G. Løvhøiden

Production of charged pions in intermediate-energy heavy-ion collisions

Abstract Cross sections for the production of π + and π − have been measured over a wide range of angles for 12 C + 7 Li, 12 C + 12 C and 12 C + 208 Pb collisions at 60 A , 75 A and 85 A MeV. Energy distributions are less steep and absolute yields of pions larger than expected from a straightforward Fermi gas nucleon-nucleon scattering prescription. The apparent velocity of the system in which the π + emission is symmetric, is closer to the nucleus-nucleus c.m. velocity than to the mean speed system. The Coulomb-corrected π − π + ratio is close to unity for 12 C + 12 C. The corresponding ratio for 12 C + 208 Pb is, however, much larger than expected from the neutron excess in 208 Pb only.

Evidence for near-stable octupole deformation in 225Ra

The low lying level structure of 225Ra, studied with the 226Ra(d, t) reaction and 225Fr β-decay, has approximately degenerate K = 12± and K = 32± bands. Hybrid absolute values of the decoupling parameters of the K = 12± bands, intermediate between the theoretical values expected for the pure Nilsson configurations, are deduced and presented as evidence for near-stable octupole deformation in the 225Ra ground state.

An investigation of the Nilsson model potential in the spherical limit: Which values for the potential parameters κ and μ should be used to describe a particular nucleus?

Abstract Shell-model states have been investigated in various regions of the nuclear chart, and single particle level schemes based on the present experimental data from transfer reactions on target nuclei with either N or Z magic are established. These level schemes are used to evaluate the κ- and μ-parameters in the Nilsson model potential. A considerable shift is found in the single particle energies with mass, and it is necessary to allow a large variation in the μ-parameter in order to reproduce this effect. The μ-values deduced for neutron states deviate significantly from the parameters recommended in previous studies. Finally, parameter values to be used with the Nilsson model potential in actual calculations on nuclear properties are proposed.

The rotational structure of 227Ra

Abstract The level structure of 227Ra has been studied using the (n, γ), (d, p) and ( t , d) reactions and the β− decay of 227Fr. A model-independent level scheme was established including 28 levels below 1500 keV. Cross sections and excitation energies have been measured for 72 levels below 2.5 MeV and analysing powers have been measured for 25 levels below 1.4 MeV. The level structure is interpreted in terms of the Nilsson model. The ground-state configuration is 3 2 + [631↑] and the first excited band [633↓] starts at 1.7 keV. Several octupole vibrations coupled to single-particle configurations are tentatively assigned above 280 keV. The neutron binding energy was determined to be 4561.41 ± 0.27 keV. The half-life of 227Fr was measured to be 148 ± 2 s.

Particle-rotor model description of the negative-parity states in the transitional 151Sm nucleus

The particle-rotor model has been used to calculate energy levels, spectroscopic factors and B(E2) values in the transitional 151Sm nucleus. Two aspects of the model are emphasized: (i) the recoil term in the particle-rotor Hamiltonian is interpreted as a many-body term and given explicit treatment and (ii) the potential parameters k and μ of the Nilsson potential are determined from the experimental single-particle energies in the spherical limit. The model is capable of giving a consistent description of the observed data.

Studies of single neutron holes in the transitional nuclei N = 83–89:: The 150, 148Nd(d, t) and 150, 148, 146Nd(3He, α) pick-up reactions

Abstract The level structures of the 145, 147, 149 Nd nuclei up to about 5 MeV excitation energy have been investigated with the ( 3 He, α) reaction at 24 MeV. Additional 17 MeV (d, t) data have been obtained for 147, 149 Nd. The angular distributions have been analyzed with standard DWBA calculations, and spectroscopic factors have been deduced. Two groups of states carrying h 11 2 single-particle strength may be associated with the 9 2 − [514] and 11 2 − [505] Nilsson orbitals. A considerable amount of high- l single-particle strength may be found in the continuum observed in the ( 3 He, α) spectra above 3 MeV in all the nuclei.

