Fission of Excited Heavy Nuclei

Abstract

ABS'TRAC'T The results of p~~blished calculations on the prompt il~rclear cascade and rluclear evaporation processes are combined with some assumptions regarding the 11uclear fission process and fission widths to calculate the average neutron-to-proton ratios of fission products from Th + %Me\. protons, Th + 87-PvIev protons, and U + 450-Mev protons. Cornparison of the results with experimental data indicates that the fission charge distribution mode characterized by the equal charge displacement hypothesis, hitherto considered a low energy phenomenon, persists in uranium and thoriu~n LIP to the highest energy studied. 'There is also some evidence that fission occurs in excited heavy nuclei before complete de-excitation by particle emission, in agreement with other experimental worli. A. IN'T'RODUC'TIOXParticle evaporation (30) and fission predominate during the de-excitation of heavy nuclei excited to energies above a few million electron volts. These two processes are expected to compete a t each step during the de-excitation in the sense of the following scheme (25): particle particle particle ~~x / I I A "~( E I~I ) 4in which a nucleus of mass A excited to energy E evaporates particles to form successively nuclei of masses A', A", etc. and energies E', E" . . . with fission as an alternative to each particle evaporation step. The decrease in excitation energy per step, E-E', El-El', etc., equals the binding energy of the particle evaporated plus, in each case, the (variable) kinetic energy it carries off. The fragments from the fission of a nucleus of a given excitation energy E will themselves share in some way an excitation energy close to E, and may de-excite by further particle emission. The relative probability that a nucleus in a given energy state de-excites via a specific reaction path is commonly described in terms of a partial "width" of the state for that reaction. Thus the competition between fission and particle emission from a given nucleus a t a given energy may be expressed as a ratio r,/I'ev,D, where is the fission width and rev,, is a sum of particle emission widthsThe ratio is expected to be a function of nuclear charge, mass, and excitation energy.Fission-evaporation competition has been the subject of a number of previous studies, among the more recent being investigations with transuranium nuclei excited t o energies up to 40 Mev (27, 28), and with thorium bombarded with protons of energies up t o 100 Mev (3). 'Afanuscript received Jzrly 22, 1958.

Nuclear charge dispersion of light-mass fission products in the fission of 235U and 238U by medium-energy protons

“…This difference in the behavior of light-and heavy-mass fission products is not yet understood, but the above data show clearly that, even at these moderate energies, the equal charge displacement hypothesis (24) does not hold. This is contrary to the conclusions drawn by Pate (25) for reactions occurring in the same energy region.…”

SEARCH FOR CORRELATIONS OF MOST PROBABLE NUCLEAR CHARGE ZP OF PRIMARY FISSION FRAGMENTS WITH COMPOSITION AND EXCITATION ENERGY

“…11 is the curve for the Cameroil masses (28), with a minimum for eq. [17] a t Z O = 37.2 a t about 96 Mev. The lower parabola is that for the Levy masses (27), with a rnini~num a t Zo = 36.6 a t about 90.5 Mev.…”

“…A nu~nber of papers (10)(11)(12)(13)(14)(15)(16), mostly a t Berkeley, examine this rule in the internlediate energy region (-50 Mev), and give it partial support. Pate (17) has, however, established a strong case against CCR and for E C D even up to very high-energy deposition before fission.…”