DISTRIBUTION OF NUCLEAR CHARGE IN THE PROTON-INDUCED FISSION OF Th²³²

Abstract

The cross sections for the independent formation of 1131, '~' c I~~, 11=, and 113' a~l d for the cumulative formation of T C '~~, IIJ3, and in the proton-induced fission of Th232 have been measurecl a t nine proton energies between 8 and 87 Me\-. From these data and from published total chain yield data, chargc distribution curves havc becn obtained. These are compared with published chargc distribution curves for ~~r a n i u m fission a t energies up to 480 Mev and together they show a shift of the most probable fission product charge toward stability and a widcning of the distribution with increasing bombarding energy. These phenomena are interpreted in terms of current models for nuclear reactions. INTRODUCTIONA voluminous literature exists on the distribution of nuclear mass in the fission process. In spontaneous fission (31) and in the fission of U233, UZ3j, and P~'~9 v i t h neutrons of thermal energy, the familiar double-peaked distribution, correspondi~lg to a preponderance of asymmetric fission modes, is observed (30). As the energy of bombarding particles is increased the mass distribution in the fission of these and other heavy nuclei becomes more and more symmetric (18, 33) and a t sufficiently high energies a singly peaked distribution is observed (19).About the nuclear charge distribution less is known. Data exist on the distribution from the fission of US3 with thermal neutrons (6, 7, 21) and with l4-3/Iev neutrons (34). Data also exist for the fission of uranium by 170-Mev protons ( I ) , of bismuth by 190-Mev deuterons (lo), and of uranium, thorium, and bismuth by 480-Mev protons (16), and there is fragmentary informati011 from other systems. T h e present work was undertaken to investigate the changes that occur in the charge distribution from protoninduced fission of T h T 3 h s the bombarding energy is raised from 8 to 90 hilev, the maximum proton energy of the McGill synchrocyclotron. This energy range is of interest in view of the substantial changes observed in the mass distribution (33). Also in this interval a change presumably begins in the nature of the initial step in nuclear reactions, from simple compound nucleus formation to a mechanism of direct interaction with individual nucleons (29). Thus a t the lower energies studied, excitation of the nuclei a t the end of the first step of the reaction will be essentially monochromatic whereas a t the higher end of the bombarding energy range, a broad spectrum of excitation energies will be produced, with corresponding complexity of the reaction products observed. Ideally, in the determination of a charge distribution, one would wish to measure the independent yields of each of the nuclides in one or more fission chains. This, ho~vever, is precluded by the short half-lives of most of the members of any given mass chain. One substitutes a determination of independent yields of accessible nuclides in chains of different mass number, followed by correlation of these data. Correlation is customarily attempted by means of semiempirical ...

Nuclear charge distribution in the region of symmetric fission of 238U by protons of energy 20–85 MeV

“…The independent formation cross sections for the nuclides l171n, l12Ag, and ll1Ag were computed directly at each energy (1,13) and are shown in Table 4. The independent cross section of l15Cd was computed from the measured independent yield of llSrnCd and the ratio of the isomeric products 5mCd/ 5gCd given by Tilbury and Yaffe (38) in this energy range.…”

“…Nevertheless, a variety of fissile systems (1)(2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14) has been studied at medium energies.…”

A STUDY OF Cd115m,g FORMED IN THE FISSION OF NATURAL URANIUM BY PROTONS OF ENERGIES 8 TO 60 MEV

“…I t has been shown that, a t thermal energies, Z,, the most probable nuclear charge, is about 3 charge units removed from stability. As the bombarding energy increases, however, 2, moves towards stability (12,13,14). In the mass 115 chain, 47Ag115 will be the most probable species formed directly in the fission process, a t low energies, with relatively large yields of 46Pdn5 and 48Cd115.…”

Nuclear charge dispersion of products in the light-mass region formed in the fission of 233U by protons of energy 20–85 MeV

“…It is the aim of the present work to contribute to knowledge concerning the division of charge in the fission pro- (1955) proposed that if the independent yields of members of a decay chain were assumed to be distributed about the most probable charge in a manner approximated by a Gaussian curve, then three points could define the charge dispersion curve. This non-theoretical choice of the form of charge dispersion curves was followed by others, including Wahl et al (1962), Pate et §! (1958), Forster et al (1966), and Rogan and Sugarman (1969).…”