J. A. Cameron

Precise DSAM lifetime measurements in 48Cr and 50Cr as a test of large scale shell model calculations

Precise DSAM lifetime values have been obtained for the positive parity yrast bands of 48Cr and 50Cr, populated with the reaction 28Si+28Si at 115 MeV bombarding energy using a Au backed target. The obtained precision relies on the well-known stopping power at high velocity and on the use of DSAM analysis procedures which avoid the sidefeeding contribution. The deduced B(E2) and B(M1) values are in very good agreement with those predicted by full fp shell model (SM) calculations. A new procedure named narrow gate on transition below has been applied for the analysis of the lifetimes. n nClear evidence was found in both nuclei for a low-lying negative parity band which has been successfully described by SM in the fp shell extended to include a hole in the 1d32 orbital. Also in this case the derived B(E2) and B(M1) values are in good agreement with the SM calculations.

Collective properties of 48Cr at high spin

Abstract High-spin states of the nucleus 48 Cr have been studied via particle-γ-γ spectroscopy, following the 28 Si( 28 Si,2α) reaction. A 44-element particle-detector array was used to isolate 48 Cr residues and to reduce γ-ray Doppler broadening. The collective band built upon the ground state has now been firmly established to spin 16 + , the highest possible in the isolated f 7 2 shell, and lifetimes of the four highest states have been measured from Doppler shifts. Although some of the ground-state band properties are well reproduced by recent fp -shell model calculations, a sharp reduction in E2 transition rates at the backbend is not.

High-spin states in the mirror nuclei 49Cr and 49Mn

Abstract The γ spectra of the neutron-deficient nuclei 49 Mn and 49 Cr have been observed using the reactions 12 C( 40 Ca, p2n) 49 Mn and 12 C( 40 Ca,2pn) 49 Cr at E (Ca)=160 MeV.Radiation from 49 Mn and 49 Cr was identified by detecting recoil ions in a 0° mass separator. Data from γ-γ, recoil-γ and recoil-γ-γ coincidence spectra and recoil-γ angular correlations allowed the level scheme of 49 25 Mn 24 to be deduced up to 6 MeV (19/2 − ).The levels of the mirror nucleus 49 24 Cr 25 were observed to over 10 MeV (27/2 − ). Relative Coulomb energies for the mirror states were found to have a smooth J dependence. The rotation-like level spacing at low spin is interrupted at J =19/2 − , and this is interpreted as an alignment effect.

Isovector pairing in odd-odd N = Z 50Mn

High-spin states in the odd–odd N=Z nucleus 5025Mn have been investigated. A sequence of states up to Jπ=6+ has been assigned as the T=1 analogue of the yrast band in 5024Cr for the first time. The differences in energy between levels in these bands are interpreted in terms of rotational alignments and the effect they have on the Coulomb energy of the nucleus. Comparisons with shell model calculations show that the Coulomb energy difference between the T=1 analogue structures is an important indicator of the competition between isovector pairing modes in N=Z nuclei and their isobars.

Channel selection for high spin γ-ray spectroscopy studies via total energy measurements in fusion-evaporation reactions

Abstract A channel selection method for high spin γ-ray spectroscopy studies based on the measurement of the total energy of all radiations (both charged particle and γ-ray) emitted in heavy-ion fusion reactions is presented. The method is applicable to all reactions in which charged-particle evaporation from the compound system dominates, and is particularly effective in isolating the weakly populated low particle multiplicity channels that leave the final nucleus with the greatest spin and excitation energy. The method is illustrated using data taken with the 8π γ-ray spectrometer and the miniball 4π charged-particle detector array at the Chalk River Tandem Accelerator Superconducting Cyclotron (TASCC) facility. Channel-to-total ratios are improved over those obtained with charged-particle detection alone by factors as large as 46 without significant loss of statistics for the selected channel.

Mirror nuclei at high spin in the f72 shell

New results for the Coulomb energy differences in the A=47 mirror nuclei 47Cr and 47V for yrast states up to spin 312ℏ are presented and shown to agree with earlier predictions made within a deformed, crancked shell model approach. The observed total difference in Coulomb displacement energies at low and high angular momenta is also explained within the spherical shell model. The first example of alignment in an N=Z nucleus, 48Cr, is reported.

Two-neutron excitations in 26Mg and 30Si

Abstract The reactions 24 Mg(t, p) 26 Mg and 28 Si(t, p) 30 Si have been studied at an incident energy of 18 MeV. Angular distributions have been measured for strongly excited states up to an excitation energy of about 10.5 MeV. A comparison with DWBA calculations using transition amplitudes resulting from a full (sd) shell model calculation generally shows good agreement up to an excitation energy of about 6 MeV, although a few states show significant discrepancies. At higher excitation energies the natural-parity states of the (f 7 2 d 5 2 ) multiplet, and the 2 + and 4 + states of the (f 7 2 ) 2 configuration can be identified in both 30 Si and 26 Mg. The 6 + state is identified in 30 Si, but not in 26 Mg. The expected 0 + states are not clearly identified in either case, although relatively strong transitions with “uncharacteristic” angular distributions are observed near the expected location of these states. DWBA calculations with a (f 7 2 ) 2 form factor suggest that these may in fact be the 0 + states of the (f 7 2 ) 2 configuration.

Coulomb displacement energies and bandcrossing phenomena

Abstract The angular momentum dependence of Coulomb energies is explored in the context of a cranked shell model approach. It is observed that a bandcrossing corresponding to the alignment of protons results in a substantial decrease in the Coulomb energy. Recently measured Coulomb displacement energies in 49 Cr and 49 Mn are shown to be consistent with this rotational alignment interpretation.

Cross-section measurements for 7Li + 11B AND 7Li + 13C reactions at low energies

Abstract The 7 Li + 11 B and 7 Li + 13 C reactions have been studied at incident energies below and around the Coulomb barrier, by measurement of the cross sections for the characteristic γ-rays emitted by the residual nuclei. The nα, αα and np emission channels are found to be enhanced with respect to the statistical compound nucleus predictions, while the nn and single-particle emission channels are not enhanced. An unusual energy dependence is observed for the 7 Li + 11 B → nn 16 O 7.12 MeV channel cross section, suggesting the existence of a broad resonance.

Cross-section measurements for the 9Be + 9Be, 7Ll +11B and α + 14C reactions at low energies☆

Abstract The 9Be + 9Be, 7Li+11B and α +14C reactions, leading to the 18O compound state in the excitation energy range between 25 and 31 MeV, have been studied by measurement of the cross sections for the characteristic g-rays emitted by the residual nuclei. The cross sections for the two-particle emission channels (nn, np, na, aa) are much larger in the two heavy-ion reactions, 9e+9Be and 7Li + 11B, than in the a + 14C reaction. It is suggested that the large enhancement in the nn and np channels, which do not include any transfer processes, indicates that 9Be+ 9Be and 7Li + 11B form a largely deformed molecular state, from which only low-energy particles can be emitted. A broad resonance is observed at ∼ 28 MeV 18O excitation energy in the 9Be+9Be→ nn16O 7.12 MeV, 7 Li + 11 B → nn 16 O 7.12 MeV and 9 Be + 9 Be → np 16 N 0.397 MeV reaction channels.