G.M. Crawley

Systematics of the excitation of M1 resonances in medium heavy nuclei by 200 MeV proton inelastic scattering

Abstract In a series of seventeen nuclei ranging from 51 V to 14 Ce, broad resonance structures are observed at energies between 8 and 10 MeV, nearly mass independent. These resonances have very forward peaked angular distributions which imply that they are populated by an angular momentum transfer of zero. This together with the observed excitation energies suggests an M1 character for these resonances. In 51 V, 58 Ni, 60 Ni, 62 Ni, a sharp peak located at an excitation energy above the threshold for neutron emission is interpreted as a part of the T 0 + 1 component of the M1 resonance. Cross sections are given for all the M1 resonances. For 58 Ni, 90 Zr, 92 Mo, 120 Sn and 140 Ce, an “attenuation” factor for the cross sections is extracted in a DWIA calculation assuming simple shell-model structures for these resonances.

On the nature of low-lying collective 1+ states in the heavy deformed nuclei 154Sm, 156Gd and 164Dy and in the f72 shell nucleus 46Ti

Abstract High resolution inelastic proton scattering at 201 MeV on 154Sm, 156Gd and 164Dy is used to clarify the nature of low-lying magnetic dipole states previously observed in inelastic electron scattering. An upper limit on the spin contribution to these collective magnetic dipole excitations is presented. The roles of spin and convection currents are also studied by inelastic proton and electron scattering to the low-lying 1+ state at 4.319 MeV in the f 7 2 - shell nucleus 46Ti. A substantial orbital contribution is observed in all cases.

A survey of the (3He, 7Be) reaction at 70 MeV

Abstract A study of the ( 3 He, 7 Be) reaction has been undertaken using a 70 MeV 3 He beam. By surveying a wide range of target nuclides, namely 12, 13 C, 16 O, 24, 26 Mg, 40, 42, 44 Ca, 58, 60, 62, 64 Ni, 90 Zr, 120, 124 Sn, 144 Sm and 206 Pb, systematics of the α-clustering phenomenon were investigated. In addition, masses and energy levels of 60 Fe and 120 Cd were measured. The 7 Be particles were detected in a single wire proportional counter backed by a plastic scintillator in the focal plane of an Enge spectrometer to ensure adequate particle identification. Total energy resolution as small as 140 keV full width at half maximum was obtained, although in most cases the target thickness limited the energy resolution to larger values. Differential cross sections as low as 20 nb/sr were measured. The finite range programs LOLA and LOLITA were used to calculate differential cross sections for comparison to data, assuming the reaction to proceed by a direct α-transfer. The spectroscopic factors which were extracted show a marked decrease with increasing atomic mass number, implying a decrease in surface α-clustering for heavier nuclei.

Excitation of giant resonances by inelastic

Inelastic /sup 3/He scattering at 71 MeV on twelve nuclei ranging from / sup 27/Al to /sup 209/Bi shows an enhancement of the continuum very similar to that observed in electron and proton scattering. The effect is ascribed to a giant multipole state. The strength of the excitation indicates an E2 character for thc state. (auth)

sup 3

Abstract Using the A1200 beam analysis device at the National Superconducting Cyclotron Laboratory, we have measured the halflives of several nuclei along the rp-process path near the proton-drip line. Halflife results for 61 Ga, 63 Ge, and 65 As (0.15±0.03 s, 95 −25 +23 ms, and 0.19 −0.07 +0.11 s, respectively) and their implications for the rp-process are presented.

He scattering

Abstract New data for the elastic and inelastic scattering of 24.4 MeV protons from 6, 7Li are presented and a summary of all the available data for the elastic, inelastic, and charge exchange scattering of 24–50 MeV protons from these same targets is given. The cross-section data for Ep ≅25 and 50 MeV are examined theoretically within the framework of the microscopic folding model and the distorted-wave approximation. Standard p-shell wave functions, supplemented by renormalization factors deduced from electromagnetic and β-decay data, are used to describe the target nuclei in these calculations. Results obtained using a phenomenological 1 fm range Yukawa interaction illustrate the information on proton-neutron differences contained in the data for quadrupole transitions in 7Li and differences in the energy dependence of the spin-flip and non-spin-flip isovector central components of the effective interaction contained in the data for the (p, n) reaction on this same target. Results obtained with the more realistic G-matrix interaction of Bertsch et al. give a reasonable description of the overall features of the experimental data after 20–60 % reductions in the strengths of its varous components are made. The calculations fail to provide a consistent picture of the energy dependence of the optical potential and the quadrupole transition potentials. Some difficulties are also encountered in describing the cross sections for scattering to the 0+, T = 1 level in the mass-6 systems.

