D. L. Watson

Elastic scattering of 33 MeV tritons and isospin dependence of mass-3 optical potential

Abstract Differential cross section data are presented for the elastic scattering of 33 MeV tritons from a range of nuclei from 12 C to 232 Th. These data have been analysed using a phenomenological optical model. Parameters are presented for three families of the real potential. A comparison of the triton optical model potential with those from a re-analysis of 3 He scattering from fp shell nuclei has allowed the isospin dependence of the optical model potential for mass-3 projectiles to be obtained in this mass region.

4 He Decay of Excited States in 14 C

A study of the 7Li(9Be,4He 10Be)2H reaction at E{beam}=70 MeV has been performed using resonant particle spectroscopy techniques and provides the first measurements of alpha-decaying states in 14C. Excited states are observed at 14.7, 15.5, 16.4, 18.5, 19.8, 20.6, 21.4, 22.4 and 24.0 MeV. The experimental technique was able to resolve decays to the various particle bound states in 10Be, and provides evidence for the preferential decay of the high energy excited states into states in 10Be at ~6 MeV. The decay processes are used to indicate the possible cluster structure of the 14C excited states.

Association of the 12C+12C breakup states in 24Mg with the quasimolecular resonances.

A high resoution measurement of the [sup 12]C([sup 24]Mg,[sup 12]C[sup 12]C)[sup 12]C reaction shows that the [sup 24]Mg breakup states have widths in the region of 100 to 250 keV. The energies, widths, and spins of these states are seen to be consistent with those of the [sup 12]C +[sup 12]C quasimolecular resonances which occur in the same region of excitation in the [sup 24]Mg nucleus.

Consistent description of the (t, 3He) reaction for A = 12−89

Abstract New differential cross section data for seven target nuclei were analysed using a microscopic model. Particular care was taken to ensure that the absolute magnitude of the cross sections was correctly measured. The predicted direct or one-step (t, 3 He) reaction did not account for the magnitude of the data except for forward angles and A ⩾40. The calculations were estended to include the contributions from two-step or sequential single-nucleon transfers. Calculations were made using the second-order DWBA and zero-range or full finite-range forms. In the latter the non-orthogonality between the basis states in the two-step transfers was included. The transfer amplitudes were deduced from comparisons of predicted angular distributions with data obtained from the (t, α) and (t, d) reactions, and from amplitudes determined by repeating previously reported shell-model calculations. The full finite-range DWBA calculations which included non-orthogonality terms, accounted for both the shape and magnitude of the data within a factor of 3 in the worst case. The (t, α) reaction angular distributions are, however, poorly predicted.

High spin resonances in 12C+12C scattering

Abstract We report on new experimental data which reveal high spin resonances in 12 C+ 12 C scattering. We have measured excitation functions for the 12 C [ 12 C, 12 C(3 1 − ) + 12 C(3 1 − )] reaction over the centre of mass energy range E c.m. =31 to 45 MeV. Resonances have been observed at E c.m. =32.5, 37.5 and 43 MeV. Detailed angular correlation measurements have been performed at the energy corresponding to the peak of each resonance. From an analysis of the angular correlations we suggest a dominant angular momentum of J =22 at the highest energy. We interpret the results in terms of an α cluster model of superdeformed states in 24 Mg.

Quadrupole and octupole collectivity in 148Nd

Abstract The role of quadrupole and octupole collectivity in the shape-transitional nucleus 148 Nd has been studied by Coulomb excitation using beams of 58 Ni and 92 Mo, and a beam of 148 Nd (using a 208 Pb target). The extracted E1, E2 and E3 matrix elements involving states up to 12 + ·in the ground band and 13 − in the negative-parity band are presented, and compared to calculations that assume a vibrational and rotational octupole nature for the negative-parity band. The positive-parity ground-band states are well described in terms of a prolate deformed shape with Q 20 ≈ 400 e fm 2 ( β 2 rms ≈ +0.18). The present results suggest a vibrational octupole nature for the low-spin negative-parity states, with an intrinsic moment Q 30 ≈ 1500 e fm 3 ( β 3 rms ≈ 0.12). The E2 and E3 matrix elements connecting these bands to the β- and γ-vibrational bands (and within these bands) are also presented, and compared to calculations incorporating the coupling between the rotational and vibrational modes. These calculations describe reasonably well the E2 matrix elements involving the gamma band, but do not reproduce the measured E2 matrix elements for the beta band, implying a complicated intrinsic structure for the beta band. The strong enhancement of the measured E3 matrix elements connecting the negative-parity band to the beta band could be indicative of a significant component of the two-phonon octupole vibration in the wavefunction of the so-called beta band.

