B. Herskind

Coulomb excitation of vibrational states in deformed even nuclei

Abstract Low-lying states in deformed even rare-earth nuclei have been studied by Coulomb excitation with 43.5 MeV oxygen ions. Gamma rays from thin oxide targets (≈0.3 mg/cm 2 ) or thin metal targets (0.5–1.2 mg/cm 2 ) were observed in coincidence with back-scattered ions. The yields were normalized on the intensity of the scattered particles. Double groups of gamma rays ascribed to the 2 +′ → 0 + and the 2 +′ → 2 + transitions from K = 2 states were observed in Nd 150 (1.07 MeV), Sm 152 (1.09 MeV), Sm 154 (1.44 MeV), Gd 156 , Gd 158 , Gd 160 (1.02 MeV), Dy 160 , Dy 162 (0.89 MeV), Dy 164 (0.77 MeV), Er 164 (0.86 MeV), Er 166 , Er 168 , Er 170 (0.93 MeV) and Yb 176 (1.27 MeV), where the energies in the parentheses are given for 2 +′ states not observed earlier. Gamma rays ascribed to the 0 +″ → 2 + , 2 +″ → 0 + and 2 +″ → 2 + transitions from K = 0 states were observed in Nd 150 (0.69 MeV), Sm 152 , and Gd 154 (0.68 MeV), where the energies are for the 0 +″ states. Gamma rays in Sm 154 and Gd 156 suggest K = 0 states at 1.10 and 1.06 MeV, respectively. The B (E2 values determined from the yields range from 2 to 8 single-particle units, being roughly inversely proportional to the excitation energies for the levels. The theory of multiple Coulomb excitation for the different bands was in qualitative agreement with the experimental observations.

Half-lives of first excited 2+ states (150 < A < 190)

The half-lives of the first excited 2+ states of the following even nuclei in the region 150 1 for these deformed nuclei by an average value of 12%.

Gamma decay after heavy ion reactions studied by first and higher order multiplicity measurements

Abstract First and higher order multiple coincidence rates have been measured for the γ-rays from 16 O induced reactions on targets of 148,150 Nd at various bombarding energies. Average values and shape parameters for the γ-ray multiplicity distributions have been extracted from the data. The multiplicities are connected to angular momentum through complete fusion cross sections, and conclusions are drawn on spin distributions of entry states in the final nuclei as well as on the decay pattern from these states.

Signature-dependent ml and e2 transition probabilities in 155ho and 157ho

Abstract 155 Ho and 157 Ho have been populated in the reactions 141 Pr( 18 O,4n) and 146 Nd( 15 N, 4n) at 85 and 74 MeV, respectively. In both nuclei bands built on the 7 2 −[523] configuration were established to spin values considerably above the first backbend. A signature dependence in the excitation energies as well as in the ratio of M1 to E2 transition rates is observed below, but not above, the backbend in both nuclei. In 157 Ho lifetimes were measured with the recoil-distance method. The ΔI = 2; E2 transition probabilities obtained show very little variation with either signature or spin and no irregularity at the backbend. The signature dependence and strong rise in the ratio B (M1)/ B (E2) observed at the backbend in 157 Ho therefore must be caused by the B (M1) values. A signature dependence in the B (E2, I → I −1)/ B (E2, I → I −2) ratios also found in 157 Ho below the backbend is mainly the result of signature dependence in the ΔI = 1 ; E2 transition rates. Qualitatively, most of the features observed can be explained by nonaxial deformations, which change from large negative to slightly positive values of γ at the backbend.

Single-neutron states at high spins in ytterbium nuclei: Evidence for the quenching of static neutron pair correlations

Abstract The decay schemes of 167,168,169 Yb are established to the largest angular momenta yet known in stably-deformed rare-earth nuclei using the 124 Sn( 48 Ca,3–5n) reaction. The systematics of the resulting spectrum of single-neutron states in these and neighbouring Yb isotopes at large and small angular momenta are discussed with regard to that expected in the presence and absence of static neutron-pair correlations. This comparison, together with cranking-model and “gauge-space” analyses, provides evidence for the effective disappearance of static neutron-pair correlations for the odd- N isotopes at ħω ⩾ 0.38 MeV. A composite empirical spectrum of single-neutron states is constructed for the “unpaired” regime and is compared with cranking calculations.

ANGULAR DISTRIBUTIONS FOR INELASTIC DEUTERON SCATTERING FROM SAMARIUM.

Abstract The elastic and inelastic scattering of 12 MeV deuterons from targets of 148 Sm, 150 Sm, 152 Sm and 154 Sm has been investigated. Angular distributions for the elastic group and for the more prominent inelastic groups were measured by means of solid state counters. Supplementary information was obtained from high-resolution magnetic spectrograph data. The elastic angular distributions are smooth and without diffraction structure. The absolute cross section at back angles decreases with increasing mass number. This effect can probably be ascribed to differences in the inelastic excitations since the distributions for the sum of elastic and inelastic intensity are almost identical for all the nuclei. The shape of the angular distributions for the collective 2 + , 4 + and, when observed, for the 6 + levels, for each angular momentum is very similar in all the nuclei, although the character of the levels changes from vibrational to pure rotational. The same is true for the distributions of the strongly excited 3 - levels. It is thus indicated that the angular distributions give a simple determination of the multipolarity of the excitation, the tendency being a decrease in forward-backward ratio with increasing multipolarity.

