G. Kiese

Point defects in alkaline-earth fluorides after thermal neutron irradiation

AbstractSingle crystals of MgF2, CaF2, SrF2 and BaF2 have been irradiated with polarized, thermal neutrons at temperatures between 10 K and 300 K. The neutron capture process

Cross-relaxation processes of polarizedβ-active nuclei in various crystalline solids

^{19} F(\vec n, \gamma )^{20} \vec F

Low-temperature spin-lattice relaxation of8Li in the glass Li2O·2SiO2

creates polarized, β-active20F(τβ=16s) probe nuclei, which are displaced by the γ recoil and thus produce some nearby point defects. These defects have been studied by their influence on20F NMR signals being monitored via the asymmetric β-decay radiation of the polarized20F nuclei.From our results we conclude that in the fluorites CaF2, SrF2 and BaF2 thermal neutron irradiation leads to the formation of anion Frenkel pairs, the predominant intrinsic defect in such crystals. Correlated recombination of these close Frenkel pairs with activation enthalpies of about 0.2 eV was observed at temperatures around 80 K. A second annealing stage occurs near 220 K corresponding to an activation enthalpy of about 0.6 eV. Possible annealing mechanisms responsible for this stage are discussed. In tetragonal MgF2 two annealing stages were found at 13 K and 60 K corresponding to 0.037 eV and 0.17 eV, respectively. The stage at 60 K possibly reflects migration of the H entre whereas the interpretation of the 13 K stage remains an open question.

8Li spin relaxation in liquid Li alloys with compound-forming tendency

Polarized116In (I=1,T1/2=14.1 s) nuclei were produced by capture of polarized thermal neutrons in undoped InP and InSb and in InSb∶Te single crystals. As a consequence of the nuclear reaction charged and paramagnetic point defects were produced with a rate of about 0.8 per absorbed neutron. Spin-lattice relaxation rates of the116In probe nuclei were measured at temperatures between 1.3 K and 90 K, as well as NMR line profiles and magnetic field dependences of the nuclear polarization up to 0.63 T. Electric field gradients, produced by the defects, could be determined, and defect induced additional relaxation processes were observed. Further, characteristic phonon frequencies and spin-phonon coupling constants for the undisturbed crystal lattices could be derived.

8Li SPIN-Lattice relaxation in the liquid alloys Li-Bi and Li-Pb

Cross-processes of neutron activated, polarized nucleiI with surrounding stable host nucleiS are observed in diverse solids, using an in-beam NMR technique where the polarizedI nuclei are produced by capture of polarized thermal neutrons. The polarization is determined by measuring the 0°–180° asymmetry of theβ-decay radiation of these nuclei. The following cross-processes are studied:8Li−6Li innatLi metal;8Li−7Li in7Li metal;8Li−6Li innatLiF (single crystal);8Li−93Nb in7LiNbO3 (single crystal);20F−19F in MgF2 (single crystal). Two applications are presented: a) Determination of the temperature dependence of the electrical quadrupole coupling constant of93Nb in LiNbO3; b) Observation of radiation induced point defects in MgF2 and their thermal annealing. All measurements are performed without any radiofrequency irradiation.

Structure and annealing of cation Frenkel defects in silver halides after irradiation with thermal neutrons

Abstract In stage I Li-graphitide, we try to determine the nature of interactions between Li nuclear spins and their neighborhood by variable temperature measurements of the Li nuclear spin lattice relaxation time T1. Two methods are used: 7Li pulsed NMR; and β-radiation detection of 8Li-nuclear polarization produced by polarized neutron capture. Both methods are in qualitative agreement, but a quantitative discrepancy appears in the low-temperature range. Results are discussed in relation with several mechanisms: contact interaction and Korringa relation, quadrupolar mechanism and diffusion modulated dipolar interaction.

8Li spin-lattice relaxation in liquid LiSn and LiSi alloys

Nuclear spin-lattice relaxation in Li20·2Si02 glass below 200 K has been studied using the asymmetric Β-decay radiation of polarized8Li (T1/2=0.Bs) nuclei produced by capture of polarized neutrons. Transients of the8Li polarization follow an exp(−√E/T1) law. The dependence of the spin-lattice relaxation rate ⊥¯11 on temperature T and magnetic field B can roughly be described by T¯11∼T/B. The interpretation is based on the assumption that for8Li, contrary to7Li in the same glass, spin-diffusion is absent and that each probe nucleus is coupled by quadrupolar interaction to an individual distribution of nearby centres typical of glasses. The fluctuation of these centres causing relaxation may be induced by either a multi-phonon or a thermally activated motional process.

β NMR on8Li in the superiqnic conductor Li3N

In liquid Li-Bi, Li-Pb, Li-Sn and Li-Si alloys the spinlattice relaxation rate T1−1 of Β-active8Li (T1/2=0.8s) probe nuclei has been measured over wide concentration (c) and temperature (T) ranges. T1−1 is governed by hyperfine contact interaction with the conduction electrons. Near the compositions Li3Bi, Li4Pb, Li4Sn and Li4Si both (T1T)−1 and d(T1T)−1/dT versus c show marked extrema. These data, when combined with Knightshift and/or conductivity values, inform on the prolongation of the electron “residence time” and the reduction of the density of states at the Fermi surface. There is progressive tendency to electron localization and compound formation in the sequence Li4Pb, Li4Sn, Li3Bi with mainly ionic bonding in the case of liquid Li3Bi.