J. Milsted

Nuclear properties of 242Cf, 243Cf, 244Cf and 245Cf☆

Abstract Irradiation of 242 Cm and 244 Cm by 3 He ions produced two new isotopes of californium; 242 Cf emits a 7.35 ± 0.01 MeV α-particle group and has a half-life of 3.2 ± 0.5 min, 243 Cf emits 7.06 ± 0.01 and 7.17 ± 0.01 MeV α-particle groups and decays with a half-life of 12.5 ± 1.0 min. New values for the α-particle energies and half-lives were obtained for 244 Cf and 245 Cf.

Energy levels of 243Am

Abstract The energy levels of 243Am have been investigated by studying the γ-rays and conversion electrons of 243Pu and α groups of 247Bk by highre-solution semiconductor detectors. Some new γ-rays were identified and the multipolarities of the 381.7 and 356.4 keV transitions were measured to be M1. The half-life of the 84.0 keV level was measured by a delayed β-γ-ray coincidence technique and found to be 2.34±0.07 ns. Nine α groups belonging to 247Bk decay were identified and the favoured transition was found to populate a level at 267 keV above the ground state. The Nilsson-state assignments to the 243Am levels agree with the previous assignments. Coriolis interaction calculations were performed to explain the contraction of the 5 2 + (642↑) band and the branching ratios of the intensities of the γ-rays de-exciting the 7 2 + (633↑) band to the rotational members of the 5 2 + (642↑) band. The calculated values of the level spacings the γ-ray intensity ratios and α intensity ratios were found to be in good agreement with the respective observed quantities.

Decay properties of berkelium-249 and californium-249☆

Abstract The following decay properties have been measured: the alpha/beta branching ratio of 249Bk, (1·45±0·08) × 10−5, the alpha half-life of 249Cf, 345±15 yr, the spontaneous fission half-life of 249Bk, (1·87±0·09) × 109 yr, and the spontaneous fission half-life of 249Cf, (6·87±0·33) × 1010 yr, which appears to be anomalously long. Observations of the alpha particle spectra of these two nucleides are also reported.

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.

THE ALPHA HALF-LIFE OF BERKELIUM-247 A NEW LONG-LIVED ISOMER OF BERKELIUM- 248

Abstract A Bk 247 sample was prepared by intensive helium ion bombardment of a curium sample containing Cm 246 . The alpha half-life was determined by a mass-spectrometric isotopic dilution method, using Bk 249 as the diluent. The value obtained was 1380±250 y, considerably shorter than previous estimates. The isotopic analyses disclosed a species of mass 248 in constant abundance in three samples analysed over a period of about 10 months. This was ascribed to an isomer of Bk 248 with a half-life greater than 9 y. No growth of Cf 248 was detected, and a lower limit for the β − half-life can be set at about 10 4 y. No alpha activity attributable to the new isomer has been detected; the alpha half-life is probably greater than 300 y.

High-resolution α-spectroscopy of 253Es and 255Fm

Abstract Alpha-particle spectra of 253Es and 255Fm have been measured with a magnetic α-spectrometer. Isotopically enriched samples prepared in the Argonne isotope separator were used for these measurements. Precise energies and and 255Fm α-groups have been determined. Two new α-groups have been decay of 255Fm. The relative intensities of α-groups populating members of are found to be in good agreement with the theoretical values. For the favored theoretical calculations cannot reproduce the experimental intensities. The intensities of α-group feeding the 5 2 + [642] band in 249Bk has been by a Coriolis interaction calculation.

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.