VOLUME 82, NUMBER 3 PHYSICAL REVIEW LETTERS 18 JANUARY 1999 Magnetic Moment of the 1 2 Ground State in 18 N Measured with a New b Level Mixing Nuclear Magnetic Resonance Technique G. Neyens, 1 N. Coulier, 1 S. Teughels, 1 G. Georgiev, 1 B. A. Brown, 2,3 W. F. Rogers, 4 D. L. Balabanski, 1, * R. Coussement, 1 A. Lépine-Szily, 5 M. Lewitowicz, 6 W. Mittig, 6 F. de Oliveira Santos, 6 P. Roussel-Chomaz, 6 S. Ternier, 1 K. Vyvey, 1 and D. Cortina-Gil 7 1 University of Leuven, Instituut voor Kern-en Stralingsfysika, Celestijnenlaan 200 D, B-3001 Leuven, Belgium 2 Department of Physics and Astronomy and National Superconducting Cyclotron Laboratory, Michigan State University, East Lansing, Michigan 48824 3 Department of Physics, University of Oxford, Oxford OX1 3PU, United Kingdom 4 Westmont College, 955 La Paz Road, Santa Barbara, California 93108 5 Institute of Physics, University of Sao Paulo, C.P. 66318, 05389-970 Sao Paulo, Brazil 6 Grand Accelerateur National d’Ions Lourds, B.P. 5027, F-14076 Caen Cedex 5, France 7 Gesellschaft f ur Schwerionenforschung, D–64291 Darmstadt, Germany (Received 27 August 1998) The 18 N ground state magnetic moment jmj  0.13515m 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 MeVnucleon. 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. Compari- son of experimental and theoretical nuclear moments can allow refinement of the interaction parameters [1] and pro- vides a test of the models [2]. It becomes especially inter- esting 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 emis- sion is asymmetric with respect to the nuclear spin direc- tion, 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 j1m and j2m quantum states, m  I z ). Producing spin-polarized nuclei has been a challenging experimental problem. Several techniques have been developed to po- larize 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 j1m and j2m levels) [10]. In that case, a b-asymmetry measurement is possible only if polarization is induced by the applied hyperfine in- teractions. 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 fragments, we apply in this work for the first time a method [11,12] which induces spin polarization starting from a spin-aligned ensemble of for- ward selected fragment nuclei. The breaking of the sym- metry is obtained by applying noncollinear static magnetic dipole and electric quadrupole interactions. The inter- action with a static magnetic field allows the determi- nation of the magnetic moment (n B  gm N Bh), while the interaction with an electric field gradient induced by a crystal lattice allows the determination of the quadrupole moment (n Q  eQV zz h, for nuclei with I $ 1). At cer- tain values of the static magnetic field, polarization is induced due to the mixing of two hyperfine levels (jm quantum states). A resonant onset of b asymmetry as a function of the magnetic field is measured, from which the ratio of the magnetic to quadrupole moment of the nuclei can be derived. This technique is known as the “level mixing resonance” (b-LMR) technique and is de- scribed extensively in Refs. [11] and [13]. By combining this method with a modified “nuclear magnetic resonance” (b-NMR) technique [14,15], both the magnetic moment 0031-9007 99  82(3)  497(4)$15.00 © 1999 The American Physical Society 497