Speaker
Dr
Aaron Hurst
(Lawrence Livermore National Laboratory)
Description
The wave-function composition for the low-lying states in 29Na was explored by measuring their electromagnetic properties using the Coulomb-excitation technique. A beam of radioactive 29Na ions, postaccelerated to 70 MeV using ISAC-II at TRIUMF, bombarded a 110Pd target with a rate of up to 600 particles per second. Six segmented clover detectors of the TIGRESS gamma-ray spectrometer were used to detect deexcitation gamma rays in coincidence with scattered or recoiling charged particles in the segmented silicon detector, BAMBINO. A reduced transition matrix element |<5/2+ ||E2|| 3/2+>| = 0.237(21) eb was derived for 29Na from the measured gamma-ray yields for both projectile and target. This first-time measured value is consistent with the most recent Monte Carlo shell-model calculation (MCSM) of Utsuno et al., predicted to be 0.232 eb [1]. This is suggestive of a strongly-mixed first-excited state comprising a 30 ~ 40% admixture of 2p-2h configurations in the wave function, and also provides evidence for the narrowing of the sd-pf shell gap from 6 MeV for stable nuclei to ~3 MeV for 29Na.
This result can also be interpreted at the phenomenological level. Within the framework of the rotational model and assuming a prolate deformation, the transition quadrupole moment, Q_{t}= 0.524(46) eb, is deduced from the measured transition matrix element for 29Na. This value also bears good agreement with the above MCSM calculation, Q_{t}= 0.513 eb [1]; a calculation utilising an effective interaction based on a shell-model space incorporating the full sd space and the two lower orbits of the pf space, with the inclusion of the cross-shell mixing terms in the effective Hamiltonian. Contrasting behaviour in the static and dynamic-nuclear properties of 29Na, arising from differences in the underlying single-particle configurations of the ground and excited states, may explain the difference between the present measurement and that of an earlier experimental result using beta-NMR spectroscopy, Q_{0}= 0.430(15) eb [2]. This intrinsic quadrupole moment, derived from the ground-state spectroscopic quadrupole moment, 0.086(3) eb, also compares well with the MCSM calculation, Q_{0} = 0.455 eb.
[1] Y. Utsuno et al., Phys. Rev. C 70, 044307 (2004).
[2] M. Keim et al., Eur. Phys. J. A 8, 31 (2000).
Summary
This work describes a Coulomb-excitation measurement that has quantitatively established the role of normal (sd) and intruder (pf) configurations in the first excited state in 29Na. In addition, this measurement describes a significant accomplishment in Coulomb excitation of low-intensity radioactive ISOL beams with only a few hundred particles per second, while maintaining the same beam quality as stable-beam experiments.
Author
Dr
Aaron Hurst
(Lawrence Livermore National Laboratory)
