The structure of28Si above 10 MeV excitation energy III: Level scheme and shell model interpretation

Abstract

Abstract. The 2*Si level scheme up to 14.5 MeV excitation energy is reevaluated using information from two preceding papers. It consists of approximately 250 levels which are almost completely characterized according to the quantum numbers 1, re, T of the levels. The properties of positive-parity states are compared to the predictions of shell model calculations within the complete s-d basis space using the unified s-d shell Hamiltonian. A spectrum of 48 experimental T = 1 states between 9.3 and 16 MeV is reproduced with a rms deviation of only 150 keV. A calculation of radiative widths and 7-decay modes which uses free-nucleon 9-factors yields excellent agreement with experiment and confirms that quenching of M1 transitions is only marginal in 2ssi. The detailed shell model analysis of the T = 0 spectrum is extended to the limiting energy where T = 1 wave function admixtures, not contained in the theory, become important experimentally. This happens at 6-8 MeV above the yrast state, depending on the spin value. Altogether it appears that a spectrum of 171 levels below 14.5 MeV, which have positive or unassigned parity, is almost completely accounted for by the model. Apparent intruder states from outside the s-d shell space are observed at Ex=lO945keV (U=4 +) and 12860 keV (U= 6 +) and are interpreted as members of a K S = 0 + rotational band.

α clustering in Si28 probed through the identification of high-lying 0+ states

“…Extrapolating the candidate superdeformed band in 28 Si from Refs. [3,8] suggests that the bandhead should lie at around 9.3 MeV. The 9.71-MeV state is the only 0 + state in this excitation energy region (from around 8.8 MeV to 10 MeV) and is the only observed candidate for the band-head.…”

Status of the Mg24(α,γ)Si28 reaction rate at stellar temperatures

“…All states without an excitation-energy uncertainty have uncertainties below 1 keV as per Ref. [12]. assignment, this disagrees with the branching ratios determined from the measurement of Strandberg et al, in which no transition to the ground state from this resonance is observed [4].…”

“…The sources of resonance strengths are the direct measurements performed by Smulders and Endt [8], Lyons [9] and Strandberg et al [4]. Spectroscopic information is available from the γ-ray spectroscopy data obtained in 27 Al(p, γ) 28 Si and 24 Mg(α, γ) 28 Si reactions of Brennesien et al [10][11][12], the 28 Si(α, α ′ ) 28 Si data of Adsley et al [13], the 28 Si(p, p ′ ) 28 Si data of Adsley et al [14], and the 28 Si(e, e ′ ) 28 Si data of Schneider et al. We briefly summarise these experimental data and the information obtained from them below.…”

“…The E x = 11.142-MeV state was observed in γ-ray decays following 24 Mg(α, γ) 28 Si and 27 Al(p, γ) 28 Si reactions [10][11][12]. As assignment of J = 0−2 was made on the basis of the feeding from a J π = 1 + excited state.…”

“…This data table is based on Ref. [12] with additional level assignments from Ref. [13] and ENSDF [25].…”