Magnetic Moment of the1−Ground State inN18

Abstract

The 18 N ground state magnetic moment jmj 0.135͑15͒m N has been measured using a modified b nuclear magnetic resonance technique. The value is compared to shell-model calculations. Spin-aligned 18 N projectile fragments were produced in the fragmentation of 22 Ne at 60.3 MeV͞nucleon. Polarization of the nuclear spins was resonantly induced by a combined magnetic dipole, electric quadrupole, and radio frequency interaction. This is the first application of a new method that allows production of polarized nuclei from spin-aligned projectile fragments, allowing one to measure b asymmetries. The method opens a new range of applications to study static dipole and quadrupole moments of exotic nuclei near the drip lines. [S0031-9007(98)08259-3] PACS numbers: 21.10. Ky, 23.20.En, 24.70. + s, 27.20. + n Nuclear moments provide a stringent test for nuclear models because, in general, they are extremely sensitive to the single-particle structure of valence particles. Comparison of experimental and theoretical nuclear moments can allow refinement of the interaction parameters [1] and provides a test of the models [2]. It becomes especially interesting if one can test nuclear theories close to the drip lines where the single-particle structure is not well established. Since the development of intermediate energy accelerators, followed by in-flight high-resolution recoil spectrometers [3,4], it has become possible to produce exotic nuclei in very clean conditions and in sufficient amounts to allow nuclear moment measurements. Moments of b-decaying ground states are measured via the asymmetric emission pattern of the decay electrons (positrons). e 1 ͞e 2 emission is asymmetric with respect to the nuclear spin direction, due to parity violation in the b decay. However, to obtain asymmetric b decay, the ensemble of the desired nuclei needs to be spin polarized (differently populated j1m͘ and j2m͘ quantum states, m ͗I z ͘). Producing spin-polarized nuclei has been a challenging experimental problem. Several techniques have been developed to polarize short-lived (ms , t , s) b-unstable nuclei after the production process, such as tilted foil polarization [5,6] or optical pumping [7,8]. When exotic nuclei are produced in a projectile fragmentation reaction, spin polarization can be obtained by the reaction itself, provided recoil fragments are selected at a nonzero angle with respect to the primary beam [9]. On the other hand, selecting projectile fragments in the forward direction is much more straightforward, es-pecially at high beam energies where much higher yields are obtained at forward angles. Fragments emitted parallel to the primary beam direction are not spin polarized, but spin aligned (equal population of j1m͘ and j2m͘ levels) [10]. In that case, a b-asymmetry measurement is possible only if polarization is induced by the applied hyperfine interactions. This procedure requires interactions that break the up-down symmetry in the quantum system.To measure the magnetic and quadrupole moment of b-decaying projectile fragment...

Nuclear structure ofN18and the neighboringN=11isotones

“…This underlines the importance of the proton-neutron valence interaction. We note, the full shell model calculation gives a ground state magnetic moment µ = 0.2890 µ N in comparison to the measured values, |µ| = 0.3279 (13) µ N [13] and |µ| = 0.135 (15) µ N [14].…”

“…Previous experiments to study 18 N used beta-decay [11][12][13][14] and charge-exchange reactions [15,16]. Reference [11] also carefully examines shell model results and observed levels in 18 N. The only information on excited state transition rates came from the beta-decay study [12] that suggested, based on intensity balance arguments, a long mean lifetime for the 18 N first excited state of greater than 600 ns, in disagreement with both shell model predictions and neighboring systematic trends.…”