B. Bader

(n, γ)-Induced point defects in InP and InSb studied by β-radiation detected nuclear magnetic resonance

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.

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

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.

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

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.

On the use of a gradient resonance neutron spin flipper

Abstract A realization of a gradient resonance spin flipper for cold neutrons on an uninterrupted neutron guide is described. Measurements of the spin flip probability are reported. The device is part of an in-beamβ-NMR spectrometer, working in the external neutron guide laboratory at the research reactor FRJ-2, KFA-Julich.

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

The nuclear spin-Lattice relaxation rate T 1 −1 of β-active 8Li (T1/2 = 0.8 s) has been measured in Liquid Li-Bi and Li-Pb alloys. T1−1 as function of concentration and temperature shows marked deviations from metaLLic behaviour near the compositions Li3Bi and Li4Pb. Combining the relaxation data with Knight shift and/or conductivity data the mean ‘residence time’ τe of the conduction electrons at a nuclear site and the density of states at the Fermi surface N(EF) are obtained relative to the free-electron values. The results show that in Liquid Li-Bi the tendency towards electron Localization and compound formation is significantly stronger than in Li-Pb. The melt Li0.8Pb0.2 seems to become metallic above 1400 K.

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

Polarised 110Ag (I=1, T12/=24.6 s) nuclei were produced in the reaction 109Ag (npol, gamma )110Ag in AgCl and AgBr single crystals. The hyperfine interaction of the 110Ag nuclei with radiation-induced point defects has been studied. Positions and shapes of nuclear magnetic resonance curves could be explained assuming a random distribution of positively and negatively charged point defects. From this analysis and from additionally measured migration enthalpies, derived from annealing stages, the defects could be identified as cation Frenkel pairs. The nuclear quadrupole moment of 110Ag was determined as mod Q mod =0.24*10-24 cm2 by measuring the spin-lattice relaxation time of the 110Ag nuclei.

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

Abstract The spin-lattice relaxation time, T1, of 8Li has been measured in a number of liquid LiSn and LiSi alloys, both as a function of temperature and of applied magnetic field. The relaxation rate, 1/T1, in liquid LiSn plotted as a function of concentration exhibits a minimum at about 28 at.% Sn. This is qualitatively explained in terms of electron charge transfer from Li to the less electropositive Sn. Varying the concentration and the temperature, deviations were observed from the relation ση≌ constant (σ denotes the electrical conductivity and η the Korringa enhancement). In liquid LiSi, the spin lattice relaxation rate is a decreasing function of the Si concentration up to approximately 43 at.% Si.

Nuclear Spin-Lattice Relaxation in Liquid Lithium-Germanium Alloys*

In liquid Li-Ge alloys with concentrations up to 55 at% Ge the spino lattice relaxation rate 1/T^ of 6-active Li nucleT was measured as a function of temperature T. The relaxation rate decreases rapidly when Ge is added to Li, up to about 20 at% Ge; for higher Ge-concentrations 1/Tj is approximately constant. Combining 1/T^ with L1 Knight shift data the enhancement factor n of the measured rate relative to the Korringa rate was obtained, n exhibits maxima at 20 and 50 at% Ge. The maximum at 20 at% Ge is probably due to the formation of a liquid octet compound similar to liquid Li^Pb. The maximum in n at the equiatomic composition is unexpected according to recent tight-binding calculations in which the stability of tetrahedral clusters was considered as a function of the size of the alkali ion.

β NMR on8Li in the superiqnic conductor Li3N

Polarized8Li nuclei were produced in a Li3N single crystal by irradiation with polarized neutrons, Β-ray detected NMR signals and spin-lattice relaxation of8Li were observed between B and 300 K. In Li3N there are two non-equivalent Li sites. The corresponding two quadrupole split NMR spectra could be resolved. From the measured relaxation rates activation enthalpies for two diffusion processes were deduced.

Annealing of (n, GAMMA) induced point defects in the silver halides

Abstract β-active 110Ag nuclei have been produced in AgF, AgCl, AgBr by capture of polarized thermal neutrons in the reaction 109Ag(n pol,y) 110Agpol. The capture process produces in the vicinity of the 110Ag nuclei many point defects which have been studied by β-NMR on 110Ag. A detailed analysis of the NMR line shape showed that 30–40% of the 110Ag nuclei see an undisturbed lattice whereas 60–70% see a lot of Ag+ vacancies and interstitials. In the case of AgCl and AgBr defect annealing has been observed in the temperature range 10–25 K.