H. Röpke

The γ-decay of25Mg

The reactions24Mg(d,p γ) and27Al(d,α γ) have been used to study the main γ-decay modes of all bound and several unbound levels in25Mg up toEx=8 574 keV. The relevance of the data with respect to the rotational model is discussed.

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.

Deformed states of high spin in42Ca

The42Ca levels at 4,715 and 6,633 keV excitation energy have been investigated using the39K(α,pγ reaction atEα=14 and 15 MeV. From particle-γ-ray angular correlations the spin assignmentsJ(4,715)=6, 4 andJ(6,633)=8, 6, 4 have been obtained. Lifetime measurements using the Doppler-shift attenuation method yieldedτ (4,715)=120±46 fs andτ(6,633)=52±21 fs. Both levels have positive parity and decay by enhancedE2 transitions. They are interpreted as theJπ=6+ and 8+ members, respectively, of theKπ=0+ rotational band which has theEx=1,837, 2,423 and 3,250 keV states as further members. The enhancement of inbandE2 transitions is 50−16+35W.u. (6→4) and 63 W.u. (8→6) respectively. The intrinsic quadrupole moments which have been derived on the basis of the coexistence model, areQ0=1.13−0.16/+0.37b(8→6) andQ0=1.36±0.25b(6→4), respectively. TheJπ=10+ member of the rotational band has possibly been observed as a level at 8,856±5 keV excitation energy.

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.

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.

The structure of28Si above 10 MeV excitation energy I: gamma-decay modes and radiative widths of levels

The systematics ofγ-decay modes and radiative widths of highly excited states in28Si has been extended by takingγ-ray spectra on 60 resonances of the27Al(p,γ) reaction in the range 1097 keV≤Ep≤ 4492 keV (12643 keV≤Ex≤ 15915 keV) and on the24Mg(α,γ) resonances atEα=3355, 3431, 4003 keV (Ex=12860, 12925, 13 415 keV). Theγ-decay modes of levels in the sub-resonance region (Ex=10−12.5 MeV) were studied with both the27A1(p, γ) reaction and the27A1(d, nγ) reaction at Ed=4,5,6 MeV. Information on radiative widths of levels was obtained by measuring the resonance strengthsSγ=(2I+1)ΓpΓγ/Γ for 52 resonances of the27Al(p, γ) reaction with an emphasis on prospective T=1 states.