Abstract
We analyzed mass-yield distributions from fission of , , , , and with 14.8-MeV neutrons to study the details of the fission distribution as a function of target mass number at a fairly low excitation energy. The fission yields of individual products, both cumulative and independent, vary smoothly and as expected with uranium target mass number. The over-all distribution of yields becomes significantly narrower as the uranium mass number is increased. The average neutron emission associated with the very-low-yield products is about 3.5, significantly less than the value of about 4.5 for the fission fragments as a whole. This difference may be due to an increase in energy required for reactions leading to the low-yield products. The peak/valley ratio increases by a factor of more than 2 as the mass number is increased from 233 to 238, and decreases linearly with increasing excitation energy, with the even-mass and odd-mass uranium isotopes defining lines that are approximately parallel and separated by 1.4 MeV. This difference can be explained either by an odd-even effect in the neutron-evaporation/fission competition as a function of target mass number, or by an odd-even effect in the ratio of symmetric to asymmetric fission as a function of mass number. One consequence of the rapid change in peak/valley ratio with mass number is that fission of with 14.8-MeV neutrons would be expected to produce a flat-topped mass-yield distribution, with a peak/valley ratio of about unity, while fission of lower-mass uranium isotopes would be expected to be predominantly symmetric. The average value of was found to be -0.16 ± 0.02 for the shielded product and -0.20 ± 0.01 for . This difference may be due to the change in prefission neutron emission with uranium mass number. All of the available data on fractional chain yields for fission of at MeV were used to estimate for MeV. The scatter in the values is too large to allow a smooth curve to be drawn relating as a function of mass number. It was found, however, that values are larger on the average for heavy-fragment products () than for light-fragment products () over this range of excitation energy. The well-known function for thermal-neutron fission of , together with the and relationships reported here, can be used to estimate values and fractional chain yields for fission of the various uranium isotopes with 14.8-MeV neutrons.
- Received 23 June 1972
DOI:https://doi.org/10.1103/PhysRevC.6.1827
©1972 American Physical Society