A.M. Friedman

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.

Energy levels of 237U

Abstract The alpha decay of 241 Pu has been investigated using high-resolution semiconductor detectors in conjunction with coincidence techniques. The observed α-particle and γ-ray transistion energies and intensities were found to be consistent with the previously known energy levels of 237 U. The half-life of the 5 2 + (622↑) state (at 160.0 keV) in 237 U has been measured by an α - γ delayed coincidence technique and found to be 3.1 ± 0.1 nsec. The half-life of the 274-keV level, which de-excites by emission of a 114 ± 1 keV γ-ray has also been measured by an α-γ delayed coincidence method and has a value of 155 ± 6 nsec. Three alpha groups populating the 274 keV state and its rotational members were identified in an α-spectrum measured in coincidence with γ-rays emitted 50 to 200 nsec after the emission of α-particles. By analogy with the energy levels of 239 Pu, the 274 keV level has been given a Nilsson-state assignment of 7 2 − (743↑) .

Measurement of rates of fast E1 transitions in AcRa nuclei

Abstract The half-lives of the 40.0 keV level in 225Ac, 31.6 keV level in 225Ra, and 27.4 keV level in 227Ac have been measured to be 0.72 ± 0.03 ns, 2.1 ± 0.2 ns, and 38.3 ± 0.3 ns, respectively. The three levels decay only by E1 transitions. The lifetimes were measured by delayed coincidence techniques using pure samples. The reduced transition probabilities in 225Ac and 225Ra are about two orders of magnitude larger than the values encountered in the mid-actinide nuclei. On the other hand, the transition rate in 227Ac is similar to the values measured in heavier actinides. Previous studies suggest the presence of octupole deformation in all three nuclei. Our results demonstrate that large enhancements in E1 transition rates can occur for nuclei with octupole deformation. However, the results also indicate that there is no simple relationship between the E1 rate and the octupole deformation. The half-life of the 25.4 keV state in 225Ra, which decays by a pure rotational E2 transition, has been measured to be 0.88 ± 0.04 ns. This lifetime yields a quadrupole moment of 6.47 ± 0.15 b and a β2 deformation of 0.17 + 0.004.

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.

Level structure of 225Ac

Abstract The level scheme of 225 Ac has been deduced from the study of the α-decay of 229 Pa and β − decay of 225 Ra. Sources of 229 Pa were produced by the 229 Th(d, 2n) and 230 Th(d, 3n) reactions, and the 225 Ra activity was obtained by milking a 229 Th sample. Gamma singles spectra of 229 Pa and 225 Ra were measured with high-resolution Ge detectors placed in a very low background shield. Alpha-gamma and γγ-coincidence measurements were made with 229 Pa sources. Using the ratio of the KX-ray and γ-ray intensities obtained from gated γ-ray spectra, we were able to deduce K conversion coefficients of the 115.55 and 120.80 keV transitions which gave M1 multipolarities for these transitions. Four bands were identified: K π = 3 2 − , ground; 3 2 + , 40.09 keV; 5 2 − , 120.80 keV; and 5 2 + , 155.65 keV. The half-life and alpha branching ratio, α/(α+EC), were measured to be 1.50 ± 0.05 d, and (0.48±0.05)%, respectively.

Production of Pu245 and Am245 by neutron irradiation of Pu244

Abstract Pu 245 and Am 245 were prepared by neutron irradiation of Pu 244 . The beta-decay half-lives of Pu 245 and Am 245 were found to be 10·1 ± 0·5 hours and 119 ± 1 minutes, respectively. The beta-decay energy of Am 245 was found to be 0·86 MeV by absorption methods, and γ-rays of 260, 121, and 70 keV ∗ were resolved with a sodium iodide crystal spectrometer. The thermal neutron capture cross-section of Pu 244 was determined to be 1·4 barns.

Neutron single particle states above the N=152 subshell☆

Abstract Single-particle states above the N=152 subshell have been studied by high-resolution 250Cf(d,p) reaction. All states between the 152 and 164 neutron subshell have been identified. A tentative assignment has been made for the 1 2 −[761] orbital.

Nuclear levels in 238Np

Low and high energy spectra from thermal neutron capture in 237Np have been studied over the energy ranges 25 to 650 keV and 2600 to 5500 keV. Primary transitions from neutron capture in four resonances have been observed between about 4800 and 5400 keV. Using 12 MeV deuterons, (d, p) spectra at three angles have been observed with a magnetic spectrograph. A nuclear level scheme for 238Np has been constructed by combining the results of the above measurements with previous data from a study of the 242mAm α-decay. The Nilsson model has been used to interpret the level structure. Including results from the previous α-decay study, nine rotational bands can be assigned. The Nilsson configurations (Kπ [Nn3ΛΣ]) and band-head energies are: 2+ π[642↑]−ν[631↓], 0.0 keV; 3+ π[642↑]+ν[631↓], 86.6 keV; 3− π[523↓]+ν[631↓], 136.0 keV; 2− π[523↓]−ν[631↓], 182.8 keV; 5+ π[642↑]+ν[622↑], 278.1 keV; 0+ π[642↑]−ν[622↑], 332.5 keV; 5− π[523↓]+ν[622↑], 342.6 keV; 0− π[523↓]−ν[622↑], 286.0 keV; 6− π[642↑]+ν[743↑], 301 keV. The measured (d, p) reaction cross sections are compared with theoretical calculations based on these assignments. The Gallagher-Moszkowski rule is found to be valid in the four cases where we have observed both parallel and antiparallel coupled bands with K+ = Ωp+Ωn and K− = |Ωp−Ωp|. The lowest levels of the two K = 0 bands have spin I = 1; Newby odd-even shifts can be determined in both cases.