Search for high-spin states in29Si using a neutron time-of-flight spectrometer

The²⁶Mg(α,nγ) reaction has been used in connection with a neutron time-of-flight spectrometer to search for high spin states in²⁹Si betweenE _x =8.4 and 11.4 MeV. The γ-decay of twenty levels has been observed. Nine levels have not been observed before. A candidate for theJ ^π=13/2^?,K=7/2 state has been located with theE _x =8761 keV level.     

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 atE _α=14 and 15.5 MeV. Spin-parity assignments of the observed levels were obtained fromn?γ angular correlation and lifetime measurements atE _α=14.5 MeV. Theγ-decay of the 9,164 keV level was investigated separately with the²⁷Al(p, γ) reaction at theE _p=2,160 and 2,312 keV resonances. Rotational bands withK ^π=3^? (comprising levels atE _x=6,879, 8,413, 10,188 and 12,204 keV),K ^π =5^? (comprising levels atE _x=9,702, 11,577 and 13,741 keV) andK ^π=0⁺ (comprising levels atE _x=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 theI ^π=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.     

Search for high-spin isomeric states in the rare-earth region

Several new high-spin (I>10) isomeric states in the mass region A=144–164 are located in a systematic search with ¹²C induced reactions, employing a sixteen NaI (Tl)Ge(Li) detector multiplicity filter. The proximity to the N=82 neutron shell closure strongly suggests that shell effects contribute significantly to this isomerism.     

Proton-threshold states in 28Si

The ²⁷Al(³He, d)²⁸Si reaction has been used to locate candidates for resonances in the ²⁷Al + p system residing near the proton-capture threshold in the energy region characteristic of quiescent stellar hydrogen burning. Two such states are observed at excitation energies E_x = 11.658 MeV(J^π = 2⁺) and 11.671 MeV (J^π = 1⁻). A comparison of the cross sections for the ²⁷Al(³He, d)²⁸Si and the ²⁷Al(α, t)²⁸Si reactions implies angular-momentum transfers of l = 2 and l = 3, respectively, for the two states of interest. Using this result, an astrophysically significant upper limit on the thermonuclear reaction rate has been calculated for the ²⁷Al(p, γ)²⁸Si reaction which is found to be too slow to affect the ²⁷Al abundance in red giants.     

Shape-excited states of28Si

The Hartree-Fock (HF) minima for the nucleus²⁸Si were obtained for the prolate, oblate and spherical shapes using the interaction obtained by Preedom and Wildenthal. The interaction gives rise to large energy separation between the prolate and the oblate shapes. The spherical solution is just 2 MeV above the lowest HF (oblate) minimum. The spectrum projected from the oblate HF state is in good agreement with the experimental spectrum. The transition probabilities for the different energy levels also agree reasonably well. The configuration mixing calculations performed on the basis of states projected from the three shapes indicate that there is no significant mixing of different projected states. The second 0 ₂ ⁺ state, thus obtained, corresponds to the third 0 ₃ ⁺ state in the experimental spectrum and stems dominantly from the spherical HF state. It is seen that the structure of the energy levels of²⁸Si, especially the second 0 ₂ ⁺ level is very sensitive to the two body interaction. The results are compared with those obtained using the renormalised interaction of Kuo.     

Shape coexistence and high-spin states in 28Si

The spectrum of ²⁸Si is investigated within the cranked Nilsson-Strutinsky model with all kinds of many-particle-many-hole excitations accounted for. Calculated excitation energies and quadropole moments are compared with experimental data. The recently observed backbend from 8⁺ to 10⁺ is suggested to be caused by the crossing of the oblate ground band with a prolate or slightly triaxial band having one proton and one neutron excited to the fp shell.     

The 6− analog-antianalog states of 28Si

A resonance is observed in the ²⁷Al(p, γ)²⁸Si reaction at E_p=2876±2 keV, which corresponds to an excitation energy of 14356±2 keV. The 14.36 MeV level decays to a new level at 11577±2 keV, which is turn decays to the known level at 9701.8±0.5 keV. With previous information on the 9.70 MeV level and the present γ-ray angular distributions, obtained from singles spectra as recorded by a 40 cm³ Ge(Li) detector, the spins of the three levels can be limited to J=5, 6; J=6; and J=5, respectively. Transition strength arguments based on measurements of the strength of the 2876 keV resonance and the lifetime of the 11.58 MeV level indicate that the 14.36 MeV level has J^π=6^?, T=1 and that the 11.58 MeV level has J^π=6^?, T=0.     

A weak coupling model for high-spin states

Weak coupling of a rotating core and an odd particle is used to explain the appearance of pure rotational bands, as well as decoupled aligned state bands by simple geometrical properties.     

