In order to obtain more precise information concerning the processes and defects produced in solids as a result of fast-particle irradiation such as those described we have irradiated thin films of nickel with fast neutrons.The films were evaporated in vacuum (10~5 mm Hg) on glass and quartz substrates, the thickness of the films being 200-800 A; the evaporation was carried out at different temperatures of the substrates in order to obtain different stages of the crystalline structure. During the irradiation, the samples were sealed in vacuum (10~4 mm Hg) or in oxygen. For each of the series of the irradiated samples, one test sample was put aside for purposes of comparison. The irradiation was made for 3.38 xlO 17 nvt and 9.45X10 15 nvt at the temperatures T= 59°C and T = 45°C, respectively. Before and after the irradiation, the saturation magnetization and coercive force of the samples were measured. The structure of the films was studied with the aid of the electron microscope and by electron diffraction.The technique, of obtaining the films and the methods of studying them have been described previously. 4 As a result of fast-neutron irradiation, the thin films of nickel were transformed from facecentered cubic lattices with a = 3.52 A into closepacked hexagonal lattices with a = 2.62 A, c =4.36 A. This transformation took place in the whole film for samples irradiated to 3.38xl0 17 nvt. For samples irradiated to 9.45xlO 15 nvt we obtain a mixture of two phases: face-centered cubic lattice and close-packed hexagonal lattice. The allotropic transformation took place only for samples irradiated in vacuum; the samples irradiated in oxygen were strongly oxidized and the oxidized film thus formed prevented the allotropic transformation into a hexagonal lattice. Such a phase change requires a volume enlargement and this is not possible.All the irradiated samples show great changes in magnetic properties. The magnetic properties disappear for the samples which are completely transformed into close-packed hexagonal lattices (the magnetization falls from 4irl s = 4000 gauss to Aj[I s < 6 gauss, and the coercive force falls from # c = 240 oe toi? c = 0oe).The samples, which are in part transformed into hexagonal lattices, show a drop in magnetization of about 30-50% and their coercive force increases to a certain extent; there are cases in which it does not vary at all. The increase in the coercive force is due to the large stresses which appear in the mixture of the phases.The samples irradiated in oxygen exhibited a drop in magnetization because the proportion of pure nickel phase was small and the oxidation correction could not be made in the case of strong oxidation. That is the explanation of the fact that the calculated magnetization is not the observed one. The coercive force also grows slightly as a result of the stresses.The samples irradiated in oxygen showed another phenomenon: those samples, which at the beginning had a less ordered structure, presented after the irradiation a more ordered one and the hyste...
The polygonization phenomenon observed on alkali halide surfaces, irradiated with electrons and subsequently illuminated, was studied by optical and electron microscopy. For the explanation of the nature and mechanism of this process, we have investigated the influence of light (containing the F-centre absorption band) on the development and annihilation in time of some polygonization lines, as well as the correlation of the polygonization patterns with dislocation etch pits and subgrain boundaries revealed by a selective chemical etching.Preceded by an optical bleaching effect which locally induces surface potential changes, the polygonization process is explained taking into account the mobility of unpinned charged dislocations and the surface atom migration. The disappearance of some polygonization lines may be produced by a vacancy-interstitial annihilation process.No identification was found between the polygonization patterns and the subgrain boundaries.
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