MAGNETOPLASTIC EFFECT IN IRRADIATED NACL AND LIF CRYSTALS

The effect of low x-ray irradiation doses (approximate to 10(2) rad) o n the magnetoplastic effect - the detachment of dislocations from para magnetic centers under the action of an external magnetic field B - in alkali-halide crystals has been investigated. The measurements were p erformed on LiF crystals and three types of NaCl crystals, differing i n impurity content. The dependence of the mean free path I of the disl ocations on the rotational frequency nu of a sample in a magnetic fiel d was especially sensitive to low irradiation doses. In unirradiated c rystals this dependence is a single-step dependence and is characteriz ed by a critical frequency nu(c) proportional to B-2 above which the m agnetoplastic effect is not observed. The frequency nu(c) depends only on the type of paramagnetic centers, and not on their density. Even t he lowest irradiation dose employed (<100 rad) leads to a sharp restru cturing of the dependence I(nu), converting it into a two-step depende nce (for edge dislocations) with an additional critical frequency nu(c 2), that is insensitive to the irradiation dose, and that corresponds to the appearance of magnetically sensitive stoppers of a new type und er irradiation. The initial critical frequency nu(c1), as a rule, also varies with the dose, reflecting the change in state of the impurity complexes (Ca in NaCl and Mg in LiF). Specifically, it is shown for Na Cl(Ca) crystals that as the irradiation dose increases, the frequency nu(c1) increases, gradually approaching the value nu(c2), SO that by t he time the dose is approximate to 300 rad, the dependence l(nu) once again becomes a single-step dependence, dropping sharply only for nu g reater than or equal to nu(c2). It is shown that the addition of a sma ll number of Ni atoms to a NaCl crystal makes the Ca complexes radiati on resistant, and the critical frequency nu(c1) corresponding to them initially equals nu(c2) for crystals with no Ni. The recombination kin etics of radiation defects in the case in which the samples are irradi ated under a tungsten lamp was investigated. A possible physical model of the observed dependences is discussed. (C) 1997 American Institute of Physics.