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

Abstract

The repartition of nuclear charge in fission has a narrower dispersion than almost any other property connected with the fission process. T o a crude approximation, the distribution of nuclear charge between light and heavy partners L and H leads to the most probable charges (ZP)L and (Zp)= displaced from the respective charges ZA of 8-stability by the same amount for the two fragments (Glendenin rule of equal charge displacement ECD, 1946). The existence of shell offsets in the Z,i vs. A function for different neutron-and proton-shell regions must be considered. All available data for thermal fission U235 (nt~,,F) are examined critically. The data show sudden offset-like drifts (fine structure) that may well be associated with shell properties of the products before the "neck" has dissolved. It is shown that these data eliminate naive equal charge displacement ECD, also an older competitive prescription of constant charge ratio CCR for the products, and an empirical Russian prescription (Apalin et al., 1960). The data are also examined in the light of the postulate that fission gives minimum nuclear plus coulombic potential energy (Present 1947, Fong 1955, Swiatecki-Blann 1960, and it is shown that the present mass formulas give too much uncertainty three to four &decays from stability t o give a useful test, but that shell effects in masses must be retained. Data from charged-particle fission with energy deposit up to 40-50 Mev are in reasonable accord with the low-energy data on correcting for composition and neutron boil-off. It is concluded from experiment that Zp is a single-valued function of A, known to about f 0.15 unit for low-energy fission and f 0.25 unit for medium-energy fission, and that the fine structure very probably present is a n indication of intrinsic nuclear chemistry.The early thinking on the wartime Manhattan Project about nuclear charge distribution in low-energy fission, before any yield data were available, was based on the concept (1, 2) of constant charge ratio CCR. Professor Wheeler had suggested (4) a search for positron enlitters t o see how wide the charge dispersion might be: the results of crude searches in the products of U235 (n,h,F) were, of course, negative (5). Way and Wigner (7) postulated t h a t the charge would distribute t o give a minimum nuclear potential energy, and Present (8) postulated a minimum for the sun1 of nuclear potential energy plus coulombic energy (equal to final kinetic energy). Present took considerable pains to handle the coulombic term for polarizable spheres with non-uniform proton density.The identification of shielded nuclides4 36-hour BrS2, 19-day Rbs6, and 13-day Cs136 among the fission products gave the first experimental leverage on charge distribution. The first detailed treatment of the problem (5) established the classical rule, or prescription (3, 6) of equal charge displacement E C D ; namely that the nuclear charge distribution between light and heavy partners L and H leads to most probable charges Zp displaced from stability...

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

“…At the sarne time, the minimum potential energy models were revised by Swiatecki and Blann (1960). Their predictions were shown to agree with data from low-energy fission (Coryell et al, 1961). As expected, the charge dispersion curves seemed to be distorted by the effects of shell edges (Wahl, 1958; Colby and CObble, 1961).…”

Relative Abundances for Six Delayed Neutron Groups from Reactor Neutron-Induced Fission of 281Pa

“…For the charge distribution of 231 Pa fission products, there is still lack of published data. The Z p , however, and hence the cumulative yields of the various delayed neutron emitters from 231 Pa fission, were calculated using the Z v values by WAHL et al [13] for 236 U thermal neutron fission in the expressions derived by COKYELL et al [14]. The average number of neutrons emitted in fission of 231 Pa was taken as 2.4 [5] and the constant of the Gaussian charge distribution 0.8 [13].…”

Recoil Ranges and Kinetic Energy Distribution in the Reactor Neutron Induced Fission of 241 Am

“…The values for Ζ ρ in 841 Am fission were obtained using CORYELL'S Eq. [11] and Ζ ρ values in ^U fission. The second range-energy equation used is the one given by ALEXANDER and GAZDIK [4] where the range R is related to the kinetic energy E by Eq.…”