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The effect of plastic deformation and quenching on the angular distribution of the two-photon annihilation in metals and alloys was observed by Dekhtyar et al.(1 to 3). In this work the effect of r-irradiation on the angular distribution of annihilation in tungsten single crystals is studied. The specimen was preliminary annealed at 2000 "C and irradiated with exposition 1.6xlO r (cobalt installation C-60000). During the irradiation the temperature of the specimen was approximately 50 OC. The angular distribution was measured before and after the irradiation with the apparatus described in (1). The results are shown in Fig. 1. The effect is similar to quenching (3) and opposite to plastic deformation (1, 2, 4, 5). After annealing at 1800 "C the angular distribution returned to its initial form. 9The measured angular distribution curve I(a) is approximated analytically as the sum of an inverted parabola I (a) and a Gaussian I (a) attributed to conduction electrons and core electrons, respectively (4). The ratio Si/S (Si is the area under the curve Ii( a), i = p, g : S is the total area) refers to the prhbability of positron annihilation with electrons. The redistribution of these probabilities occurs in tungsten single crystals after irradiation: the probability of positron annihilation with conduction electrons decreases and, accordingly, the probability of positron annihilation with core electrons increases. P gThe opposite effect of plastic deformation on the angular distribution in metals,shown by an increase of the annihilation probability with conduction electrons, was considered as a result of capturing of the thermalized positrons by negatively charged dislocations what yields positron annihilation in regions of reduced density of the material. The estimate points to the possibility of a positron localization in the dislocation core (4, 5). In irradiated material a local reduced density is realized near vacancies whereas the bulk material, saturated by interstitial atoms, is com-

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