Abstract
A Coulomb-excitation reorientation-effect measurement using the TIGRESS γ−ray spectrometer at the TRIUMF/ISAC II facility has permitted the determination of the 〈21+‖E2ˆ‖21+〉 diagonal matrix element in 12C from particle−γ coincidence data and state-of-the-art no-core shell model calculations of the nuclear polarizability. The nuclear polarizability for the ground and first-excited (21+) states in 12C have been calculated using chiral NN N4LO500 and NN+3NF350 interactions, which show convergence and agreement with photo-absorption cross-section data. Predictions show a change in the nuclear polarizability with a substantial increase between the ground state and first excited 21+ state at 4.439 MeV. The polarizability of the 21+ state is introduced into the current and previous Coulomb-excitation reorientation-effect analyses of 12C. Spectroscopic quadrupole moments of QS(21+)=+0.053(44) eb and QS(21+)=+0.08(3) eb are determined, respectively, yielding a weighted average of QS(21+)=+0.071(25) eb, in agreement with recent ab initio calculations. The present measurement confirms that the 21+ state of 12C is oblate and emphasizes the important role played by the nuclear polarizability in Coulomb-excitation studies of light nuclei.
Highlights
Experiment GOSIAno-core shell model (NCSM) calculations have been performed to estimate κ for the in 12C
A Coulomb-excitation reorientation-effect measurement using the TIGRESS γ−ray spectrometer at the TRIUMF/ISAC II facility has permitted the determination of the 2+1 || E2 || 2+1 diagonal matrix element in 12C from particle−γ coincidence data and state-of-the-art no-core shell model calculations of the nuclear polarizability
Considerable α-cluster triangle admixtures of 52% and 67% for the ground and 2+1 states, respectively, are predicted by fermionic molecular dynamics (FMD) calculations [12], whereas a meanfield contribution of 15% is predicted for the 0+2 Hoyle state [16]; the state crucial to fusion of three α particles in the core of massive stars
Summary
NCSM calculations have been performed to estimate κ for the in 12C. 2+1 states 0.77 presumed that all the E1 strength from the ground state was concentrated at the GDR energy [22]. 28 1− states up to 30 MeV, values of κ(g.s.) = 1.6(2) and κ(2+1 ) = 2.2(2) are predicted. 24 MeV – the centroid energy of the GDR [48]. The lowest calculated 1− state energy was set to the lowest found 1−1 state at 10.84 MeV. The integrated γ−ray yields for the 2+1 → 0+1 transitions in 12C and 194Pt have been calculated using the semi-classical coupled-channel Coulomb-excitation least-squares code GOSIA [42]. Predictions of the cross sections for populating states in 12C were calculated at fixed values 2+1 || E2 || 2+1 of 2+1 || E2 = +0.070 eb. The normalization procedure used in Ref. [29] was esa2txp+1ecppils−tiaeotdif2ot0+1no.0|dc|1euEtere2vbrem,|s|ai0nna+1derevp2adlr+1aeynti|en|ergEmby2i2nfi|+1|exd2|i|n+1iEgn,2w2t|h+1|he0e|+1|reE2ˆC+12uon|||ut|i2llEo+1cm2onbi|n--| verging with the experimental intensity ratio between target and projectile, IγT /IγP, given by, σ T W (θ )T E2 σ P W (θ )P E2
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