Relaxation and deorientation of110Ag(T 1/2=24.4s) nuclei produced by capture of polarized neutrons in the silver halides

Polarized¹¹⁰Ag nuclei are produced in the silver halides by capture of polarized neutrons at temperatures below 30 K and magnetic field strengths up to 6 kOe. The depolarization process is studied by observation of the β decay asymmetry as a function of magnetic field, temperature and of the radio frequency field strength in NMR signals. The depolarization is caused by a field dependent deorientation process and by temperature dependent spin-lattice relaxation. The deorientation is due to a succession of coupling steps of the nuclear spin with electromagnetic fields of defects generated as a consequence of the capture process, and the field dependence of the polarization can be understood as a decoupling curve. The temperature dependence of the spin-lattice relaxation is in accordance with the theory of quadrupolar relaxation above 18 K if an empirical phonon spectrum is used for the calculation. At lower temperatures the experimental relaxation rate is anomalously high, which may be due to resonance modes connected with recoil lattice defects.     

Point defects in alkaline-earth fluorides after thermal neutron irradiation

Single 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 creates polarized, -active²⁰F( =16s) probe nuclei, which are displaced by the recoil and thus produce some nearby point defects. These defects have been studied by their influence on²⁰F NMR signals being monitored via the asymmetric -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.     

In-beam study of 80Kr; Quasiparticle excitations in nuclei around mass 80

The excited states in ⁸⁰Kr have been studied in the reactions ⁷⁷Se(α, n), ⁷⁸Se(α, 2n), ⁸⁰Se(α, 4n) and ⁶⁵Cu(¹⁸O, p2n) by using in-beam γ-ray spectroscopy. In addition to γγ-coincidences, excitation functions and angular distributions, linear polarization of γ-rays and conversion electrons were also measured. All together, 32 levels have been identified up to spin 14 at an excitation energy of 6.7 MeV in ⁸⁰Kr. For 21 of these levels the mean lifetime could be determined by Doppler shift methods and by the pulsed-beam γ-timing method. The B(E2) values of 30–60 W.u., derived for many transitions, indicate strong collectivity and the existence of several band structures is suggested. Above 2.5 MeV 2-quasiparticle (qp) excitations become important. The excitation energies of ⁸⁰Kr and its neighbours ^{77, 78, 79}Kr, ⁷⁷Br and ⁸¹Rb have been analysed in terms of the cranked shell model. In ^78,80Kr two-proton excitations have been found to be responsible for the observed band crossing. Quasiparticle excitations strongly influence the pairing and stabilize the deformation. The anomalies in the negative-parity bands of ⁸¹Rb and ⁷⁷Br are interpreted as a crossing of a 3qp and a 1qp band and the relatively low frequency of the crossing point is ascribed to the blocking effect.