Measurement of the electric quadrupole moment of 32Al

doi:10.1016/j.physletb.2007.01.063

Physics Letters B

Volume 647, Issues 2–3, 5 April 2007, Pages 93-97

https://doi.org/10.1016/j.physletb.2007.01.063 Get rights and content

Abstract

The electric quadrupole moment Q for the ground state of ³²Al has been measured using the β-NMR technique. Spin-polarized ³²Al nuclei were obtained from the fragmentation of ⁴⁰Ar projectiles at $E / A = 95 MeV / nucleon$ , and were implanted in a single crystal α-Al₂O₃ stopper. The quadrupole moment was deduced from the measured quadrupole coupling constant. The obtained value, $| Q ({}^{32}{Al}_{g.s.}) | = 24 (2) mb$ , is remarkably small as compared with other Al isotopes, but is well explained by shell model calculations within the sd shell. The present result indicates that ³²Al has a normal sd-shell structure, in contrast to the neighboring $N = 19$ isotone ³¹Mg for which a strongly deformed intruder ground state has been reported to occur.

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Acknowledgements

The authors are grateful to staffs at the RIKEN Ring Cyclotron for their support during the running of the experiment. They would like to thank Dr. E. Yagi for useful help and advice with the X-ray diffraction analysis of the α-Al₂O₃ sample. The authors D.K. and T.H. are grateful for the Special Postdoctoral Researcher Program in RIKEN. This work was supported in part by a Grant-in-Aid for Scientific Research from the Ministry of Education, Science, Sports and Culture. The experiment was

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Electric quadrupole moment Q of the ${}^{33}{Al}$ ground state has been measured by means of β-NMR spectroscopy using a spin-polarized ${}^{33}{Al}$ beam produced in a projectile fragmentation reaction. The obtained Q moment, $| Q_{\exp} (^{33} Al) | = 132 (16) e mb$ , shows a significant excess from the prediction of shell model calculations within the sd shell. The result indicates sizable admixing of pf intruder configurations in the ground state, demonstrating that the $N = 20$ shell closure certainly erodes in ${}^{33}{Al}$ , a nucleus located on the border of the island of inversion. Comparison was made with predictions of the Monte Carlo shell model, and also a particle-vibration coupling model treating the neutron pairing correlations in the ground state of ${}^{33}{Al}$ . Again, a significant admixture of pf intruder configurations to the ${}^{33}{Al}$ ground state was needed in both theoretical approaches to explain the observed large Q.
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Citation Excerpt :
A purity of 90% was achieved using a 1054 μm thick 9Be wedge-degrader in the intermediate dispersive plane. Two techniques are commonly used: the continuous rf technique (CRF) [31,32] and the adiabatic fast passage method (AFP) [18,23]. Both are based on the principle of β-NMR [30].
The electric quadrupole coupling constant of the ³¹Al ground state is measured to be $ν_{Q} = | \frac{e Q V_{z z}}{h} | = 2196 (21) kHz$ using two different β-NMR (Nuclear Magnetic Resonance) techniques. For the first time, a direct comparison is made between the continuous rf technique and the adiabatic fast passage method. The obtained coupling constants of both methods are in excellent agreement with each other and a precise value for the quadrupole moment of ³¹Al has been deduced: $| Q ({}^{31}{Al}) | = 134.0 (16) mb$ . Comparison of this value with large-scale shell-model calculations in the sd and sdpf valence spaces suggests that the ³¹Al ground state is dominated by normal sd-shell configurations with a possible small contribution of intruder states. The obtained value for $| Q ({}^{31}{Al}) |$ and a compilation of measured quadrupole moments of odd-Z even-N isotopes in comparison with shell-model calculations shows that the proton effective charge $e_{p} = 1.1 e$ provides a much better description of the nuclear properties in the sd-shell than the adopted value $e_{p} = 1.3 e$ .

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Physics Letters B

Abstract

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Acknowledgements

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Coulomb excitation of <sup>31</sup>Mg

Precision measurement of the electric quadrupole moment of <sup>31</sup>Al and determination of the effective proton charge in the sd-shell