FISSION CROSS-SECTIONS FOR 243PU 250BK 247CM 245CM 254MES AND 254ES AND ODD-ODD SYSTEMATICS

Fission cross-sections measured in back-to-back ion chambers in a graphite thermal column of a reactor are: 243Pu, 196±16 barns; 250Bk, 960±150 barns; 247Cm, 108±5 barns; 245Cm, 2040±80 barns; 254mEs, 1840±80 barns; 254Es, 3060±180 barns. A more accurate beta decay half-life of 4·955±0·003 hours was found for 243Pu. Of the 12 known thermal neutron fission cross-sections for odd-neutron and odd-proton target nuclei, only one cross-section is more than a factor of 2·3 from 1350 barns. A more striking uniformity is observed in the ratio of thermal neutron fission cross-section to target spin: eight of the ten known ratios are grouped at 600±320 barns per spin unit. The dependency of fission cross-section upon target spin is also shown by the three sets of isomers (254Es, 242Am, 244Am) in which the higher fission cross-section of each pair is associated with the higher spin isomer. No simple correlation between fission cross-section and calculated compound nucleus level-density was found. © 1968.