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β-NQR with lithium isotopes

Introduction

Halo nucleus

11Li is the first-discovered and best-known example of a halo nucleus. It consists of a core of 9Li surrounded by a two-neutron cloud, or “halo”, that gives it a nuclear matter radius similar to that of a lead nucleus. While this in itself should not affect the charge radius of 11Li since the halo is composed entirely of neutral particles, experiments at TRIUMF-ISAC have measured a charge-radius significantly larger than that of 9Li[1]. This indicates that the presence of a halo somehow deforms the nuclear core—a change that would manifest itself as an increase in the nuclear quadrupole moment Q from one isotope to the next. Currently accepted values for these moments provide evidence for this, giving a ratio between the two of Q11/Q9 = 1.088(15)[2]. Unfortunately, these moments are not known very precisely, and large differences exist between the predictions of their values by different theoretical models, necessitating that a more accurate experimental measurement of this nuclear property be made. This experiment therefore sets as its ultimate goal the measurement of the Q11/Q9 ratio to as high a precision as possible, using the technique of β-NQR spectroscopy. TRIUMF-ISAC is well-suited to perform this experiment since it currently produces the highest-intensity 11Li beam in the world, and already contains all the needed experimental tools.

[1]     Sánchez et al., 2006 Phys. Rev. Lett. 96, 033002                                                                                                                                                                     [2]     Neugart et al., 2008, Phys. Rev. Lett. 101, 23502

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