H. Grupp

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

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

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.

Point defects in CaF2 following the 19F(n, γ)20F process

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.

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

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.

The quadrupole moment of 116In determined from the spin-lattice relaxation in In III–V compounds

Abstract β-radiation detected NMR signals of 8 Li nuclei have been observed in 7 Li 2 S and 7 Li 2 O. By double frequency irradiation the dipolar linewidth of the 8 Li NMR signal was suppressed and lines as narrow as 32 Hz were obtained being among the narrowest ever obtained in solids. This allowed the determination of the 7,8 Li hyperfine structure anomaly 7 Δ 8 = −7.3(1.9) × 10 −5 .

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

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.

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

Abstract Structure and annealing of (n, γ)-induced point defects are studied in CaF2 by means of nuclear magnetic resonance of the neutron-activated β-emitter 20F.