R.F. Barnes

Nuclear constants of nine transplutonium nuclides

Abstract The total half-lives of 252Cf, 250Cf, 253Cf are 2·646 ± 0·004 years, 13·2 ± 0·5 years and 17·6 ± 0·2 days. The ratios of α-emission to spontaneous fission for 252Cf, 250Cf, 253Es and 244Cm are 31·3 ± 0·2, 1260 ± 40, (1·15 ± 0·03)107 and (7·43 ± 0·01)105. The partial spontaneous fission half-life of 246Cm = (1·66 ± 0·10)107 years. The thermal neutron fission cross sections for 249Cf and 251Cf are 1735 ± 70 barns and 3000 ± 260 barns. A separate determination and a re-evaluation of the data from the MIKE thermonuclear event confirms the 60·5 ± 0· 2 day 254Cf half-life reported by others.

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.

Nuclear decay properties of light einsteinium isotopes

Abstract The light einsteinium isotopes 248 Es, 249 Es, 250 Es and 251 Es were synthesized by irradiating a 249 Bk target with 3 He and 4 He ions. Because of the short half-life of 248 Es, it was necessary to characterize this isotope by using the recoil technique, whereas the other isotopes were chemically purified before their properties were determined. The half-lives of the above nucleides were found to be 28 ± 5 min, 1.7 ± 0.1 h, 8.3 ± 0.2 h and 33 ± 1 h, respectively. A new isomer of 250 Es was discovered and its half-life was determined to be 2.1 ± 0.2 h. The energies of the main α groups for 248 Es, 249 Es and 251 Es were observed to be 6.87 ± 0.01, 6.770 ± 0.005 and 6.488 ± 0.005 MeV, respectively. The α/K X-rays ratio for 249 Es was found to be 0.007 ± 0.001 and the value for 251 Es was 0.008±0.002. Several gamma rays associated with the electron-capture decay of the einsteinium isotopes were observed. On the basis of the present investigation the ground states of 249 Es and 251 Es have been assigned to the Nilsson states 7 2 + (633↑) and 3 2 − (521↑) , respectively.

The alpha and electron capture partial half-lives of 242Am

The partial alpha half life and partial electron capture half life of the ground state of Am/sup 242/ were measured at 32,000 the J-57 engin 1600 years and 960 the J-57 engin 50 years, respectively. The alpha half life of Am/sup 242/ was found to be 7720 the J-57 engin 160 years, in agreement with the more precise specific activity measurements of Wallman et al. The estimated errors are based on standard deviations. (auth)

Nuclear properties of 254Md, 255Md, 256Md, 257Md and 258Md

Abstract The isotopes 254–258Md were produced by irradiating an Es target with 4He ions and their nuclear properties were investigated. A chromatographic separation procedure was utilized to obtain pure Md samples. The half-lives of 255Md, 256Md, 257Md and 258Md were found to be 27 ± 2 min, 75 ± 4 min, 5.4 ± 0.5 h and 56 ± 7 d, respectively, and are in agreement with previously reported values. The α/(α+EC) ratios for 255Md, 256Md and 257Md were measured to be 7 ± 1 %, 8.5 ± 0.8 % and 10 ± 3 %, respectively. The main a groups of 255Md, 256Md, 257Md and 258Md were found to have energies of 7.322 ± 0.005, 7.202 ± 0.005, 7.064 ± 0.005 and 6.716 ± 0.005 MeV, respectively. The nucleide 254Md was observed for the first time and was found to decay by EC with a half-life of 10 ± 3 min. Successive chemical separations of Fm indicate the existence of another 254Md isomer having a half-life of 28 ± 8 min.

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.

Electron capture decay of 240Am

Abstract The γ-ray and conversion electron spectra of mass-separated 240Am samples were measured with high-resolution Ge(Li) and Si(Li) spectrometers. The conversion coefficients of the 42·9, 99·0, 888·7 and 987·7 keV transitions were measured and their multipolarities were established as E2. A ce−-γ-ray coincidence experiment showed that the 888·7 and 987·7 keV transitions originate from a 1030·6 keV level. On the basis of the measured log ft value the ground state of 240Am has been given a spin-parity assignment of 3- with the configuration {n[631] 1 2 +; p[523] 5 2 −} . The 1030·6 keV level of 240Pu has been given a spin-parity assignment of 3+ with the two-quasi-particle configuration {n[631] 1 2 +; n[622] 5 2 +} . Several other low intensity γ-rays were observed. The half-life of 240Am was measured to be 50·8 ± 0·3 hr which is in agreement with previously reported values.

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.

Electron capture decay of 238Am and electric monopole transitions in 238Pu

Abstract Mass-separated samples of 238 Am (T 1 2 = 98 min ) have been used to investigate the EC decay scheme of 238 Am. The γ-ray and conversion electron spectra were measured with high-resolution Ge(Li) and Si(Li) spectrometers. Several electric monopole transitions were identified. The K/L ratios of the EO transitions are in agreement with the theoretical values of Church and Weneser. A γ-ray spectrum was measured in coincidence with the electron lines of the 44.1 keV transition in order to distinguish transitions going to the ground state from those terminating at the 2 + or 4 + member of the ground state band. From these data a decay scheme has been constructed. Assignments of levels already known from the β − decay of 2338 Np and α-decay of 242 Cm have been confirmed. Several other low-spin states have been postulated to account for the observed γ-rays and conversion electrons. From log ft value considerations the ground state of 238 Am has been given an assignment of { n [743] 7 2 − ; p [523] 5 2 − }1 + and the 962.8 keV level of 238 Pu has been assigned to { n [743] 7 2 − ; n [622] 5 2 + }1 − . The half-life of 238 Am has been measured to be 98±3 min. A 5.94 MeV α-group has been assigned to 238 Am decay. The α-branching has been measured to be (1.0±0.4)×10 −4 %.

Ion-exchange behaviour of the transuranium elements in LiNO3 solutions

Abstract The distribution coefficients (KD′s) of the trivalent actinide elements Pu, Am, Cm, and Cf between LiNO3 solutions (containing 0·005 M H+) and Dowex-1 anion resin were measured. A column separation of Am and Cm using this system was investigated at 22·5°C and 87°C and the separation factor was found to be substantially higher than that obtained with the complexing agent ammonium α-hydroxyisobutyrate and cation exchange resin.