^9$Bi(n,4n)$^2$$^0$$^6$Bi reaction were measured at 29.6, 31.8, 34.5, 36.8, 39.7 and 42.0 MeV. These newly measured cross sections are close to those predicted by the EAF-2010 and TALYS. The present studies show that the simulated two peaks of monoenergetic neutrons can be experimentally validated and can be used to measure the cross sections for the neutron induced reactions and other applications.
