States In 28Si Research Articles

Toroidal states in 28Si with covariant density functional theory in 3D lattice space

The toroidal states in 28Si with spin extending to extremely high are investigated with the cranking covariant density functional theory on a 3D lattice. Thirteen toroidal states with spin I ranging from 0 to 56ħ are obtained, and their stabilities against particle emission are studied by analyzing the density distributions and potentials. The excitation energies of the toroidal states at I=28, 36, and 44ħ reasonably reproduce the observed three resonances extracted from the seven-α de-excitation of 28Si. The possible existence of α clustering in these toroidal states is discussed based on α-localization function.

Asymmetric cluster structure and isoscalar monopole/dipole transitions of 28Si

The asymmetric cluster states in 28Si are discussed on the basis of antisymmetrized molecular dynamics. It is found that the inversion doublet bands having the 24Mg+α, 20Ne+8Be and 16O+12C cluster configurations appear in the energy range of Ex = 10 ∼ 15 MeV. It is demonstrated that the 24Mg+α, 20Ne+8Be cluster states have pronounced isoscalar monopole (IS0) and dipole (IS1) transition strengths from the ground state. This result suggests that the IS0 and IS1 strength are good probe for these cluster states.

Search for highly excited states in 28Si

The theoretical and experimental determination of superdeformed states in nuclei in the mass region A≤40 has been since a long time one of the major challenges of nuclear structure studies. Despite the considerable experimental and theoretical work dedicated to this topic, up to now superdeformed bands have been found in only two nuclei, 36Ar and 40Ca. While the experimental signature of the superdeformed nature of those states is irrefutable, their theoretical interpretation is still uncertain. In particular, it is not clear whether clusterisation is responsible of the onset of superdeformation. For this reason, we wanted to investigate an even lighter system, 28Si, where a number of theoretical calculations predict the presence of superdeformation as an effect of the cluster structure of the nucleus.

Mixed symmetry states and electromagnetic transition in the N = Z nucleus 28Si

The interacting boson model with isospin (IBM-3) has been used to study mixed symmetry states and electromagnetic transitions at low-lying states for a 28Si nucleus. The theoretical calculations show that the 24+ state is the lowest mixed symmetry state in 28Si and the 43+ state is also a mixed symmetry state.

Isotope effect on the lifetime of the 2p0state in phosphorus-doped silicon

The low-temperature (∼5 K) phonon-assisted relaxation of the 2p0 state of phosphorus donors in isotopically pure, monocrystalline 28Si has been studied in the time domain using a pump-probe technique. The lifetime of the 2p0 state in 28Si:P is found to be 235 ps, which is 16% larger than the lifetime of a reference Si:P sample with a natural isotope composition. The interaction of the 2p0 state with intervalley g-type longitudinal acoustic and f-type transverse acoustic phonons determines its lifetime. This interaction, which depends on the homogeneity of the crystal, becomes weaker in 28Si because of its more perfect crystal lattice compared to natural Si, and this leads to a longer lifetime. The difference between the linewidths of the 1s(A1) → 2p0 transition in 28Si:P and natural Si:P is more than a factor of two. It follows that linewidth broadening due to isotopic composition is an inhomogeneous process.

Search for 28Si cluster states through the 12C+16O radiative capture

The 12C+16O resonant radiative capture reaction has been studied at 5 bombarding energies around the Coulomb barrier, between Elab = 15.4 and 21.4 MeV. These experiments have been performed at the TRIUMF laboratory (Vancouver, Canada) using the Dragon 0° spectrometer and the associated BGO array. The most remarkable result is the previously unobserved decay path through 28Si doorway states of energies around 12 MeV leading to the measurement of new capture cross-sections. The feeding of specific, deformed states in 28Si from the resonances is discussed, as well as the selective feeding of 1+ T=1 states around 11 MeV.

