[en] Specific aspects of the Perturbed Angular Correlation (PAC) of gamma rays concerning its application to the study of atomic diffusion in solids are presented. PAC results recently obtained on the ¹⁸¹Ta probe in several crystalline and amorphous phases of Zr₂Ni hydrides are briefly summarized. Preliminary μSR results relative to these intermetallic hydrides are presented and compared to the PAC data

[en] The authors have surveyed the longitude range 40⁰ <= l <= 60⁰ in the J = 1-0 line of ¹³CO with the 2.5-m telescope of the Bordeaux Observatory. At b = 0⁰ the galactic disk was sampled at 15' spacings. Out of the plane, spectra were obtained at b = +-0⁰.5 and at 5' spacings in b at mod(b) <= 0⁰.25. Some major features of the l-v and b-v diagrams and variation with l of integrated intensities and latitude profiles are briefly discussed. (Auth.)

[en] The galactic plane was sampled at a 5', spacing between l=38deg and l=67.5deg in the J=1-0 line of ¹³CO with the 2.5 m telescope of the Bordeaux Observatory. An automatic analysis of the data gives 181 clouds with masses ranging from a few solar masses to a few 10⁵ M_Sun. At least two spirals can be fitted to the data. (author). 2 figs., 7 refs

[en] The quadrupole interaction experienced by the ¹⁸¹Ta nucleus placed in substitution for Zr in the Na₃Zr₂Si₂PO₁₂ compound (NASICON) has been determined from gamma-ray angular correlation experiments in the temperature range 20-700⁰C. The asymmetry of the electric field-gradient tensor vanishes for T >approx. 200⁰C, which is an indication for a non-random substitution of Si⁴⁺ for P⁵⁺ ions and a high mobility of both Na(1) and Na(2) species in the rhombohedral structure. Furthermore, the results are consistent with the idea that the transition from a monoclinic to a rhombohedral structure is accompanied by a transfer of sodium ions from static interstitial positions into partially occupied Na(1) and/or Na(2) positions. (Auth.)

No abstract available

[en] When a nucleus emits two gamma rays in cascade, an angular correlation generally exists between the directions of emission of the successive radiations. Most frequently the intermediate level of the cascade is able to interact with magnetic and/or electric fields produced by the environment, i.e. the spin and/or charge distributions in the vicinity of the emitting nucleus, and this results in a perturbation of the angular correlation PAC of the two radiations. This paper emphasies the actual potentialities of PAC spectroscopy for specific studies in chemistry or in border regions of physics, chemistry and biology. It is organized as follows: after a brief presentation of the basic principles and theory of perturbed angular correlations of γ-rays, the experimental procedure and the merits of PAC spectroscopy are discussed in a second section. The third section is focused on some recent applications of PAC in fields of interest for chemists and biochemists. The last section deals with hot atom chemistry and reports on the influence of after effects and thermal neutron capture on PAC successively. (Auth.)

[en] The diffusion of hydrogen in the crystallized hydride Zr₂NiH_3.8 has been studied by perturbed angular correlation (PAC) spectroscopy using ¹⁸¹Ta as a probe in substitution for zirconium. The temperature dependence of the quadrupole relaxation of the ¹⁸¹Ta spins between 200 K and 470 K could be properly described by assuming the jump probability of hydrogen atoms to result from the coexistence of two thermally activated processes. The values obtained for the activation energies and pre-exponential frequency factors of both processes suggest that diffusion is dominated by a tunnelling mechanism for T<300 K and by a 'classical' over-barrier jump mechanism for T>370 K. (orig.)

[en] The results of gamma-gamma angular correlation experiments performed on the ¹⁸¹Ta nucleus in calcia - and yttria -stabilized zirconias and in calcia-doped cerias between 20 to 1150⁰C are reported. At high temperatures, the angular correlation spectra indicate a relaxation of the ¹⁸¹Ta spins induced by the diffusive motions of oxygen vacancies. The slow and fast relaxation regimes are successively observed when the temperature is raised. The behaviour of the relaxation constant is fairly well reproduced by assuming the existence of fluctuations in the potential energy barrier which controls the migration of oxygen vacancies. For stabilized zirconias the mean activation energies deduced from angular correlation experiments agree with the conductivity data. The disagreement observed in doped cerias is attributed to impurity effects produced by the ¹⁸¹Ta probe itself. The local investigation of oxygen self-diffusion gives evidence for the homogeneous behaviour of these highly defective oxides at a microscopic level. The ¹⁸¹Ta relaxation data, especially the comparison of the respective data relative to aged and deaged calcia-stabilized zirconia, strongly suggest the existence of short-range ordering phenomena in these solid solutions. (author)

[en] The time-dependent perturbation factors G₂(t) of PAC are numerically calculated by diagonalizing the Blume's supermatrix when static and fluctuating electric field-gradients are simultaneously present at the nucleus. The calculated functions are fitted with the expression of G₂(t) given by isotropic models, strictly valid for asymptotic conditions. The asymptotic relations between the spin relaxation constant λ and the jump frequency w introduced in the stochastic model are found to be a good approximation in large domains of w values. (orig.)

[en] TDPAC experiments on ¹⁸¹Ta in various inorganic materials have been performed by using BaF₂ crystals as fast γ-ray scintillators. The use of such scintillators allows reaching a time resolution as low as 600 ps with both high detection efficiency and good energy resolution. (Auth.)