CUMULATIVE YIELDS OF LIGHT FRAGMENTS IN U²³⁵ THERMAL NEUTRON FISSION

Abstract

X mass spectronletric study of the relative yields of the strontiun~, yttriunl, zirconium, and 111olybdenum isotopes formed in the thernlal neutron fission of U235 has made possible a detailed examination of structure of the light mass region of the mass-yield curve. The relative yields of this work have been normalized t o 5.777'0 for Sr", and when literature and extrapolated values are talcen for the remaining chains, the fission yields total 100.87'0. IKTRODUCTIOKThere have been illany illass spectroilletric studies of the light illass fragments from the thermal neutron fission of UC3j. Tlle relative yields of the lcrypton isotopes mere first measured by Thode and Grahain (1947) and again by Wanless and Thode (1055). Glendenin et al. (1931) reported the relative yields of the zirconium, 111015 bdenum, and ruthenium isotopes; and Wiles et al. (1953) measured the relative yields of the rubidium and strontium isotopes. The first absolute cun~ulative yields for the light illass frag~nents using a illass spectrometer with the isotope dilution technique were reported by Petruska, Thocle, ancl Tomlinson (1953b). These authors normalized the relative yields of zirconium and nlolybdenunl reported b) Glendenin et al. (1951) to their absolute yielcls of strontium, rubidiunl, and Icrypton so that the total for the light mass group, including some estrapolatecl values, was 100%. Steinberg and Glendenin (1956) using the isotope dilution technique obtained absolute yields for strontium, zirconium, and molj bdenun~ which, together with relative yields for liryptoll and ruthenium and some radiochemical yields, were also forced to total 100%. Recent compilations of fission yields by I<atcoff (1 960) and Wallcer (1960) have been based largely on the latter two publications. The yields of the strontiunl isotopes were the only jields measured both by Petruslca et al. and Steinberg and Glendenin. The yield for SrSO obtained by Petruslta et al. was 5.74y0, which is in excellent agreement with the 5.80% obtained by Steinberg ancl Glendenin. The mass spectroinetric data reported both by Petruska et al. and Steinberg and Glendenin give the relative isotopic abundances of each fission product element with an accuracy of perhaps ~1 % . However, the nornlalization of the relative abundances of the isotopes of one elelllent to those of another element is clependent on isotope dilution data where further uncertainties exist. When one plots a mass-yield curve with the combined data obtained by these authors, it is found that the abundances of the isotopes of one element do not always form a ~11100th curve relation with those of another element. For example, the data of Petruslta et al. indicate structure in the region of mass 90. I t is not clear fro112 these measurements

Mass distributions for nuclear disintegration from fission to evaporation

“…It was also shown by Perry and Fairhall [12] that first-chance fission of the compound nucleus at high excitation energies can produce a single-peaked distribution. At low excitation energies up to the fission threshold, however, the [14], experimental [15], and present results at 0.03 MeV n −1 . double-peaked distributions can be produced by neutrons, this is because of the Coulomb barrier for low energy charged particles.…”

“…Figure3. Mass yield distributions for the fission of 236 U theoretical[14], experimental[15], and present results at 0.03 MeV n −1 .…”

The Anisotropic Plasma-Enhanced Chemical Vapor Deposition SiO2/Spin-on Glass Process for 0.35 µm Technology

“…It was also shown by Perry and Fairhall [12] that first-chance fission of the compound nucleus at high excitation energies can produce a single-peaked distribution. At low excitation energies up to the fission threshold, however, the Mass distributions for nuclear disintegration from fission to evaporation [14], experimental [15], and present results at 0.03 MeV n −1 .…”

“…Figure 3 shows our results for the mass yield distributions for the fission of 236 U. In order to make a better comparison, we have also plotted theoretical and experimental data [14,15] in the same figure. It can also be seen from this figure that the SMM predicts a mass yield distribution consistent with the experimental and theoretical data.…”