J. Siefert

Structure of28Si from the spectroscopy of high-spin states

A search for high-spin states in28Si has been performed byn−y coincidence measurements in the25Mg(α,nγy) reaction atEα=14 and 15.5 MeV. Spin-parity assignments of the observed levels were obtained fromn−γ angular correlation and lifetime measurements atEα=14.5 MeV. Theγ-decay of the 9,164 keV level was investigated separately with the27Al(p, γ) reaction at theEp=2,160 and 2,312 keV resonances. Rotational bands withKπ=3− (comprising levels atEx=6,879, 8,413, 10,188 and 12,204 keV),Kπ =5− (comprising levels atEx=9,702, 11,577 and 13,741 keV) andKπ=0+ (comprising levels atEx=6,691, 7,381, 9,164 and 11,509 keV) were observed. The finding of the latter band supports the idea of coexisting oblate and prolate shapes in28Si. A level at 14,643 keV excitation energy has the properties of theIπ=8+ member of the ground state band. There are additional positive-parity high-spin states which do not fit into rotational bands. All types of positive-parity states are well accounted for by shell model calculations.

Shape coexistence and other aspects of nuclear structure in40Ar

Theγ-decay of40Ar has been studied by particle-γ-ray coincidence measurements in the37Cl(α, pγ) reaction at 12 and 13 MeV bombarding energy. Particle-γ-ray angular correlations and linear polarizations ofγ-rays were measured at 12 MeV. A lifetime measurement using the Doppler-shift attenuation method was performed at 11 MeV. The coexistence of spherical and deformed states in40Ar could be concluded from the observation of aKπ=0+ rotational band which has itsIπ=0+ through 6+ members at 2,121, 2,524, 3,515 and 4,959 KeV excitation energy. The intrinsic quadrupole moment derived fromB(E2) values is ∥Q0∥=1,320−120+60 mb. Negative-parity states with high spin were observed at 4,858(5−), 4,494(5−), 4,226(4−) and 4,991 KeV(4−) excitation energy. A complete account of all levels below 5 MeV excitation energy is obtained by a model in which twod3/2 proton holes couple weakly to the42Ca levels below 4.75 MeV excitation energy.

Spectroscopy of highly excited states in29Si

Particleγ-ray coincidences have been measured in the28Si (d,pγ) reaction at 6.5 and 7 MeV bombarding energy, in the26Mg (α,nγ) reaction at 12, 14 and 15 MeV, and in the27A1 (τ,pγ) reaction at 9 MeV. Theγ-decay has been observed for all bound states of29Si and for 56 unbound states up to 12,960 KeV excitation energy. Particleγ-ray angular correlations were measured in the28Si (d,pγ) reaction at 6.5 MeV and in the26Mg (α,nγ) reaction at 12 MeV. Spin (-parity) assignments or restrictions were obtained for nearly all bound states and some high-spin states above the binding energy. The assignment of mirror levels in29Si and29P has been extended to 8.2 MeV excitation energy. The excitation energies of 41 positive-parity states are reproduced by shell model calculations. The possible existence of aKπ=5/2+ band with prolate deformation is discussed.

Structure of30Si from the spectroscopy of highly excited states

Highly excited states in30Si were investigated using the27Al(α, p γ) reaction. Proton-γ ray angular correlations were measured atEα=12, 14.1 and 15MeV. At 15MeV linear polarizations ofγ-rays were measured in coincidence with protons using a five-crystal Compton-polarimeter. Lifetimes were measured atEα=14.6 MeV using the Dopper-shift attenuation method. Unambigious spin-parity assignments were obtained for the levels at 6,865 (3+), 7,001 (5+), 7,079 (3+), 7,810 (4), 9,371 (6+), 7,613 (4−), 8,196 (5−), 8,596 (4−), 8,963 (5−), 9,111 (6−), 9,350 (4−), 9,507 (5−), 9,777 (6−), 10,188 (5−), 10,305 (3−), 10,561 (6−), 10,725 (7−), 11,477 (6−) and 11,544 (7−)keV excitation energy, respectively. The structure of30Si is understood both in terms of the shell model and the collective model. The levels at 5,487, 6,505, 8,196, 9,777 and 11,544keV, respectively, are theIπ=3− through 7− members of a well developedKπ=3− rotational band with intrinsic quadrupole moment |Q0|=350−70+250mb. There is evidence of further rotational bands, among them aKπ=3− band with |Q0|=800−80+422 mb.

