Spallation-Fission Competition in Heavy-Element Reactions:Th232+He4

Abstract

Cross sections and excitation functions have been determined for spallation and fiss-ion products from bombardments of Th 232 with helium ions (15 to 46 Mev) and u 2 33 with deuterons (9 to 24 Mev). This work extends a series of investigations of charged particle (a, d, and p) induced reactions in heavy elements (z ~ 88). Radiochemical methods were employed to isolate products corresponding to the following spallation reactions: neutron emis-, sion, (a,4n), (a,5n), (d,n), (d,2n), and (d,3n); emissionof one proton and neutrons (a,p), (a,pn), (a,p2n), and (a,p3n); and emi~sion of two protons and neutrons, (a,2p), (a,2pn), and (a,an), and (d,an). In addition, the following fission products were isolated from one or more bombardments: zn7 2 , The results show that fission is the predbminant reaction at all energies for Th 2 3 2 and to an even greater extent for u 2 33. The data for the surviving spallation products are consistent with several mechanisms of reaction, including compound-nucleus formation and evaporation, direct interactions between nucleons of the incoming helium ion or deuteron and nucleons of the nucleus, and a combination of these types of processes (direct interaction followed by evaporation). In general, the results confirm and extend previously established concepts. The fission mass-yield curves are similar to those found for other heavy target isotopes (~or elements from thorium to plutonium). The minimum in the curves in the region of mass 120 tends to disappear as helium-ion or deuteron energy is increased.The (a,pxn), (a,2pxn), (a,an), (d,n), and (d,an) products are attributed to direct interactions, with complex particles emitted in preference to a series of protons and neutrons. Thus (a,d), (a,t), and (a,tn) mechanisms would account for most of the (a,pn), (a,p2n), and (a,p3n) products, respec-In the case of the (a,t) and (a,tn) reactions, analysis of the ratio leads one to the conclusion that with 35-Mev helium ions only 9% of outgoing tritons leave the residual nucleus with sufficient energy to evaporate a neutron or undergo fission, and with 44-Mev helium ions only 20% do so. The (d,n) product probably results from the stripping reaction.

Dissipative effects in spallation-induced fission ofPb208at high excitation energies

“…Generally, conventional spallation-fission experiments are performed in direct kinematics [26][27][28][29] where the identification of the fission fragments in atomic number is a difficult task and only long-lived target materials can be studied. To overcome these difficulties, the inverse kinematics technique is better suited to unambiguously identify the products of the reaction.…”

Nuclear Reactions Induced in Tantalum by Protons of Energy Up to 84 Mev

“…According t o Lindner and Turkevich (39), the yields of the products of the reactions, formally written as (p,3pxn), are considerably higher than predicted by the cascade-evaporation model (35,36), and they state that such products are perhaps produced by t h e reaction of the (p,apxn) type in which the cu-particle is knocked out of the nucleus during the cascade step, since the evaporation stage seems hardly likely to provide enough charged-particle emission in heavy elements bombarded with 340 Mev proton. Similarly, a t lower energies below 70 Mev, Foreman et al (49) suggest that (a,xpxn) reactions are formed by direct interaction with complex emitted in preference to a series of protons and neutrons. Thus a t low energies below 100 Mev, the large yields of the (p,2pxn) and (p,3pxn) reaction products in Tala1 can best be explained by direct interaction of the incoming particle with aggregates of like H3, He" He4, He6, Li5, Li6, Li7, etc., followed by evaporation of an appropriate number of neutrons, rather than by multiple emission of particles, unless an appreciable reduction of the Coulo~nb barrier, proposed only a t high energies of excitation (50,51), occurs even a t low excitation energies (48) permitting the evaporation of charged particles.…”

High-spin states in 232U investigated with the 232Th(α, 4nγ) reaction

“…For 232U, for example, the ground-state band is known up to 6 + from work on 236pu a-decay and 232Np electron capture [5]. The peak value of the 232Th(a, 4n) reaction cross section at about 40 MeV is ~55 mb, while the fission cross section at that energy is about 1.3 b [6]. A few years ago, additional transitions have been tentatively assigned to the 232U ground-state band on the basis of an (a, 4n~,) experiment with a fission anticoincidence detector [7], though no information was obtained about the multipolarity and coincidence relationships of the transitions.…”