SEARCH FOR CORRELATIONS OF MOST PROBABLE NUCLEAR CHARGE ZP 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. To a crude approximation, the distribution of nuclear charge between light and heavy partners L and H leads to the most probable charges (Z_p)_L and (Z_P)_H displaced from the respective charges Z_A of β-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_A vs. A function for different neutron- and proton-shell regions must be considered. All available data for thermal fission U²³⁵(n_th,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 etal., 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 β-decays from stability to 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 Z_P is a single-valued function of A, known to about ± 0.15 unit for low-energy fission and ± 0.25 unit for medium-energy fission, and that the fine structure very probably present is an indication of intrinsic nuclear chemistry.