The structure of27Al

Gamma decay modes and spin assignments of levels in27Al have been investigated by proton-γ-ray angular correlation measurements in the24Mg(α, pγ) reaction at 14.2, 15 and 15.6 MeV bombarding energy and byγ-ray angular distribution measurements on the Ep=1820, 2114, 2293 and 2574 keV resonances of the26Mg(p, γ) reaction. Unique spin assignments were obtained as follows: I=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 0d5/2-1s1/2-0d3/2 shell and the unifieds-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(M1↑) below 8.5 MeV excitation energy is evaluated, using published resonance fluorescence and (e, e′) data, and quenching relative to the free-nucleon predictions is reexamined. Evidence in favour of a prolate ground state deformation of27Al and implications of this work for the astrophysically interesting26Al(p, γ) reaction are discussed.

High-spin states in26Mg

Theγ-decays of levels in26Mg have been investigated up to 12.5 MeV excitation energy by proton-γ-ray coincidence measurements in the23Na(α, pγ) reaction at 14.2 and 16 MeV bombarding energy. Lifetime-measurements, made with the Doppler-shift attenuation method, and proton-γ-ray angular-correlation measurements were performed at Eα=14.2 MeV. Many high-spin states were observed, among them levels at 6,978 (5+), 7,283(4−), 7,395(5+), 7,953(5−), 8,202(6+), 8,472(6+), 9,065(5), 9,112(6+), 9,169(6−), 9,383 (6+), 9,542(5), 9,829(7+), 9,989(6+) and 12,479(8+, 7−) keV excitation energy. The spectrum of positive-parity states and their electromagnetic properties are reproduced with good accuracy by shell-model calculations which employ a unifieds-d shell Hamiltonian and the unrestricted configuration space of the 0d5/2 1s1/2 0d3/2 shell. Members of five inferred rotational bands, withKπ=0+, 2+, 3+, 0+ and 3− have been observed up to at leastI=6. TheKπ=2+ band shows strong anomalies of excitation energies andE2 transition rates near theI=6 state. The static intrinsic quadrupole moments calculated from the shell model wave functions indicate transitions from prolate to oblate deformation within theKπ=2+ band and also the ground state band. The lowest lyingIπ=4+ state appears to be “spherical” and cannot be associated with a rotational band.

Search for missing predicted high-spin states in28Si

New shell model calculations have predicted several high-spin (Iπ=5+ and 6+) levels in28Si near 10 MeV excitation energy which are missing from or ambiguous in existing experimental studies. Angular distributions, linear polarizations and Doppler-shifts ofγ-rays have been measured for theγ-decay of theEp=1,911 and 2,073 KeV resonances of the27Al(p, γ) reaction in an attempt to discover these missing states or confirm the discrepancies between experiment and theory. The excitation energies and spin-parities of the resonances were determined as 13,424.4±0.2 keV,Iπ=5+ and 13,582.3±0.5 keV,Iπ=6+. States populated in theγ-decay of these resonances were assigned spins and parities as follows: 11,777 keV,Iπ=5+; 11,331 keV,Iπ=6+; 10,417 keV,Iπ=5+; 9,417 keV,Iπ=4+ and 8,945 keV,Iπ=5+. On the basis ofγ-ray transition rates T=1 is assigned to the 13,424 keV level and T=0 to the 10,417 and 11,777 keV levels. With the new data excellent agreement is achieved between the experimental spectrum of28Si and the new shell model predictions. These data provide evidence for aKπ=3+ rotational band comprised by the 6,276, 6,889, 8,945 and 11,331 keV levels. This band emerges also from the shell model wave functions as do theKπ=0+ bands based on the ground state and the 6,691 keV state.