Excitation Functions and Nuclear Charge Dispersion in the Fission of Uranium by 0.1- to 6.2-GeV Protons

Abstract

Cross sections for formation of cesium and rubidium isotopes produced by bombardment of uranium with protons ranging in energy from 0.1 to 6.2 GeV were measured both radiochemical^ and mass spectrometrically. Independent yields were determined for Rb 84 , Rb 86 , Cs m , Cs m , Cs 130 , Cs 131 , Cs 132 , Cs 134 , Cs 136 , and, at some energies, Rb 83 and Cs 136 . In addition, the independent yield of Ba 131 and the chain yields of Cs 126 , Cs 127 , Cs 129 , La 131 , Cs 135 , Cs 137 , Rb 83 , Rb 87 , and Ba 140 were obtained. The formation cross sections of the Cs and Ba products on the neutron-excess side of stability decrease monotonically with increasing energy above 0.1 GeV, whereas the excitation functions for independent formation of the more neutron-deficient products in the Cs-Ba region and of Rb 84 and Rb 86 all go through maxima. The proton energies at which these maxima occur fall on a smooth curve when plotted against the neutron-proton ratio of the product, with the peaks moving to higher energies with decreasing neutron-proton ratio. Under the assumption that the mass-yield curve in the region 125 <A <140 is rather flat at each proton energy, the cross-section data in the Cs region can be used to deduce the charge dispersion in this mass range. Plots of logo-vs N/Z (or Z-ZA) show symmetrical bell-shaped peaks up to a bombarding energy of 0.38 GeV, with full width at half-maximum increasing from 3.3 Z units at 0.10 GeV to about 5 Z units at 0.38 GeV, and with the peak position (Z p ) moving from Z^-1.44 to ZA-0.85 over the same energy range. At all higher energies, a double-peaked charge distribution was found, with a neutron-excess peak centered at N /Z~l.515 (Z P~Z A-1.9), and having approximately constant width and height at bombarding energies greater than 1 GeV. The peak on the neutron-deficient side which first becomes noticeable at 0.68 GeV appears to become broader and shift slightly to smaller N/Z values with increasing energy. The two peaks are of comparable height in the GeV region, and the peak-to-valley ratio is only ~2. The total formation cross section per mass number in the Cs region decreases from ~52 mb at 0.1 GeV to about 29 mb at 1 GeV and then stays approximately constant; the contribution of the neutron-excess peak above 1 GeV is about 12 mb. The neutron-excess peak corresponds in width and position to that obtained in fission by ^50-MeV protons. The recoil behavior of Ba 140 leands support to the idea that the neutron-excess products are formed in a low-deposition-energy process. The recoil behavior of Ba 131 indicates that it is formed in a high-deposition-energy process. Post-fission neutron evaporation is required to account for the observed characteristics of the excitation functions of the rubidium isotopes and the neutron-deficient species in the Cs region. The correlation between neutron-proton ratios and positions of excitation function maxima is semiquantitatively accounted for if fission with unchanged charge distribution, followed by nucleon evaporation, is assumed.

CHARGE DISTRIBUTION AND RECOIL PROPERTIES IN THE FISSION OF $sup 238$U.

“…The charge dispersion determined for products with 125<A<140 formed from uranium exhibits two distinct peaks of approximately equal height on either side of the beta stability line (1,2). This finding contrasts with the results below about 0.7 Gev.…”

“…This finding contrasts with the results below about 0.7 Gev. (1,8,9). where the charge disperdion is single-peaked.…”

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

“…A further assumption is that the total chain yields can easily be determined. Friedlander et al (1963), in studying the fission of uranium with high-energy protons, proposed that the abscissa be drawn in terms of N/Z (neutron-to-proton ratio) rather than (Z-ZA)' thus avoiding the discontinuity of ZA at the neutron shell edges.…”

“…A further assumption is that the total chain yields can easily be determined. Friedlander et al (1963), in studying the fission of uranium with high-energy protons, proposed that the abscissa be drawn in terms of N/Z (neutron-to-proton ratio) rather than (Z-ZA)' thus avoiding the discontinuity of ZA at the neutron shell edges.They also assumed the mass-yield curve to be flat in the narrow mass region of the products studied. The actual independent yields could then be plotted directly to give isotopic charge dispersion curves.…”

Analysis of Thick-Target Recoil Studies of High-Energy Nuclear Reactions.

“…The results of this calculation are summarized in Table I In order to apply this relation to the fragments of interest it was necessary to determine the effective nuclear charge with which they are produced at [22][23][24] GeV energies. This infotmation was obtained from charge dispersion measurements and is summarized in Table I …”