The structure of27Al

Abstract

Gamma decay modes and spin assignments of levels in 27A1 have been investigated by proton-y-ray angular correlation measurements in the 24Mg(~, PY) reaction at 14.2, 15 and 15.6 MeV bombarding energy and by y-ray angular distribution measurements on the Ep = 1820, 2114, 2293 and 2574 keV resonances of the 26Mg(p, 7) reaction. Unique spin assignments were obtained as follows: 1=3/2 for the 5827 keV state, I=5/2 for the 6115, 6465, 6765, 7577, 7721, 8097, 8136, 8324 keV states, I=7/2 for the 5433, 6533, 7413, 8037, 8442, 8586 keV states, I = 9/2 for the 5418, 6512, 6713, 7997 keV states, I = 11/2 for the 5500, 6948, 7400, 8396 keV states. The level scheme and electromagnetic properties of levels are compared with the results of shell model calculations which use the complete configuration space of the 0d5/2-1 s 1~2-0d3/2 shell and the unified s-d shell Hamiltonian. The agreement is good to excellent and extends into the region of high level density above 7 MeV excitation energy. The total B(M1T) below 8.5 MeV excitation energy is evaluated, using published resonance fluorescence and (e, e') data, and quenching relative to the flee-nucleon predictions is reexamined. Evidence in favour of a prolate ground state deformation of 27A1 and implications of this work for the astrophysically interesting 26Al(p, 7) reaction are discussed.

The structure of28Si above 10 MeV excitation energy III: Level scheme and shell model interpretation

“…Thus the 11 +, T = 1 ~ 11 +, T = 0 transition remains exceptional. A similar situation has been observed in the case of 27A1 [6]. Here we observed quenching of the M1 transition between the d3/a and ds/z single-particle states while there was no evidence of quenching otherwise.…”

“…The comparison of Table 2 does not extend to transitions between T = 1 states since they are very weak and difficult to trace experimentally in the presence of isospin impurities of states. As in previous work [6,7] we enumerate levels with a Branching ratios are rounded to integer values, hence branches < 0.5% are omitted. If experimental levels decay to negative-parity states we cite experimental radiative widths and branching ratios which are renormalized to account for decay to positive-parity states only.…”

“…In the lighter s d shell nuclei the N = 1 ~ N = 2 excitations are lower in energy, in the heavier nuclei it is the N = 2--+ N = 3 excitations. In a first series of experimental-theoretical studies which dealt with the properties of 2ssi [5], 27A1 [6] and 26Mg I-7] levels, mostly below 10 MeV, the success of the calculations in mid-shell nuclei was established. In 28Si the excitation energies and electromagnetic properties of the I < 4 levels below 10 MeV and of the I > 5 levels which were known at that time were successfully reproduced.…”

The structure of 32S II. Shell model interpretation

“…Their influence is only felt in the spectrum of I π = 0 + , T = 0 levels above 9.5 MeV. Apart from this fact shell-model calculations within the untruncated s-d basis space are able to reproduce the complete spectrum of positiveparity states in 32 S with an rms deviation of 200 keV, which is the accuracy previously observed [3][4][5][6][7][8][9] in the A = 21 − 28 region. The experimental-theoretical agreement is not confined to levels on or near the yrast line, which have in part been used to adjust [2] the matrix elements of the residual interaction.…”

“…The latter one is introduced with the goal of obtaining a universal Hamiltonian which can describe all nuclides of the shell at a comparable level of accuracy. Extensive tests of model predictions have been performed for a series of nuclides between A = 21 and A = 28 [3][4][5][6][7][8][9] yielding excellent agreement with experimental data over unprecedentedly large ranges of excitation energy. The favourable experimental situation which is now achieved for 32 S allows an extension of our tests to a nucleus in the upper half of the s-d shell where new matrix elements of the residual interaction come into play.…”