F. Glatz

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.

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.

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.

Density of high-spin states in38Ar and42Ca

Theγ-decay modes of38Ar levels withEx≦11,630keV and of42Ca levels withEx≦10,036keV have been studied using the35Cl(α, pγ) reaction at 16MeV and the39K(α, pγ) reaction at 15.14 MeV, respectively. In both nuclei the number of states withJ≧6 exceeds fifty. Weak coupling calculations of the Bansal and French type reproduce the density of high-spin states. The success of the model implies that the excitations of up to four particles from thed3/2 into thef7/2 shell play a role in both nuclei. The structure of deformed states was found to be predominantly 4p/s 6h in38Ar and 4p/s 2h in42Ca, respectively.

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.

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.

High-spin states in48Ti

The45Sc(α, p γ) reaction has been investigated atEα=11, 12 and 13MeV. Theγ-decay of 198 levels in48Ti up to 8,323 keV excitation energy has been observed. High-spin states were investigated by proton-γ ray angular correlation measurements atE=11 and 13MeV and by DSAM lifetime measurements atE=11 MeV. From the combined evidence spin (-parity) assignments were obtained for the levels atEx=8,323 keV (J= 10,8,6), 8,091(12, 10, 8, 6), 7,668(10, 8), 7,427(9, 7), 7,374(11, 9, 7), 6,906(10, 8, 6), 6,172(8+,6+), 6,102(10+,8+), 6,039(6), 6,034(9+, 7+), 5,630(7), 5,197(8+), 5,169(7+), 5,155(5), 4,404(5), 4,398(6+) and 4,046keV (5). Most of the ambiguous spin assignments become unique if the 8,091 keV level hasJ=12, an assumption which is favoured by its excitation function. The level spectrum thus obtained is well reproduced by shell model calculations in the pure (f7/2)8 configuration space. Discrepancies exist in the reproduction ofγ-decay modes. The reason is found in low-lying high-spin intruder states which include the 7,427 and 8,323 keV levels. The spectrum of negative-parity states is understood qualitatively by a comparison with46Ti and42Ca.

High-spin states in38Ar

Theγ-decay of 9338Ar levels between 6.84 and 10 MeV excitation energy has been measured using the35Cl(α,pγ) reaction atEα=14 MeV. A previous measurement of the same reaction could be exploited in the light of the present results, yielding spin(-parity) assignmentsJπ(7,070)=5-, J(7,507)=7,5,Jπ(8,077)=7+, J(8,129)=5,6 and J(8,488)=7,5. Theγ-decay modes of yrast levels, which have been previously observed in heavy ion induced reactions, are revised leading to major changes in their spin-parity assignments. New candidates for theJπ=8- and 9+ levels were found in the levels at 9,644 and 9,928 keV excitation energy, respectively. A comparison of yrast levels with shell-model calculations is satisfying.

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.