Coulomb excitation of even Sm isotopes

Abstract Low-lying excited states in the 148 Sm, 150 Sm, 152 Sm and 154 Sm nuclei have been studied by means of Coulomb excitation with oxygen ions. Gamma rays from thin metallic targets were detected in coincidence with backscattered oxygen ions in NaI(Tl) crystals. In the case of 148 Sm and 150 Sm singles, gamma-ray spectra were also recorded by means of a Li-Ge detector. Gamma rays originating from the states at 0.551 MeV (2 + ), 1.162 MeV (3 − ), 1.181 MeV (4 + ) and 1.455 MeV (2 + ) were observed in the 148 Sm spectra and from the states at 0.334 MeV (2 + ), 0.741 MeV (0 +′ ), 0.774 MeV (4 + ), 1.046 MeV (2 +″ ), 1.072 MeV (3 − ) and 1.193 MeV (2 +′ ) in the 150 Sm spectra. The B (E2) values between several of the positive-parity states were derived from first- and second-order perturbation theory. The B (E2) values obtained for the 3- states were 0.37 ± 0.03 and 0.31 ± 0.03 e 2 × 10 −72 cm 6 for 148 Sm and 150 Sm, respectively. For the lowest octupoe bands in 152 Sm and 154 Sm, the B (E2) values were 0.14 ± 0.03 and 0.11 ± 0.03 e 2 × 10 −72 cm 6 , respectively.

Proton configuration dependence of neutron alignments and transition probabilities in 165Lu

Abstract High-spin states in 165 Lu have been populated in the 153 Eu( 16 O, 4n) reaction. An excitation function measurement was made to assign correct γ-rays to transitions in 165 Lu. A level scheme is constructed from the studies of γγ coincidences, and the measurement of γ-ray angular distributions. Nine decay sequences are assigned to 165 Lu. At low spin, these sequences are identified with the 5 2 + [402], 1 2 + [411], 1 2 − [541], 7 2 + [404] and 9 2 − [514] Nilss determined and the variation in the crossing frequency is discussed. ΔI = 1 transitions observed between the favoured and unfavoured decay sequences of four of the five configurations are also discussed.

The

Abstract The g-factor of the 12+ two-quasiparticle isomeric state, ¦(v i 13 2 ) −2 ¦ 12+ , in 206Pb has been measured by the time-differential perturbed angular correlation method. The result obtained is g(12+, 206Pb) = − 0.155 ± 0.004. Three different methods are applied to deduce the orbital g-factor of the neutron around the 208Pb core, namely (i) the M1 core polarization model, (ii) the configuration mixing model, and (iii) a combination of the M4 transition rate in 207Pb with the present result. The gleff (neutron) deduced in the three different ways are in good internal agreement: (i) − 0.036 ± 0.010, (ii) − 0.040 ± 0.03 and (iii) −0.034 ± 0.010. Although it is hard to estimate the overall uncertainty, it is probably significant that the anomaly in the orbital g-factor of the neutron has opposite sign from that of the proton, and that the size is smaller. From known g-factors in the lead region the polarization part δgs and δgs of the g-factor of the neutron can be derived. These values are δgs = 2.77 ± 0.15 and δgs = − 3.30 ± 0.22. By comparing the experimental g-factors for the i 13 2 proton and neutron we find that the part of the g-factor deviating from the Schmidt line has an isovector character. This means that δgs and δgs should have the same magnitude but opposite sign for the proton. Calculated g-factors based on this conclusion agree well with the experimental g-factors for the h 9 2 and i 13 2 protons.

g

Abstract High-spin states in 163Yb have been studied by the 149Sm(18O, 4n)163Yb reaction with 84 MeV 18O ions from the NBI tandem accelerator. Gamma-gamma coincidence data were accumulated with an array of five Compton-suppressed Ge(Li) detectors resulting in an extension of our knowledge of high spin members of familiar rotational bands as well as of weakly populated side bands. A γγ time coincidence experiment, conversion electrons and angular distribution measurements were performed. Four side bands are observed besides the yrast cascade, which is followed up to its 49 2 + member. The yrast band of the 163Yb nucleus has been interpreted, following the systematics in this region of neutron deficient rare-earth nuclei, as the favoured member of a decoupled i 3 2 neutron band. The negative-parity band ( −, +)1 based on the 3 2 − [521] state was reestablished. The other signature of this band is the newly introduced band ( −, − )1. The signature splitting in the negative-parity band is observed to be small; however, for large zω, band ( −, − )1 lies lower in energy than band ( −, +)1; which is a unique observation in this mass region. The quasiparticle routhians and alignments obtained from these data are compared with results of cranked shell-model calculations and similar data for the heavier and lighter Yb nuclei. The rotational effects on M1 transitions connecting unfavoured members of the band, based on the i 13 2 orbital, with the favoured members are demonstrated.