W.T. Milner

Coulomb excitation of vibrational-like states in 166−170Er

Abstract Direct E2 and E3 Coulomb excitation of 2+ and 3− states with 14 MeV 4He ions on isotopically enriched targets of 166Er, 168Er and 170Er has been measured by means of γ-ray spectroscopy. Several 2+ states and 3− octupole vibrational states were identified in each nucleus. The B(Eλ, 0 → J = λ) is obtained for excitation of each state, and information is given on the reduced transition probabilities for the different decay modes of these states. The experimental results are compared with theoretical predictions of nuclear models describing these states.

Coulomb excitation of states in 238U

Abstract Twenty-two states in 238 U have been observed with 18 MeV 4 He ions on a thick target. Eight 2 + states between 966 and 1782 keV and three 3 − states are populated by direct E2 and E3, respectively. The remaining states are either weakly excited by multiple Coulomb excitation and /or populated by the γ-ray decay of the directly excited states. Spin assignments are based on γ-ray angular distributions. Reduced transition probabilities have been deduced from the γ-ray yields. The B (E2) values for excitation of the 2 + states range from 0.10 to 3.0 W.u. (281 W.u. for the first 2 + state). For the 3 states, the B (E3, 0 → 3 − ) values are 7.1, 7.8, and 24.2 W.u. Several of the 2 + states have decay branches to the one-phonon states with B (E2) values between 27 and 56 W.u. which are an order of magnitude larger than the B (E2) values between the one- and zero-phonon states. This disagrees with our present understanding of collectivity in nuclei if these 2 + states are considered to be collective two-phonon excitations. However, the excitation energies of these 2 + states with respect to the one-phonon states are only 1.3 to 1.6. The B (E1) values for 17 transitions between the positive- and negative-parity states range between 10 −3 and 10 −7 W.u. The B (E1) branching ratios for many of these transitions have large deviations from the Alaga-rule predictions. These deviations can be understood by the strong Coriolis coupling between the states of the one-phonon octupole quadruplet in deformed nuclei. The general features of the experimental results for the B (E3) values are reproduced by the microscopic calculations of Neergard and Vogel when the Coriolis coupling between the states of the octupole quadruplet is included.

Coulomb excitation of states in 229Th

The Coulomb excitation of states in 229Th has been observed using 17.0-MeV 4He2+ ions. For the 5/2+[633] ground-state rotational band, we deduced an intrinsic quadrupole moment, Q20, of 8.816 ± 0.090 eb and an intrinsic hexadecapole moment, Q40, of 3.69 ± 0.72 eb2. This Q40 value for 229Th is 43% larger than that for the even-even neighbor, 230Th, and is the largest thus far observed in the actinide region. Large E3 matrix elements are extracted for states at 512, 562, and 611 keV, which supports the assignment of these states as members of a Kπ = 0− octupole-vibrational band built on the 5/2+[633] ground state. This 0− octupole excitation occurs at much higher energy than the one inferred from earlier decay studies that is built on the 3/2+[631] state. This suggests state-dependent octupole correlations consistent with octupole shape transitional models of 229Th.

Coulomb excitation of states in 232Th

Abstract Twenty-five states in 232 Th have been observed with 18 MeV 4 He ions on a thick target. Eleven 2 + states between 774 and 1554 keV and three 3 − states are populated by direct E2 and E3, respectively. The remaining states are either weakly excited by multiple Coulomb excitation and/or populated by the decay of the directly excited states. Spin assignments are based on γ-ray angular distributions. Reduced transition probabilities have been deduced from the γ-ray yields. The B (E2) values for excitation of the 2 + states range from 0.024 to 3.5 W.u. (222 W.u. for the first 2 + state). For the 3 − states, the B (E3,0 → 3 − ) values are 1.7, 11, and 24 W.u. A possible two-phonon state at 1554 keV, which is nearly harmonic, decays to four members of the one-phonon states, to the ground-state band, and to the K = 0 − octupole band. The B (E2) value for excitation of this state is 0.66 ± 0.05 W.u. and the B (E1) values for decay of this state are (2 and 6)×10 −4 W.u. The B (E2) values between two- and one-phonon vibrational states range between 16 and 53 W.u. which are an order of magnitude larger than the B (E2) values between the one- and zero-phonon states. This disagrees with our present understanding of collectivity in nuclei if this 2 + state is considered to be a collective two-phonon excitation. The 2 + states at 1477 and 1387 keV, which are also nearly harmonic, are possible candidates with two-phonon structure. The agreement between the experimental results and the microscopic calculations by Neergard and Vogel of the B (E3,0 → 3) for the 3 − members of the one-phonon octupole quadruplet is satisfactory when the Coriolis coupling between the states with K and K ± 1 is included. The B (E1) branching ratios for transitions from the 3 − and 1 − states to the ground-state band have large deviations from the Alaga-rule predictions. These deviations can be understood by the strong Coriolis coupling between the states of the octupole quadruplet in deformed nuclei.

Coulomb excitation of 115Sn

Abstract Coulomb excitation of the nucleus 115 Sn was studied with beams of 4 He and 16 O. Level energies, spins, mean-lives and B (E2) and B (M1) transition probabilities were obtained. Spin 3 2 + states were observed at 497.35 and 1280.08 keV and spin 5 2 + states were observed at 986.54 and 1416.78 keV. A state of 612.79 keV was observed to be indirectly excited by decay of the Coulomb excited states. Eleven B (E2) values and nine B (M1) values were obtained for the transitions between the low-lying states. In contrast to previous particle transfer results which suggested a clear distinction between shell-model and collective 3 2 + and 5 2 + states, our results suggest the collective strength is shared by the two 3 2 + and two 5 2 + states.

High-spin states in Ca 42

Levels in /sup 42/Ca excited by the /sup 28/Si(/sup 16/O, 2p) reaction have been investigated with E16O = 36--42 MeV. ..gamma..-ray angular distributions and ..gamma gamma.. angular correlations relative to the beam direction have been determined. Level energies, decay modes, spins, parities, and ..gamma..-ray radiative admixtures were extracted. Level energies in keV and preferred J/sup pi/s are: 4099.9 (5/sup -/), 5491.4 (5/sup +/), 5744.1 (5/sup -/), 6144.5 (6/sup +/), 6408.8 (7/sup +/), 6553.9 (7/sup -/), 7368.7 (6/sup -/ or 8/sup -/), and 8297.9 (J/sub 7369/ +- 1). These spin-parity assignments differ from those reported recently by Warburton, Kolata, and Olness. (AIP)

High-spin states in /sup 42/Ca. [J,. pi. ]

Levels in /sup 42/Ca excited by the /sup 28/Si(/sup 16/O, 2p) reaction have been investigated with E16O = 36--42 MeV. ..gamma..-ray angular distributions and ..gamma gamma.. angular correlations relative to the beam direction have been determined. Level energies, decay modes, spins, parities, and ..gamma..-ray radiative admixtures were extracted. Level energies in keV and preferred J/sup pi/s are: 4099.9 (5/sup -/), 5491.4 (5/sup +/), 5744.1 (5/sup -/), 6144.5 (6/sup +/), 6408.8 (7/sup +/), 6553.9 (7/sup -/), 7368.7 (6/sup -/ or 8/sup -/), and 8297.9 (J/sub 7369/ +- 1). These spin-parity assignments differ from those reported recently by Warburton, Kolata, and Olness. (AIP)