Th²³² (n,2n)Th²³¹ CROSS SECTION FROM THRESHOLD TO 20.4 MEV

Abstract

The excitation curve for the reaction Th232 (n,2n) Th23l has been measured by the activation method from the threshold energy, 6.34 Mev, to 20.4 Mev, relative to the known cross section for the S32(n,p)P32 reaction. Monoenergetic neutrons were obtained from the D ( d , n ) He3 and T (d,n) He4 reactions employing a Tandem Van de Graaff accelerator. From threshold to 9.0 Mev, the (n,2n) cross section rises rapidly, reaching its maximum value of 1.88f 0.09 barns in the region of 9.5 to 11.0 Mev. Above 11.5 Mev the (n,2n) cross section decreases due t o competition of the (n,3n) and (n,2nf) reactions and a t 20.4 Mev it has a value of 0.226f 0.01j barns. INTRODUCTIONAlthough many measurelnents of fast neutron cross sections (n,2n), (n,p), and (n,a) have been made with 14-Mev neutrons, relatively few data have beell published over a range of neutron energies for which these reactions have appreciable cross sections. Due to the scarcity of data on neutron cross sections as a function of energy and the lack of agreement among existing results, a program has been initiated to measure fast neutron cross sections from threshold energies to about 21 Mev. These cross sections will be determined whenever possible with a n accuracy of f 5%.The recent development of the Tandem Van de Graaff accelerator now makes it possible to obtain measurements of fast neutron cross sections over a greater energy range.By accelerating deuterons to a precise energy, the reactions D (d,n) He3 and T (d,n) He4 can be used t o produce monoenergetic neutrons a t any energy from about 5 t o 21 Mev.Results have been published on the cross section for absorption reactions of fast neutrons with Th232. T h e (nly) reaction has been measured up to 14 Mev (1) and the fission cross section has been measured by several investigators from threshold energies up to 11 Mev (1, 2) and a t 14.5 Mev (1, 3). However, data on the significant (n,2n) cross section are only available a t a few energies (4, 5). T h e present investigation was undertaken t o determine the cross section for the reaction ThZ3"n,2n) ThZ3l from threshold, 6.34 Mev, to 20.4 Mev. The reaction (n,p) P3? was used to monitor the fast neutron flux. After the irradiation, thorium was purified radiochemically, and the as well as the P32 produced were determined by absolute beta counting. The (n,2n) cross section a t a particular neutron energy was then calculated from the ThZ3l-and Pa?-activity measurements and the lcnown cross section for the sulphur (n,p) reaction (6). E X P E R I M E N T A L METHOD A . Neutron SourcesNeutrons with energies froin 6 t o 11.6 Mev were obtained from the reaction D (d,n) He3 employing the Chalk River Tandem Van de Graaff accelerator. A gas target cell similar to that described by Johnson and Banta (7) and illustrated in the lower part of Fig. 1 was used to contain the deuterium. The bombarding deuteron beam was magnetically analyzed, focused with a set of quadrupole magnets, and collimated by a 0.125-in. gold aperture. Deuterons scattered off the edge of the...

THE S32 (n,p) P32 REACTION AS A FAST-NEUTRON FLUX MONITOR

“…For cross-section measurements above 12 Mev, neutrans were produced using the T (d,n) He4 reaction. This irradiation assembly was also described in a previous paper (6). Zirconium-tritium targets (1.2 cm3/cn12 of tritium a t NTP) were bombarded with 125 kev deuterons from a Texas Nuclear Neutron Generator, and with 4.00 RIev deuterons from the tandem accelerator.…”