Spallation-Fission Competition in Heaviest Elements; Helium-Ion-Induced Reactions in Uranium Isotopes

Abstract

A radiochemical study of f i s s i o n and s p a l l a t i o n products produced by bombardment of U 233, $35 , and U238 with 18-46 Mev helium ions has been made.As i n the case of s i m i l a r studies using isotopes of plutonium as t a r g e t s , most of the reaction c r o s s section i s taken up b y f i s s i o n . Also, t h e pronounced increase of the t o t a l cross section f o r (a,xn) reactions with increasing mass number of the t a r g e t t h a t was observed f o r plutonium t a r g e t s i s observed f o r uranium t a r g e t s .Excitation functions f o r (a,2n), (a,3n), and (a,kn) reactions a r e i nt e r p r e t e d i n terms of compound nucleus formation and f i s s i o n competition a t the various s t a g e s of t h e neutron evaporation chain. The importance of neutron binding energies on t h e competition between f i s s i o n and neutron emission i s stressed. An e x i s t i n g model f o r neutron evaporation following compound nucleus formation has been extended t o include t h e e f f e c t of f i s s i o n competition.

Evaporation of 3 to 8 Neutrons in Reactions betweenC12and Various Uranium Nuclides

“…li'ormula (1) must then be modified to include this effect. Fission may take place at each step in the cascade and the cross section can then be written as: 3,4 vlhere r nand r l' are levE?l widths for neutron emission and fission~ respectively. Again other modes of decay have been ignored during the cascade.…”

Mass spectrometric determination of Rb fission yields from 3He and 4He induced fission of 238U and 232Th

“…Different runs were normalized to a common isotope. The results were averaged and normalized to 1 Table 2. For comparison, the recommended cumulative yields for thermal neutron fission of '.…”

“…The fission of 'j8U and ' "Th by moderate energy helium ions MeV) has been studied previously by both radiochemical (1)(2)(3)(4)(5)(6) and mass spectrometric techniques (7,8), to determine cumulative and independent yields of various fission products. Cumulative yields of fission products at or near stability are used to determine the m a~s -~i e l d curves whereas independent yields are used to determine charge distributions.…”

Survey of Tritium-Producing Nuclear Reactions

“…20,21 The (a,p) reaction is usually thought of as a knock-on reaction. [20][21][22] In which case, if the (a,/) is also a knock-on reaction, its cross section should be smaller than the (a,p) cross section, because the configuration x-p for the target nucleus should be more probable than a y-t configuration in the heavy-element region. Therefore, it would appear that in the heavy-element region the direct-interaction component of the (a,t) reaction is mainly the stripping of one proton from the helium ion.…”