J. Lerner

Measurement of prompt neutron fission yield (v̄p) in thermal neutron fission of 232U, 238Pu, 241Pu, 241Am, 242mAm, 243Cm, 245Cm and in spontaneous fission of 244Cm

Abstract The prompt neutron fission yield (v p ) has been measured in thermal neutron fission of 232 U, 238 Pu, 241 Pu, 241 Am, 242m Am, 243 Cm, 245 Cm and in the spontaneous fission of 244 Cm. A coincidence method yielded ratios to the v p values of the standards 233 U, 235 U, 239 Pu and 252 Cf, with fission fragments detected in an ionization chamber at close to 100 % efficiency, and with fission neutrons detected using four ZnS(Ag) methyl methacrylate discs (Hornyak buttons). The small variation of neutron detection efficiency with fission neutron spectrum differences was calibrated with the four standard nucleides.

Fission cross-sections for 243Pu, 250Bk, 247Cm, 245Cm, 254mEs and 254Es, and odd-odd systematics☆

Abstract Fission cross-sections measured in back-to-back ion chambers in a graphite thermal column of a reactor are: 243 Pu, 196±16 barns; 250 Bk, 960±150 barns; 247 Cm, 108±5 barns; 245 Cm, 2040±80 barns; 254m Es, 1840±80 barns; 254 Es, 3060±180 barns. A more accurate beta decay half-life of 4·955±0·003 hours was found for 243 Pu. 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 ( 254 Es, 242 Am, 244 Am) 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.

Some new properties of 254Es and 255Es

Abstract Alpha decay of the 39.3 h 254m Es isomer was observed and further studies showed that this decay populates a 2 − excited state in 250 Bk, but decay to the 2 − ground state is highly hindered. The partial alpha half-life of 254m Es is 1.52±0.1 y. A lower limit to the spontaneous fission half-life of 254 Es was set at 2.5 × 10 7 y. The total decay and partial spontaneous fission half-lives of 254 Fm were redetermined as 194.4±0.1 min and 228±1 d, respectively. The pile neutron cross sections of 253 Es to produce the two isomers of 254 Es were measured as σ c = 13 b ( 254 Es ), σ c = 338 b ( 254 m Es ) . The partial alpha half-life of 255 Es has been measured as 1.38±0.08 y and the partial spontaneous fission half-life as 2440±140 y. The rotational band structure observed in the alpha decay of 255 Es is consistent with a 7 2 + spin.

Alpha-decay properties of 247Cf, 248Cf, 252Fm and 254Fm

Abstract Alpha-decay properties of 247 Cf, 248 Cf, 252 Fm and 254 Fm were measured using thin massseparated sources. Alpha spectra were measured with Au-Si surface barrier detectors and a magnetic spectrometer, and these were used to determine energies and intensities of α-groups. The energies and intensities are: 6.296 (95 %), 6.238 (5 %) in 247 Cf decay; 6.258 (80.0 %), 6.217 (19.6 %), 6.118 (0.4 %) in 248 Cf decay; 7.039 (84.0 %), 6.998 (15.0 %), 6.904 (0.97 %), 6.759 (0.023 %) in 252 Fm decay; 7.192 (85.0 %), 7.150 (14.2 %), 7.050 (0.82 %), 6.898 (0.0066 %) in 254 Fm decay. Gamma-ray spectra of mass-separated 252 Fm and 254 Fm samples were measured with a high-resolution Ge(Li) spectrometer, and the 2 + → 0 + and 4 + → 2 + transitions were observed in the decays of both nuclei. The half-lives of 247 Cf and 252 Fm were measured to be 3.11 ± 0.03 h and 25.39 ± 0.04 h, respectively. The α branching ratio α (α + EC ) for 247 Cf was found to be (3.5±0.5) × 10 −2 % and the fission/α ratio was measured to be (2.3 ± 0.2) × 10 −3 % which corresponds to a fission half-life of 125 ± 8 y.

Alpha decay of 247Cm

Abstract An enriched sample of 247Cm (99.4 %) has been used to investigate the α-decay scheme of 247Cm. Seven α-groups with energies and intensities 5.265 (13.8 %), 5.210 (5.7 %), 5.145 (1.2 %), 4.983 (2.0 %), 4.941 (1.6 %), 4.868 (71.0 %) and 4.818 (4.7 %) were observed. Gamma-singles and αγ coincidence spectra showed the presence of 278.0, 287.5, 346.0 and 402.4 keV γ-rays. A two-parameter coincidence experiment established that the sources of the 402.4 and the 287.5 keV γ-rays were 4.868 and 4.983 MeV α-groups, respectively. The multipolarity of the 287.5 keV transition was found to be M1. The K conversion coefficient of the 402.4 keV γ-ray indicates an E1 multipolarity for this transition. The levels at 287.5 and 402.4 keV have been assigned to the 5 2 + (622) and 9 2 − (734) Nilsson states, respectively. The half-life of 247Cm was found to be (1.56±0.05) ×107 y from the α pulse analysis and mass spectrometric analysis of the sample.

Half-life of 85Kr☆

Abstract The half-life of 85 Kr has been measured by observing the decay of the 0·7 MeV β-activity over a period of ten years, yielding the value 10·76 years. This is somewhat higher than all of the values previously recorded in the literature. Accuracy of the method and possible sources of systematic error are discussed.

Alpha-decay of 39.3 h 254mEs isomer

Abstract Mass-separated samples of 254mEs were used to investigate its α-decay scheme. Twelve α-groups were identified in a spectrum measured with the Argonne magnetic α-spectrometer. The favored α-transition populates a 211.8 keV level in 250Bk and four members of its rotational band were observed. A three-parameter α-γ time coincidence experiment showed that the 211.8 keV level decayed by three routes: two were prompt and one was delayed. The delayed γ-rays (50.07, 71.30, 90.7, 104.0, 126.0 and 175.7 keV) decayed with a 42±2ns half-life. The energies of the prompt γ-rays were 79.90, 96.3, 177.3 and 211.8 keV. From an α-ce− coincidence experiment the multipolarity of the 211.8 keV transition was deduced to be E1. The α-branching was determined from an α-singles spectrum and was found to be 0.33±0.01%. From the results of the present investigation and the known properties of 250Bk and 254mEs, the 211.8 keV state in 250Bk and the 254mEs ground state were given { n [622] 3 2 + ; p [633] 7 2 + 2 + assignment. Other levels in 250Bk were postulated at 104.0 (and its rotational member at 125.3), 131.9 and 175.3 keV and these were given tentative assignments of n [620] 1 2 + ; p [521] 3 2 − 1 − , n [620] 1 2 + ; p [633] 7 2 + 3 + and n [613] 7 2 + ; p [633] 7 2 O + (with I = 1), respectively. The splitting energies between the parallel and antiparallel coupled states were calculated with a Gaussian potential for the residual neutron-proton interaction and were found to be in agreement with the experimental values. A precise measurement of the energies and intensities of γ-rays and K X-rays associated with the 254mEs β− decay was also made. The Fm Kα2 and Kα1 energies were found to be 115.280±0.015 and 121.065±0.015 keV, respectively.

Nuclear properties of 251Bk

Abstract A new isotope, 251 Bk, has been isolated and found to decay by β − particle emission with a half-life of 57·0 ± 1·7 min. Gamma rays of 34 and 140 keV were observed, and 94 and 184 keV gamma rays are probably present. The apparent absence of observable gamma rays at 58·5 and 81 keV indicates that the ground state of 251 Bk is probably not 7 2+ as might be expected from analogy with 249 Bk.