Mass and Energy Distributions in the Spontaneous Fission of Some Heavy Isotopes

Abstract

A back-to-back semiconductor counter system was used to study the energy and mass distributions of the fission fragments in the spontaneous All distributions (including those of the odd-mass isotope E 2 53) are rather similar, but not identical with the standard ci5 2 . The mean prompt kinetic energy of the fragments increases with z, and this trend is compared with the results of other related e~eriments. The asymmetry of the mass distributions shows only small differences between the isotopes studied here.Some theoretical aspects of the asymmetries are discussed. The variance (widths) of nearly all the distributions increases with Z and seems to increase with A. A new semiempirical correlation between the variances and z 2 /A is proposed and interpreted on a qualitative basis.The effects of neutron emission on the results are briefly discussed and the values of the mean total kinetic energy release are corrected for the effect of neutron emission. The value of such a study is to provide experimental data from which some general features of the fission process may be deduced and against which theories may be tested, both generally and in detail.Prior to this work, very little information has been available concerning details.of spontaneous-fission isotopes other than Cf 2 5 2 . 1 Therefore such properties as the mass-yield curves, kinetic-energy distributions, and their widths remained to be studied for a number of heavy nuclei.Such investigations have become possible during the past few years because of advancements in several directions. The first involved production of larger amounts of certain heavy isotopes by multiple neutron capture in high-flux reactors. The second involved application of semiconductor counters for energy measurements. The third development was the availability of multidimensional pulse-height analyzers which allow one to record and store information correlating, :fbr example, two energy measurements from .each event.The fourth advancement is the ready availability of computers for calculating the results of a very large number of measurements.-2- UCRL-10506In the experiments reported here, the energies of both fragments from spontaneous-fission events were studied for several isotopes. From these energies the masses of the fission fragments and total kinetic energies were calculated. The properties of the measured distributions were compared with those of the isotope Cf 2 5 2 , measured under the same conditions. Trends in the p~operties as functions of Z and A and certain fission paramete.rs·::were studied. II. EXPERIMENTAL PROCEDURESTranscurium isotopes used in these experiments were produced by longtime irradiation of curium isotopes with neutrons. As a result, elements as heavy as fermium were produced. 2 The production pa.ths are shown schematically in Table I. The elements were separated on ion-exchange columns.3 $pecial care was taken to remove all contaminations to such a level that their influence could be neglected in the final results. The isolated and purified isoto...

Fission modes ofFm256andFm258in a microscopic app

“…As for the spurious contribution from centerof-mass vibrations to the energy, thoroughly analyzed by Bender et al in Ref. [26], only the traditional one-body contribution E (1) K (leading simply to a renormalization factor 1 − 1/A in the kinetic energy) is considered since the SkM* force parameters were fitted within this framework. As we can see in Tab.…”

“…Before the work of Brandt and collaborators [1] in 1963, very little was experimentally known about detailed spontaneous-fission properties of isotopes other than 252 Cf. These authors measured the mass-yield and kinetic-energy distributions for the spontaneous fission of 254 Fm and obtained a well-marked asymmetric massyield curve.…”

Fingerprints of Heavy-Element Nucleosynthesis in the Late-Time Lightcurves of Kilonovae

“…For the spontaneous fission of 254 Cf, the kinetic energy EKinetic carried by the fission fragments is taken from Ref. [49]. No data is available for the neutron and photon effective energies but they are expected to be much smaller.…”

Mass Symmetry in the Spontaneous Fission ofFm257

“…tWork performed in part under the auspices of the One of the significant aspects of spontaneous fission (SF) of the heavy nuclei is that the fragment mass distribution is strongly asymmetric for A ^ 254. From measurements of fragment kinetic energies for a series of nuclides ranging from 248 Cm up to 254 Cf and 254 Fm, Brandt et al 1 note that the average mass of the heavy fragment remains relatively constant at A = 142 ± 1, while that of the light fragment increases. If this feature were to continue, a true symmetric distribution would not be reached until A approached 284.…”