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The new isotope 12.3-min ' Cf was produced via the ' Cf(t,p) reaction, and a new 43-min isomer of ' Md was produced via the ' Es(a,n) reaction. The fragment mass and kinetic energy distributions from the spontaneous fission of" Cf were found to be very similar to those from the spontaneous fission of lighter Cf isotopes. The mass division is primarily asymmetric, and the average total kinetic energy is 189.8+0.9MeV. The 43-min "Md presumably decays by electron capture and provides an opportunity to study the mass and kinetic energy distributions from the spontaneous fission of. the 380-p,s Fm daughter. The observed narrow, symmetric mass distribution and the most probable total kinetic energy of 238+3 MeV are similar to those reported for the spontaneous fission of" Fm but show a sharp increase in symmetric mass division and total kinetic energy compared to Fm and the lighter Fm isotopes. No such abrupt change in properties was observed for "Cf, which, like "Fm, has 158 neutrons. The marked difference between the spontaneous fission properties of the heavier Fm isotopes and those of other spontaneously fissioning nuclides is compared to some theoretical predictions. RADIOACTIVITY, FISSIONCf (SF); measured T&~&, fragment-fragment coin; deduced TKE, mass distribution. Md; measured''~g2, . EC decay to 5 Fm (SF); measured fragment-fragment coin; deduced TKK, mass distribution.

The neutron-induced fission cross section of Am has been measured over the energy region from 10 eV to -20 MeV in a series of experiments utilizing a linac-produced "white" neutron source and a monoenergetic source of 14.1 MeV neutrons. The cross section was measured relative to that of U in the thermal (0.001 to -3 eV) and high energy (1 keV to -20 MeV) regions and normalized to the ENDF/B-V 2 5U(n, f) evaluated cross section. In the resonance energy region (0.5 eV to 10 keV) the neutron flux was measured using thin lithium glass scintillators and the relative cross section thus obtained was normalized to the thermal energy measurement. This procedure allowed a consistency check between the thermal and high energy data. The cross section data have a statistical accuracy of -0.5% at thermal energies and in the 1-MeV energy region, and a systematic uncertainty of -5%. We confirmed that Am has the largest thermal fission cross section known with a 2200 m/sec value of 6328 b. Results of a Breit-Wigner sum-of-single-levels analysis of 48 fission resonances up to 20 eV are presented and the connection of these resonance properties to the large thermal cross section is discussed. Our measurements are compared with previously reported results.

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We measured the (n, 2n) cross sections for neutron energies from 13.7 to 15.1 MeV on 239 Pu and 241 Am by α-and γ -spectroscopy measurements of the post-irradiation 238 Pu/ 239 Pu and 240 Am/ 241 Am ratios relative to the (n, 2n) cross sections of 196 Au and other calibration reactions. Statistical errors on the measurements were not more than a few percent. Cross-section values ranged from 228 to 214 millibarns for the reaction on 239 Pu and from 260 to 222 millibarns for the 241 Am reaction for neutron energies from 13.7 to 15.1 MeV.