Properties of effective interactions and the excitation of [formula omitted] states in 28Si

Cross-section and analyzing power data from ( p , p ′ ) scattering to the 6 − states at 11.58 and 14.35 MeV in 28Si, taken with 80, 100, 134, and 180 MeV protons, have been analyzed using a distorted wave approximation with microscopically defined wave functions. The results of our analyses of those data, taken in conjunction with the analysis of an M6 electron scattering form factor, suggest that in the specification of the two states, ∼ 25 % and ∼ 36 – 40 % respectively of the strengths of isoscalar and isovector 0 d 5 2 −1 – 0 f 7 2 excitations from the ground state as described by a Skyrme–Hartree–Fock model. The energy variation of data also suggests that the non-central components of the effective interactions at 80 and 100 MeV may need to be enhanced.

The 12C( 16O, 12C 12C) α reaction at Ebeam=75 MeV

The 12C( 16O, 12C 12C) α reaction has been measured at E beam=75 MeV, for 24Mg emission from 0 to 180 degrees. This energy coincides with an enhancement observed in earlier measurements of the 12C+ 16O system. The properties of the angular correlations of the reaction products are found to be consistent with a resonance in 28Si at E x=48.9 MeV, with spin J=17±1. The decay properties of the states in 24Mg, populated following α-decay of the resonance, suggest that the state in 28Si possesses a triaxial structure linked to a ( sd) 8( fp) 4 configuration.

Mesonic effects deduced from good-resolution ( 3He, t) reaction in comparison with inelastic scatterings

Gamow-Teller (GT) states excited by a charge-exchange reaction on N = Z nuclei with ground-state isospin T = 0 have isospin T = 1, while M1 states excited by inelastic scattering of electrons or protons can have either T = 0 or T = 1. The latter are the isobaric analog states of the GT states. By comparing the GT states observed in the good-resolution ( 3He, t) reaction at 150 MeV/ u with the M1 states observed in inelastic proton reactions, the isospin of each 1 + state in 28Si was determined. Furthermore, by comparing results from electron inelastic scattering induced by electro-magnetic M1 operator, different response of nucleus depending on different interaction is investigated. Based on these comparisons among different probes, we discuss (1) the contribution of the “ Δ- h excitation” mechanism in the quenching of magnetic excitations; (2) the effects of meson exchange currents; and (3) the orbital contribution in the M1 operator.

Search for highly deformed shape isomers in 28Si, 24Mg and 20Ne linked by successive alpha decays

Measurements of the 12C( 16O, 20Neα)α, 12C( 16O,αα) 20Ne, 12C( 16O, 8Be) 20Ne and 12C( 16O, 16Oα) 8Be reactions have been performed at E c.m. = 27.0 MeV. For decays proceeding through resonant states in the intermediate 24Mg nucleus, angular correlation measurements have enabled spin assignments to be made and the dominant entrance channel angular momentum to be determined. The results are inconsistent with recent similar experiments which were interpreted as arising from successive α-decays from deformed shape isomeric states in 28Si and 24Mg.

Dilepton decay from excited states in 28Si populated via different isospin entrance channels.

Nuclear states in $^{28}\mathrm{Si}$, with an initial excitation energy E*=50 MeV, were populated via the isospin T=0 reaction $^{4}\mathrm{He}$${+}^{24}$Mg and the mixed-isospin $^{3}\mathrm{He}$${+}^{25}$Mg reaction. In both reactions the dilepton (${\mathit{e}}^{+}$${\mathit{e}}^{\mathrm{\ensuremath{-}}}$) and photon decay yields were measured concurrently, up to transition energies of 30 MeV. Both the dilepton and the photon yields are well described by a modified version of the statistical decay model, which includes both the photon and dilepton decays of giant resonances built on excited nuclear states. An excess of counts in the ${\mathit{e}}^{+}$${\mathit{e}}^{\mathrm{\ensuremath{-}}}$ spectrum, over the converted photon yield, is observed in the energy region 17--22 MeV in the $^{3}\mathrm{He}$ induced reaction, possibly indicating an entrance channel effect. \textcopyright{} 1996 The American Physical Society.