Reduced transition probabilities for high-spin states in 232Th

The lifetimes and reduced transition probabilities for spins 6⁺ ? 18⁺ in the ground-state rotational band of ²³²Th have been measured by recoil-distance, Doppler-broadened lineshape, and multiple Coulomb excitation methods using ⁸⁴Kr and ¹³⁶Xe projectiles. Within the limits of experimental uncertainty, transition probabilities from all of the methods are consistent with those of a good rotor for spins 6⁺ ? 18⁺. The descrepancy with the prediction of simple β-band mixing and centrifugal stretching models is discussed.     

g Factors for high-spin states of238U

Calculations of the g-factor and the moment of inertia J for high-spin states of²³⁸U are performed using the model which is based on the cranking Hartree-Fock-Bogolyubov (CHFB) theory. Importance of the state-dependent pairing force on 3(ω²) is investigated.     

Spectroscopy of high-spin states in 211,212,213Fr

The level structures of ²¹¹Fr, ²¹²Fr and ²¹³Fr have been observed to high spins, ~ 28 /kh, (and excitation energies ~ 8 MeV) using a variety of γ-ray spectroscopic techniques. The structure of these nuclides is discussed in terms of couplings of single-particle states through empirical shell-model calculations. Good agreement with experiment is obtained. In ²¹²Fr and ²¹³Fr core-excited configurations are required to explain the properties of the highest states. A number of long-lived states were observed in each nucleus some of which decay by enhanced E3 transitions. The E3 transition strengths are discussed.     

On the ferromagnetism of high-spin states

It is established on the basis of the idea of strong interaction within the unit cell that ferromagnetic instability is possible in a system with hopping between d-electron states of atoms that are in high-spin states and form a bcc lattice. The phase diagram of the existence of ferromagnetic ordering as a function of the degree of filling of the d shell is constructed. Pis’ma Zh. éksp. Teor. Fiz. 68, No. 4, 275–280 (25 August 1998)     

Alpha decay of isobaric analogue states in24Mg and28Si

Theα-decay of isobaric analogue states (which are forbidden by isospin selection rules) excited in²⁴Mg and²⁸Si through preton capture by²³Na atE _p=677 and 739 keV and by²⁷Al atE _p=295, 327 and 405 keV, respectively, have been studied using solid state track detectors. The ratio ofα-decay widths of the two resonance states in²⁴Mg to the state at 1.632 MeV (2⁺) in²⁰Ne yields the value 0.065 for the mixing parameterε and the value 4.01 keV for the Coulomb matrix element responsible for the isospin mixing in²⁴Mg. In²⁸Si the measurement of theα-decay widths of the three resonance states resulted in the determination of the proton, gamma and alpha partial widths which comprise the total width of the resonance states. Limits have been set for the value of the two mixing parameters involved in this case. Upper limits of 16 and 39 keV have been obtained for the Coulomb matrix elements responsible for the isospin mixing in²⁸Si.     

Angular correlations in 28Si + 28Si resonances

Recent particle—particle—γ coincident measurements on a ²⁸Si+²⁸Si resonance have suggested “vanishing spin alignments”. New analyses of the spin alignments with a molecular model are presented. It is clarified that due to a triaxial deformation of the total system a wobbling mode (K-mixing) appears to give rise to spin disalignment.     

Deformation effects in the 28Si+12C and 28Si+28Si reactions

The possible occurrence of highly deformed configurations is investigated in the ⁴⁰Ca and ⁵⁶Ni di-nuclear systems as formed in the ²⁸Si + ¹²C, ²⁸Si reactions by using the properties of emitted light charged particles. Inclusive as well as exclusive data of the heavy fragments and their associated light charged particles have been collected by using the ICARE charged particle multidetector array. The data are analysed by Monte Carlo CASCADE statistical-model calculations using a consistent set of parameters with spin-dependent level densities. Significant deformation effects at high spin are observed as well as an unexpected large ⁸Be cluster emission of a binary nature.     

Lifetime study of high-spin states in 104,105In

In the neutron deficient isotopes _104,105In lifetimes of high spin states in the range from 0.5 ps to 600 ps have been measured in a coincidence recoil distance Doppler shift (RDDS) experiment. By combining the Doppler shift attenuation (DSA) and RDDS methods at few μm flight distances, the problem of delayed feeding has been avoided and very short lifetimes in the range 0.5–0.8 ps have been determined. Shell model calculations with strong restrictions for the neutron orbitals reveal good agreement with experimental level energies and still fair agreement for most measured B(M1) and B(E2) values. Received: 30 June 1998     

Properties of unbound high-spin states in27Al

Selectively excited high-spin states in²⁷Al have been located by the reaction¹²C(¹⁶O,p)²⁷Al. The excitation functions of these states have been measured at incident energiesE _cm=18.7 to 30.1 MeV at intervals of 150 keV. They exhibit maxima of presumably non-statistical origin. At four different energies the subsequent decay of the²⁷Al states has been studied by detecting the final heavy recoils with the Munich recoil spectrometer in coincidence with the proton emitted in the first stage of the reaction. Using this new method branching ratios of theγ-, α-, andn-decay have been measured. Based on angular correlation arguments spins up to 27/2? have been determined within the experimental accuracy of 1–2?. The observations suggest a superdeformed shape of²⁷Al at least in some of the states.