Elektrische Quadrupolwechselwirkungen von²⁰F im MgF₂-Einkristall

Abstract

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.