M. Grupp

Elektrische Quadrupolwechselwirkungen von20F im MgF2-Einkristall

Polarized20F nuclei (T1/2=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 ¦e2q 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.

Nuclear moments of the β-emitters 108Ag and 110Ag

Abstract Nuclear magnetic resonance measurements were carried out on neutron activated 108, 110 Ag nuclei in AgCl resulting in the magnetic moments μ( 108 Ag) = 2.6727(10) n.m. and μ( 110 Ag) = 2.7111(10) n.m. The values were determined relative to μ( 8 Li) in LiF and are uncorrected for atomic diamagnetism or chemical shifts. From measurements of the quadrupolar relaxation rates in AgCl at several temperatures, the ratio of the quadrupole moments was found to be ¦ Q ( 110 Ag)/ Q ( 108 Ag)¦ = 1.49(7). The results are satisfactorily explained with help of the unified model.

Multiple quantum NMR transitions of8Li (T1/2=0.84 s) in single crystals and powders of LiTaO3, and the quadrupole moment of8Li

Multiple quantum nuclear magnetic resonance (NMR) transitions were observed on polarized8Li nuclei, which were produced by capture of polarized neutrons in a single crystal of LiTaO3. The asymmetric8Liβ-radiation distribution was used for the detection of NMR. A quadrupole moment ratio ¦Q(8Li)/Q(7Li)¦=0.78(1) was determined. Saturation of multiple quantum transitions in nuclear radiation detected NMR may lead to a reduction in measuring time of up to two orders of magnitude, as compared to single quantum detection methods. The measured spectra agree well with an exact lineshape calculation. The same measurements were also performed on a LiTaO3 powder sample. This was done to test a method to obtain quadrupole coupling constants from high field NMR multiple quantum powder patterns, which are easily detectable, also for higher nuclear spins. This latter method may be applicable also to conventional NMR detection techniques.

Structure and annealing of (n, γ)-Induced point defects in CaF2

Neutron activated20F nuclei (T1/2=11 s) in CaF2 serve for two purposes: first they produce well defined point defects by their (n, γ)-production mechanism, second, they act as probes to detect the properties of these defects in aβ- andγ-ray detected nuclear magnetic resonance experiment. Below about 70 K an interstitial defect is observed due to a quadrupolar splitting of the20F-NMR line. The defect induced electric field gradient is oriented along the 〈111〉-axis of the crystal and causes a quadrupole coupling constante2qQ/h=−2.0(1) MHz at the20F site. The defect anneals above 70 K with an Arrhenius like temperature dependence corresponding to an activation energyEM=0.17(4) eV.

Sign of nuclear electric quadrupole coupling constants in solids fromγ-ray anisotropy measurements after capture of polarized neutrons

An experimental method is presented which allows the determination of the sign of nuclear electric quadrupole interactions in solids. Activated target nuclei with a purely dipolar spin polarization are produced by capture of polarized thermal neutrons. The quadrupole coupling of the nuclei to the electric field gradient tensor in the target crystal converts this (dipolar) polarization partly into a (quadrupolar) alignment, which can be measured by the anisotropy of theγ-ray emission in a succeeding nuclear transition. The sign of the alignment created in this reorientation process depends on the sign of the electric quadrupole interaction. The reorientation effect can be enhanced by selective induction of nuclear magnetic resonance transitions. The method has been applied to measure the sign of the crystal electric field gradient (efg) in tetragonal MgF2. Further, the sign of an efg in cubic CaF2 originating from a19F interstitial adjacent to the activated20F probe nucleus has been determined. The method is in principle applicable to a considerable number of nuclides.