H. J. Stöckmann

Optical pumping of free atomic ions with hyperfine structure

Free atomic ions with hyperfme structure have been oriented m He buffer gas by absorption of circularly polarized light. ΔF=±1 hyperfine transitions have been measured in the 62S1/2 ground state of135Ba+ and137Ba+ with an accuracy of 1 part in 107.

(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.

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.

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

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.

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.

Very narrow nuclear magnetic resonance lines of 8Li(T12 = 0.84 s) in solids by double frequency irradiation, and the hyperfine structure anomaly 7Li8Li

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 .

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.

Nuclear magnetic resonance of 8Li in LiNbO3 and the quadrupole moment of 8Li

Abstract Nuclear magnetic resonance measurements were carried out on neutron activated 8 Li (T 1 2 = 0.84 s ) nuclei in a single crystal of LiNbO3. The quadrupolar splitted NMR spectrum, detected via the 8Li β radiation, could be resolved. The quadrupole coupling constant was determined to |e2qQ/h| = 43 ± 3 kHz at room temperature. Comparison with the coupling constant of 7Li in LiNbO3 leads to the value of the quadrupole moment |Q( 8 Li )| = 32 ± 6 mb .

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.

Static electric quadrupole interaction of the β-emitter20F produced by capture of polarized neutrons in MgF2 single crystals

Abstract Polarized 20F nuclei ( T 1 2 = 11.2 s, I =2 ) have been produced in a tetragonal MgF2 single crystal by capture of polarized neutrons. Nuclear magnetic resonance has been observed via the asymmetric β decay in an intermediate magnetic field of 4350 Oe, yielding the quadrupole constant |e 2 qQ/h | = 5.77 ± 0.02 Mhz and |Q( 20 F )| = 0.012±0.002 b .