Quenching of Stretched Magnetic Transitions

Abstract

Quenching of the MQ transition strengths in inelastic scattering to 6" states in 28 Si is predicted when the a\/ 2 shell-model space is enlarged to include the s\/2 single-particle orbital. Nucleon transfer cross sections to the yrast (/, T) = (6 ", 0) and (6', 1) states and B (Ml) between them are also predicted to decrease when the extended space is used. The calculations suggest that the data may be understood in terms of only nucleon degrees of freedom once an adequate model space is used.PACS numbers: 23.20. Ck, 21.60.Cs, 25.80.Fm In both the (Id,2s) and l/ 7/2 shells inelastic scattering experiments 1 " 6 on even-even nuclei populating the high-spin "stretched" magnetic states have cross sections very much smaller than predicted by the (^5/2*)^ [dg^"" 1 ®/,/^-. T and (A/ 2 n )-* [/ 7/2 ' 1 " 1 (8)^9/ 2 ] 8 -; r models. This has led to the speculation that nonnucleonic degrees of freedom may have to be introduced to account for the observed quenching. In this note, we examine the effect of increasing the size of the model space and show that, for 28 Si, the inclusion of the 2s 1/2 single-particle level substantially reduces the predicted cross sections. Other properties of these 6" levels are examined and, in all cases, use of the larger space improves the agreement between theory and experiment. Although significant deviations from experimental results still exist, the calculations suggest that when a still larger model space is used the remaining discrepancies may become small.The simplest model of 28 Si is one in which the 0 + ground state is described by the configuration (^5/2 12 )/=o;r = o and the (A? 1 ) = ( 6~> 0) anc * (6",1) states have the form [d 5/2 n £)/ 7/2 ] / = 6 -. r . With this model one predicts equal matrix elements for the transition operator [a 7 / 2;1 / 2 *® a 5 / 2;1 / 2 ] 6M . T0 which governs inelastic scattering between the ground state and these two 6" levels,

Fragmentation of stretched spin strength inSi28

“…The curves show the strength function for inelastic scattering calculated by AL (dashed curve) in a dsn,s\/2 basis (Ref 13). and in this work (solid curve) after allowing up to four particles in the ^3/2 orbit compared to "data points" representative of the observed scattering strength.…”

“…Measured properties of the observed 6~ states in 28 Si compared to results from the simple model (find ^2), the AL model (fmld5/2S1/2I n ) from Ref 13,. and this work.…”

“…1 show the result of our calculation after 24 Lanczos iterations for each isospin compared to the AL results. 13 Following the Livermore approach, 20 the curves in Fig. 1 are based on the strength of Z 2 for the individual eigenstates spread by a Gaussian of width {(H-{H)) 2 ) xn or 100 keV, whichever is larger.…”

Pion scattering to 8/sup -/ stretched states in /sup 60/Ni

“…Systematic shell model studies, which include configuration mixing in the ground and excited states, predict too much cross section by at least a factor of two for many of the observed transitions. 9 Including meson exchange currents in the analysis of the electron scattering data to stretched states results in yet further disagreement between experiment and theory. 10 The principal means of investigating high-spin particle-hole states has been by inelastic electron scattering.…”

Fragmentation of stretched spin strength inN=Zsd-shell nuclei

“…The challenge has been to extend such relatively complete calculations to the sd shell (and beyond) so that systematic comparisons can provide additional tests of the model. In an earlier shell-model study in the sd shell, Amusa and Lawson [29] (hereinafter to be called AL) examined the effect of limited configuration mixing on several aspects pertaining to the single particle transition 1ds/z~1f7/q in Si. Liu and Zamick [26] have made related studies using the rotational model with Coriolis mixing.…”