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.

Di-lepton yield from the decay of excited 28Si states

The first dilepton yield measurements from excited nuclear states obtained with a new Positron-Electron Pair Spectroscopic Instrument (PEPSI) are reported. Nuclear states in 28Si, with an initial excitation energy E ∗ = 50 MeV , were populated via the isospin T = 0 reaction 4He + 24Mg and the mixed-isospin 3He + 25Mg reaction. In both reactions the dilepton (e +e −) and photon decay yields were measured concurrently. An excess of counts in the e +e − spectrum, over the converted photon yield, is observed in the energy region above 15 MeV. An analyses is discussed whereby the observed excess counts are assumed to represent the isoscalar E0 strength in excited nuclear states.

Evidence for superdeformed shape isomeric states in 28Si at excitations above 40 MeV through observations of selective particle decays of 16O+12C resonances in 8Be and alpha channels.

Excitation functions of the reactions $^{12}\mathrm{C}$${(}^{16}$O${,}^{8}$Be${)}^{20}$Ne and $^{12}\mathrm{C}$${(}^{16}$O${,}^{4}$He${)}^{24}$Mg leading to several states have been measured in the range ${\mathit{E}}_{\mathrm{c}.\mathrm{m}.}$=25.7--38.6 MeV. From cross channel correlation analysis two intermediate resonance structures at ${\mathit{E}}_{\mathrm{c}.\mathrm{m}.}$=26.9 and 29.5 MeV corresponding to excitation energies of 43.7 and 46.2 MeV in $^{28}\mathrm{Si}$ have been identified. Angular distribution measurements for $^{12}\mathrm{C}$${(}^{16}$O${,}^{4}$He${)}^{24}$${\mathrm{Mg}}_{\mathrm{g}.\mathrm{s}.}$ have also been made at the lower resonance at ${\mathit{E}}_{\mathrm{c}.\mathrm{m}.}$=26.9 MeV which enable spin assignment of ${\mathit{J}}^{\mathrm{\ensuremath{\pi}}}$=${14}^{+}$ to this resonance. Striking observation of preferential $^{8}\mathrm{Be}$ decay of both the resonances to the 8-particle--4-hole states in $^{20}\mathrm{Ne}$ is interpreted as evidence for the resonances to be highly deformed shape isomeric configurations in $^{28}\mathrm{Si}$ expected from deformed shell-model calculations.

The shell model for T = 0 and T = 1 states in the middle of the sd shell

Spectroscopic properties of several hundreds of T = 0 and T = 1 states of A = 28 and 30 nuclei have been compared with the results of a shell-model calculation (untruncated sd shell). For A = 28 the calculation also includes γ-ray branching and mixing ratios, lifetimes, spectroscopic factors, proton widths and log ft values for the 28P(β +) 28Si decay. Uncertainties in spin, parity and isospin determinations have been resolved and parent-analogue relations have been established. Errors have been spotted in the analysis and evaluation of experimental data. A clear indication has been found of intruder states in 28Si and 30P. The present work has provided a good idea as to the reliability of the shell model in the middle of the sd shell. With previous results for A = 26 taken along one arrives at an average error of 145 keV in the theoretical energies. The averaged absolute spread in the ratio of theoretical over experimental strengths amounts to a factor 1.9 for isoscalar E2 transitions, and to a factor 2.3 for isovector Ml transitions.

Study of the M1 isovector strength in 28Si

Inelastic electron scattering measurements up to 1.5 fm-1 of strong M1 transitions to the 10.900 MeV, 11.446 MeV and 12.330 MeV states in 28Si, which form the bulk of the 28Si M1 isovector giant resonance, have revealed striking differences in their transition form factors, and have allowed information on the configurations contributing to each transition to be deduced. Full 2s-1d shell model calculations have also shown large differences in the form factors of the lowest Jpi =1+, T=1 states, but do not agree, in detail, with the experimental data.

