W. Brendler

Evidence for the coexistence of spherical and deformed states in38Ar

The38Ar levels at Ex=5350, 7289 and 9341 keV have been investigated using the35Cl(α,pγ) reaction at Eα=l4 and 14.5 MeV. From particle-γ-ray angular correlations the spin assignments J(5350)=4, J(7289)=6,4 and J(9341)=8,6,4 have been obtained. Life-time measurements using the Doppler-shift attenuation method yielded τ(9341)=106±25 fs and τ(7289)=77±30 fs, while the lifetime τ(5350)=200±50 fs was known previously. All levels have positive parity and decay by enhanced E2 transitions. Hence we propose that they are the Jπ=4+, 6+ and 8+ members, respectively, of a Kπ=0+ rotational band which has the Ex=3377keV, Jπ=0+ and the 3937 keV, Jπ=2+ levels as further members. The enhancement of inband E2 transitions is 30−6+10 W.u. (4→2), 35−14+30 W.u. (6→4) and 20–36 W.u. (8 → 6), respectively, yielding an average intrinsic quadrupole moment Q0=850−125+200 mb. The moment of inertia is given by h22θ=92 keV. The present data are in good agreement with the predictions of a deformed state in38Ar that coexists with the spherical states.

The structure of25Mg

Gamma-decay modes and spin(-parity) assignments of levels in25Mg have been systematically investigated up to 10 MeV excitation energy by particle-γ-ray angularcorrelation measurements with the24Mg(d, pγ) reaction at 6.5 MeV bombarding energy and with the22Ne(α,nγ) reaction at 11.8, 12.5, 14.4 and 15.5 MeV bombarding energy. A level scheme has been established which is comprehensive up to 8.3 MeV excitation energy forI≦9/2 and up to 10 MeV for 9/2<I≦15/2. The excitation energies and electromagnetic properties of more than 70 states for which positive parity has been established or is most probable are compared with the results of shell-model calculations which use the complete configuration space of theO d5/2 — 1s1/2-O d3/2 shell and the unifieds-d shell Hamiltonian. The agreement is good to excellent. The first intruder states are located near 6.8 MeV excitation energy. The collective properties of25Mg beyond the well established rotational bands are investigated using both the new experimental information and theB(E2)s obtained from the shell model. The spectrum of25Mg is completely rotational for the first five to six MeV above the yrast line. Shell modelB (M 1)s reflect the Nilsson model structure of25Mg in great detail. The prospectiveIπ=9/2−, 13/2−, and 15/2− members of the established negative-parity,K=1/2 band are found in levels atEx=7801, 9410, and 8896 keV.

High-spin states in27Al

The24Mg(α, p γ) reaction has been studied atα-particle energies of 13, 15, and 15.2 MeV. Using method II of Litherland and Ferguson spin assignments or restrictions thereof could be made to the27Al levels atEx=5500 keV (I=11/2, 7/2), 6950 keV (I=11/2, 7/2), 7226 keV (I=9/2), 7286 keV (I=9/2–13/2), 7399 keV (I=11/2, 7/2) and 7443 keV (I=13/2, 9/2). Using the Doppler-shift attenuation method a lifetimeτ=20–50 fs was obtained for theEx=7399 keV level and a limitτ<20 fs for the levels atEx=7443, 7286, 7226, 6950, 6718, and 6287 keV, respectively. Further spin assignments in27Al were obtained by a study of the26Mg(p, γ) reaction at the Ep=2174 keV resonance. Fromγ-ray angular distributions the resonance spin was determined as I=9/2 (previous assignmentI=7/2) and the spin of theEx=5959 keV level was determined asI=7/2. Tentative spin assignments were made to the levels atEx=6537 keV (I=7/2) and 6718 keV (I=9/2). From the combined evidence of the24Mg(α, p) and26Mg(p, γ) reactions the spin of the 6287 keV level was found to beI=7/2. The results are compared with shell model calculations and the Nilsson model.

New aspects of the rotational model in25Mg

Spin assignments have been made to the25Mg levels in theEx=5–6 MeV region fromγ-ray angular distributions measured in the22Ne(α, n y) reaction atEα=8.0 and 8.8 MeV and fromp-γ angular correlations measured in the24Mg(d, p γ) reaction atEd=6 MeV. Unique spin assignments ofJ=7/2, 5/2, 5/2, and 9/2 could be made to the levels atEx=5005, 5511, 5851, and 5967 keV, respectively. Ambigious assignments have been made to the levels atEx=5245, 5524, 5785, 6032 keV (J=11/2, 7/2), 5455 keVJ=13/2, 9/2), and 5738 keV (J=3/2, 5/2). The present data confirm previous assignments ofJ=1/2 to the levels atEx=5108 and 5466 keV, respectively. Lifetime estimates have been obtained, using the Doppler-shift attenuation method, for the levels atEx=5245 keV (τ=30–60 fs), 5785 keV (τ=50–100 fs), 5967 keV (τ=50–100 fs), and 5455 keV (τ>1ps). All other levels in theEx=5–6 MeV region haveτ<60 fs. A breakdown of theK-selection rule has been observed in theγ-decay of some high spin states, indicating a deviation from the strong coupling model.

The structure of27Mg from the26Mg(d, pγ) reaction

The excited states of27Mg have been investigated up toEx=7976 keV using the26Mg(d, pγ) reaction. A measurement of proton-γ-ray angular correlations atEd=6.5 MeV has yielded spin-parity assignments or restrictions of levels up toEx=6508 keV and accurate excitation energies and branching ratios of levels up toEx=7976keV, respectively. The Doppler-shift attenuation ofγ-rays has been measured atEd==5.5 MeV yielding information on the lifetimes of levels up toEx=6009keV. The properties of positive-parity levels up toEx=5 MeV are successfully reproduced by recent shell-model calculations. The rotational model is also applicable to the description of collective properties without reaching the overall agreement of the shell model. The negative-parity states are well described by the rotational model.

Lifetime measurements in25Mg

The mean lives of25Mg levels have been studied using the Doppler-shift attenuation method in the24Mg(d, pγ) reaction atEd=6 MeV. Except for theEx=6873 keV level, all levels between 2 MeV excitation energy and the neutron binding energy could be investigated, yielding 11 mean lives and 30 limits thereof. A large number of multipole transition rates could be deduced and several parity assignments be made. The relevance of the data with respect to the rotational model is discussed. A serious breakdown of theK-selection rule, which has been proposed earlier for some high-spin states, is confirmed.