Single-proton states in 151Pm

Abstract The 152 Sm ( t , α) 151 Pm reaction was studied using 17 MeV polarized tritons from the tandem Van de Graaff accelerator at the Los Alamos Scientific Laboratory. The α-particles were analyzed using a Q3D magnetic spectrometer and detected with a helical-cathode position-sensitive counter. The overall resolution was ∼ 18 keV FWHM. Measurements of the 150 Nd( 3 He, d) 151 Pm reaction were made using 24 MeV 3 He beams from the McMaster University tandem accelerator. The deuteron spectra were analyzed with a magnetic spectrograph using photographic emulsions for detectors, yielding a resolution of ∼ 13 keV FWHM. By comparing the measured angular distributions of ( t , α ) and ( 3 He, d) cross sections and ( t , α ) analyzing powers with DWBA predictions it was possible to assign spins and parities to many levels. The present results confirm earlier assignments of rotational bands based on the low-lying 5 2 + [413], 5 2 − [532], 3 2 + [411] and 1 2 + [420] orbitals. In addition, states at higher excitation have now been assigned to the 1 2 + [411] and 7 2 + [404] orbitals, and members of the 3 2 + [422], 5 2 + [402], 3 2 − [541] and 7 2 − [523] bands are tentatively proposed. The spectroscopic strengths can be explained reasonably well by the Nilsson model when pairing and Coriolis mixing effects are included.

Studies of 158, 160, 162, 164, 166Dy levels with the (t, p) reaction

Abstract Angular distributions of the 156, 158, 160, 162, 164 Dy(t, p) reactions have been measured using 17 MeV tritons from the McMaster University Tandem Van de Graaff accelerator. The reaction products were analyzed with a magnetic spectrograph and detected with photographic emulsions, resulting in overall peak widths of 15–20 keV (FWHM). Levels populated with L =0 transitions were identified from the unambiguous angular distributions, and at least one previously-unknown I π = 0 + state was found in each nuclide studied. New I π =0 + levels were found at energies of 1269, 1549, 1743, and 2000 keV in 158 Dy, 1457 and 1709 keV in 160 Dy, 2126 keV in 162 Dy, 1774 keV in 164 Dy, and 1149 keV in 166 Dy. Also, the previously proposed 0 + assignment for the 1655.4 keV level in 164 Dy has been confirmed. For the neutron-rich nuclide 166 Dy there was previously no information on excited states listed in the Nuclear Data Sheets, and many levels have been located in the present study, including the gamma band and an excited K π =0 + band. The 1457 keV 0 + state in 160 Dy is a possible candidate for the bandhead of the previously reported S-band. The fraction of the L =0 strength which feeds excited states is unsually high in several cases, and particularly for 164 Dy, in which the total L =0 strength to excited levels is ∼35% of that for the ground state. This can be explained qualitatively in terms of a “sub-shell closure” corresponding to the gap in the Nilsson diagram at N =98, and supports the earlier explanation of large L =0 strengths to excited states in the 161 Dy(t, p) 163 Dy reaction as being due to this gap. Most of the L π =0 + excited states observed in previous (p, t) studies were not populated in the present work, thus ruling out a pairing vibrational interpretation for these levels.

The nuclear structure of 225Ra

Abstract The level structure of 225 Ra has been studied using the (d, t) reaction with both unpolarised and polarised deuterons and the β − decay of 225 Fr. Cross sections and excitation energies have been measured for 41 levels below 1800 keV in the (d, t) reaction, and levels up to 724 keV have been established from the β − decay measurements. The level structure is interpreted in terms of the Nilsson model.

A comparison of the (p, α) and (t, α) reactions in the shape transitional region near N ≃ 90

Abstract A comparison has been made between the (p, α) and (t, α) reactions leading to 147, 149, 151 Pm and 153, 155 Eu. A strong correlation was observed between the cross sections in the two reaction types except for the results leading into 149 Pm. Direct three-nucleon microscopic and cluster transfer reaction calculations and also two-step sequential transfer reaction calculations were made for the (p, α) angular distributions. Several sets of α optical model and bound state parameters were tried. The shape of the predicted angular distributions were found to be determined by the α optical model and bound state parameters. The two-step sequential transfer calculations reproduced the magnitude of the 158 Gd(p, α) data within a factor of two for all three parameter sets tried.