Studies of the generator coordinate method within the complex VAMPIR model

A set of deformed states generated with the VAMPIR approach is used for a generator coordinate method description of the 0 + states in 28Si. As model space the 1s0d shell was used, and as effective hamiltonian the Chung-Wildenthal interaction. Thus a comparison with the “exact” shell-model solutions was possible. Potential-energy functions, overlaps between different deformation points and the hamiltonian matrices are discussed for three different types of VAMPIR solutions: with full symmetry projection, with particle-number projection only and without any projection. The validity of the “horizontal expansion” of the hamiltonian matrix is examined.

Low-energy resonances in 25Mg(p, γ) 26Al, 26Mg(p, γ) 27Al and 27Al(p, γ) 28Si

Gamma-ray decay schemes have been measured with bare and Compton-suppressed Ge detectors at low-energy resonances ( E P < 340 keV) in the (p, γ) reactions on 25Mg, 26Mg and 27Al. Altogether 58 new decay branches have been observed and a new 26Mg(p, γ) 27Al resonance has been found at E P = 154.5 ± 1.0 keV. The new branchings lead to J π ; T determinations (or limitations) for two states in 26Al and four states in 28Si. The absolute strengths of the 25Mg(p, γ) 26Al and 26Mg(p, γ) 27Al resonances have also been obtained, and the uncertainties of the stellar rates, deduced from the available data for both reactions, are significantly reduced. Some astrophysical consequences are discussed.

Gamow–Teller strength functions from scattering experiments

We present here recent [Formula: see text] results from TRIUMF that are relevant to the determination of spin-flip isovector strength functions in nuclei. Distortion factors needed for the extraction of nuclear-structure information have been deduced from cross sections and analyzing powers in elastic scattering for several energies and targets. Nonrelativistic optical potentials obtained by folding effective nucleon (N)–nucleus interactions with nuclear densities are found to overpredict both elastic and reaction cross sections, whereas Dirac calculations that include Pauli blocking are in good agreement with the data. Spin observables (Snn and Ay) for the quasi-elastic region in 54Fe[Formula: see text] at 290 MeV provide some evidence for the reduction of the effective proton mass predicted in relativistic mean-field theories as a consequence of the attractive scalar field in the nuclear medium. The energy dependence of the effective N–nucleus interaction at small momentum transfers has been investigated using isoscalar and isovector 1+ states in 28Si as probe states. We find that the cross sections for the isovector transitions are in good agreement with predictions for the dominant Vστ part of the Franey–Love interaction. Gamow–Teller (GT) strength functions have been obtained in 24Mg and 54Fe from measurements of both cross sections and spin–flip probabilities Snn. The spin-flip cross sections σSnn are particularly useful in heavier nuclei to discriminate against a continuous background of ΔS = 0 excitations. In the (s, d) shell where full shell-model wave functions are available, the GT quenching factors [Formula: see text] are in good agreement with those from recent (p, n) and (n, p) experiments. We show that a state-by-state comparison of (p, p′) and (e, e′) results has the potential of identifying pionic current contributions in (e, e′). The GT quenching factors in 54Fe are smaller than in the (s, d) shell probably because of severely truncated shell-model wave functions, particularly those of the nuclear ground state.

The reaction 26Mg( 3He, n) 28Si at 23 and 45 MeV

Angular distributions of neutron groups from the 26Mg( 3He, n) 28Si reaction have been measured at bombarding energies of 23.1 and 45.5 MeV. The experimental results are compared with DWBA and coupled-channel calculations. The zero-range DWBA calculation using shell-model two-nucleon spectroscopic amplitudes explains rather well most of the experimental data of the lowest four states in 28Si, but it is necessary to take two-step processes into account to reproduce all of them satisfactorily. A strong neutron group observed at about 6.9 MeV in residual excitation is described as one composed of unresolved four groups, of which three are attributed to positive-parity states in 28Si predicted in this energy region by the shell-model calculation and one to the 6.88 MeV 3 − state in 28Si.