Elektrische Quadrupolwechselwirkungen von 20 F im MgF 2 -Einkristall

Polarized20F nuclei (T 1/2=11s,I=2) have been produced in a tetragonal MgF2 single crystal by capture of polarized neutrons. Nuclear magnetic resonance transitions have been induced and observed via the asymmetricΒ decay to20Ne, yielding the quadrupole coupling constant ¦e 2 q Q/h¦=5.77(2)MHz, the quadrupole moment ¦Q(20F)¦=0.064(12) b and the asymmetry parameter of the electric field gradientη=0.317(2). In order to study possible lattice defects produced by the recoil displacement due to the captureγ-rays NMR signals were registered both at room temperature and at 12 K. Width and depth of the resonance curves indicated that at room temperature nearly all20F ions occupy normal lattice sites with an undisturbed environment. At 12 K, however, where no annealing occurs, this is only true for about half of the20F ions which are stopped directly at defect free sites. The other half of the20F ions at 12 K shows resonance frequencies shifted by defect interactions up to the order of some 100 kHz. Furthermore two simple methods are reported which allow a quick estimate of the strength of the quadrupolar interaction: (1) observation of theΒ decay asymmetry versus the magnetic field strength (decoupling curve), (2) broad-band modulation of the inducedrf field and observation of theΒ asymmetry versus the depth of the modulation.