High resolution inelastic electron scattering on 28Si at low momentum transfer and isovector m1 strength in 28Si

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

“…We introduce hitherto unknown levels below 14 MeV excitation energy at 8953, 11572, 13 105, 13 117, 13 201, and 13 710 keV. The closely spaced levels at 11 138 keV [21] and ll 148 keV [14] which have been lumped together by the compiler, reappear as the 11 142 and 11 148 keV states. Above 14 MeV excitation energy we construct the level scheme with the aid of original literature.…”

“…An asterisk is indicative of excitation energies obtained (with keV accuracy) in a preceding paper [10]. Errors are given only if:they exceed 1 keV b Quantum numbers without reference have been obtained from the compilation of [12] for E~ < 14 MeV and from [13] for higher energies ~ h Quantum numbers and/or excitation energies adopted from c [14], a [15], ~ [17], ~ [18], g [19], h [12] i From 7,decay modes of [10] k From Table 4 of [11] 1From Table 2 of [11] reAssignments of J, 7z, T from Table 5 of [11] "The T-assignment is obtained :in Table 5 of [11] ~ I = = 0 +, 1 + assignment which is based on an observed L = 0 transfer in the 26Mg(~', n) reaction [20] should not be taken for granted since the transfer could lead to the 12 301 keV level…”

The structure of28Si above 10 MeV excitation energy II: Assignments of quantum numbers

“…The shell model predicts decay of the 0 +, T = 1 level to the 1779keV state in agreement only with the properties of the 13 040keV level ru from [11] Agreeing U assignments from [12,8] *The T = 1 assignment to the 13 583 keV level rests on the agreement of branching ratios with shell model predictions for the 6~, T = 1 state [4]. The 2SA1 level scheme provides no partner of the 2sSi state except for the 4385 keV level The percentages are based on a comparison of ,/-decay modes with shell-model predictions given in a subsequent paper The notations ~z = u and ~ = n stand for unnatural and natural parity of a level, respectively WThe transition is attributed to the T = 0 component of the 13 425/13 557keV analog state The transition is attributed to the T = 0 component of the 13 979/14 0t2 keV analog state Y Calculated from the (e, e') data of [21] Private communication by P.M. Endt zero. The neglect of possible E2 contributions to the ~/-decay widths has a marginal influence on the resulting value of the M1 strengths.…”

“…The nearby 11 657 keV, U = 2 + state has a convincing T = 0 assignment [1] fThe T = 1 strength may be shared between the 12241 keV level and the 12016keV, J = 3 levels which have very similar ,/-decay modes gI ~ from [1] hU from [4] iA small ( < 8%) percentage is possibly found in the 11 867keV level. For details see text J The corresponding E1 strength exceeds 3 mW u and thereby the upper limit of isoscalar transitions kThe known level scheme of 2sSi provides no other candidate 1U from [12] raThe T = 1 assignment is independent of the spin assignment and follows, for J~ = 0 +, 2 +, from an isovector M1 character of the present transition or, for J~ = 1-, from an isovector M1 character of a transition to the 8905 keV, J~ = 1-state nU from [21] o Agreeging U assignments from [12,11] PU from [8] q The known 2 sSi spectrum provides U = 0 + states at 12 977 and 13 040 keV excitation energy. The shell model predicts decay of the 0 +, T = 1 level to the 1779keV state in agreement only with the properties of the 13 040keV level ru from [11] Agreeing U assignments from [12,8] *The T = 1 assignment to the 13 583 keV level rests on the agreement of branching ratios with shell model predictions for the 6~, T = 1 state [4].…”

“…Hence the upper limit of 5 W u for isovector transitions can be applied r In Table 5 we establish a T = 1 character of the 11 572 keV state which would also be valid in the case of the I = 6 spin alternative. Hence the upper limit of 5 W u for isovector transitions can be applied Obtained by assuming a conservative limit Sp,.~ > 0.1 eV for the 14471 keV level derived using the resonance strengths and radiative widths of the preceding paper [2] and partial widths for ground state decays from (e,e') work [21]. Branching ratios were also taken from the preceding paper amended by data from [12].…”