Lifetimes of low-lying states in 148Nd

Abstract The recoil-distance method following Coulomb excitation has been used to determine the mean lifetimes of the 2 + , 4 + , 6 + , 8 + , 5 − , and 7 − states in the ground band and lowest negative-parity band of 148 Nd. The lifetimes of the 2 2 + , 0 2 + , and 2 3 + states were also measured. The E2 matrix elements, derived from the lifetimes, show a stable intrinsic quadrupole deformation, while the B (E1) values are typical of a nucleus having strong octupole correlations.

α-decay of excited states in 11C and 11B

Abstract Studies of the 16O(9Be, α7Be)14C and 7Li(9Be, α7Li)5He reactions at E beam = 70 MeV have been performed using resonant particle spectroscopy techniques. The 11C excited states decaying into α + Be 7 ( gs ) are observed at 8.65, 9.85, 10.7 and 12.1 MeV as well as possible states at 12.6 and 13.4 MeV. This result is the first observation of α-decay for excited states above 9 MeV. The α + Li 7 ( gs ) decay of 11B excited states at 9.2, 10.3, 10.55, 11.2, (11.4), 11.8, 12.5, (13.0), 13.1, (14.0), 14.35, (17.4) and (18.6) MeV is observed. The decay processes are used to indicate the possible three-centre 2 α + He 3 ( H 3 ) cluster structure of observed states. Two rotational bands corresponding to very deformed structures are suggested for the positive-parity states. Excitations of some observed T = 1 / 2 resonances coincide with the energies of T = 3 / 2 states which are the isobaric analogs of the lowest 11Be states. Some of these states may have mixed isospin.

Comparison of resonances observed in the 12C(24Mg, 12C12C)12C and 12C(20Ne, 12C12C)8Be reactions

Abstract A study of the 12 C( 24 Mg, 12 C 12 C) 12 C reaction has revealed discrete states at excitation energies between 20 and 30 MeV in 24 Mg. Angular correlation analyses of some of the lower energy states confirms the report by earlier authors of monotonically increasing spin values consistent with a rotational sequence. Comparison with breakup observed in the 12 C( 20 Ne, 12 C 12 C) 8 Be reaction suggests that the same states in 24 Mg are populated, while the difference in laboratory cross sections in the two reactions is interpreted as evidence for the association of the breakup states with a prolate hyper-deformed configuration appearing in theoretical calculations for 24 Mg.

Spectroscopy of the light rare-Earth nuclei 133Pm and 133Sm: Configuration-dependent pairing and evidence for strong residual proton-neutron interactions in neighbouring odd-N and odd-Z nuclei

Abstract The low-lying, rotational band structures of the highly neutron-deficient light rare-earth nuclei, 133Sm and 133Pm have been investigated using the 40Ca + 96Ru reaction at a beam energy of approximately 180 MeV. Five new bands have been observed in both nuclei. 133Sm possesss two strongly coupled bands which are thought to be built upon the v h 11 2 [523] 7 2 − and v d 5 2 [402] 5 2 + Nilsson configurations. In addition to the previously reported v i 13 2 intruder band, there is also evidence for a second decoupled, negative-parity structure in 133Sm based on the v h 9 2 [541] 1 2 − orbital. In 133Pm evidence is presented for both signature partners of the π h 11 2 band. The expected h 11 2 neutron alignment in these bands has not been observed. It is suggested that this provides evidence for a strong pn interaction. A strongly coupled band built on the π g 7 2 [413] 5 2 + configuration has also been established. Both signature partners of this band show clear evidence for the alignment of a pair of h 11 2 protons followed by a pair of h 11 2 neutrons. A strongly coupled three quasi-particle configuration has also been identified which feeds into the unfavoured signature of the π h 11 2 band. The results are discussed in terms of cranked shell model, total routhian surface calculations and the systematics of the A ∼ 135 region.