Halflife measurements of the rp-process nuclei 61Ga, 63Ge, and 65As

Abstract Valence and deep-lying neutron-hole strengths corresponding to orbits near and well below the Fermi surface have been observed in high-resolution studies of the 144, 148, 152 Sm( 3 He, α) and of the 144, 148, 150, 152, 154 Sm(p, d) reactions at 70 and 42 MeV bombarding energy, respectively. The explored excitation energy range was 28 MeV for the ( 3 He, α) experiment and about 12 MeV in the (p, d) study. Complete angular distributions have been measured in both cases and the data was analyzed within the framework of the distorted waves Born approximation theory of direct reactions. For the neutron closed shell target ( 144 Sm), in addition to the well-known fragmentation of the 2d 5 2 and 1g 7 2 valence-hole strength, a new bump observed around 7.6 MeV excitation energy is excited in both reactions. This structure corresponds to the 1g 9 2 inner-hole strength in 143 Sm and the analysis of the ( 3 He, α) data suggests that more than 50% of the l = 4 strength can be found between 6 and 12 MeV. When one goes to the heavier Sm isotopes, the energy spacing between valence-hole states located above and just below the N = 82 shell decreases strongly and disappears in 151 Sm as a result of increasing deformation. Combining good energy resolution and detailed analysis of the two reactions, rather complete spectroscopic information is obtained for the valence-hole strength distributions. With regard to inner-hole states, the energy spectra exhibit a narrow structure whose centroid energy decreases from 4.4 to 2.9 MeV when the mass number increases from A = 147 to A = 153. The main peak displays an asymmetric shape with an extremely large high-energy tail. The 1h 11 2 hole strength is split into the Nilsson Orbitals. The narrow bumps are found to carry a large fraction of the l = 5 and l = 2 hole strengths in 147,149,151,152 Sm isotopes. In the high-energy tail of the structures one observes overlapping and increasing spreading of the g 7 2 , 2d 5 2 and possibly 1g 9 2 inner-hole strengths due to the disappearance of the N = 82 shell gap between N = 83 and N = 89 neutron numbers. The experimental hole strengths distributions are compared where possible to the predictions of the quasiparticle-phonon nuclear model or to the simple Nilsson model.

Nucleon scattering from light nuclei: (I). The targets 6Li and 7Li☆

Abstract Observations down to 2° in the 48 Ca(p,p′) reaction at 201 MeV reveal 0 + →1 + strength with certainly only at the previously known excitation of 10.22 MeV. Compared to DWBA calculations, the strength of this state is only ∼ 30%.

Neutron-hole strength distributions in heavy nuclei: (II). The 144, 148, 152Sm(3He, α) 143, 147, 151Sm and the 144, 148, 150, 152, 154Sm(p, d) 143, 147, 149, 151, 153Sm reactions

Abstract The even-even cadmium isotopes have been studied by the (p, t) reaction at 42 MeV. A broad resonance-like structure is observed between 6 and 7 MeV excitation energy which has similar properties to the feature observed in (p, t) reactions in the tin isotopes. A second smaller bump at higher excitation energy appears to arise from pickup of two particles from deep orbits.

Observation of l=0, spin-flip transitions in 48Ca☆

Abstract Neutron-hole strengths corresponding to orbits near and well below the Fermi surface in the 121, 123, 129 Te isotopes have been observed in high resolution studies of the ( 3 He, α) and of the (p, d) reactions at 70 and 42 MeV bombarding energy, respectively. Complete angular distributions have been obtained in the ( 3 He, α) experiments whereas a few angles were recorded in the case of (p, d) reactions. Several new levels are observed up to 3 MeV excitation energy in the 121, 123, 129 Te isotopes. A detailed analysis of the results from the two reactions has allowed rather unambiguous l determination of the transferred angular momentum, and spectroscopic factors have been extracted for the observed levels. A strong fragmentation of the 2d 5/2 and 1g 7/2 hole strengths is evidenced. At higher excitation energy, the 1g 9/2 inner-hole strength is still observed as a giant-resonance-like structure whose excitation energy and width increase with mass number. About 40 to 60 % of the total l = 4 hole strength is found to lie in the gross structure region (4–10 MeV). The results of the analysis are compared to the theoretical predictions of the quasiparticle-phonon coupling nuclear model.