The positron annihilation method is effectively used in studying radiationinduced defects in solids, in particular f o r semiconductor crystals. In /1 to 31 the positron mean lifetime was measured at low temperatures after high temperature annealing of Si crystals irradiated by neutrons, electrons, and protons.12 Ge p-type crystals of Q = 35 ! J cm, Ga doped (NGa = 6.7xlO ~m -~) have been studied in the present note. These crystals have been irradiated by 4.4 MeV electrons f r o m the "Microtrone" accelerator (fluence 0 = 9 . 7 6~x l 0 nealing has been carried out.

The first information about radiation defect formation in Si, irradiated by subthreshold electrons was discussed in /l/. It was suggested that the defect formation cross-section was determined by the ionisation cross-section of the intrinsic atomic shells in Si. More effective defect formation should b e expected in Si at irradiation by super dense subthreshold electron pulses. In paper /2/, for instance, the rate of A-centre production was obtained to be about two t i m e s g r e a t e r when passing to the nanosecond range of pulse duration a t irradiation by a supercurrent accelerator of 4 A/cm current density than under continuous irradiation. In the present experiment the electron energy was higher than the threshold one. At present the method of positron annihilation, which is very sensitive to deep defect states of vacancy type, is widely used in semiconductor radiation physics. 2In the present paper Si p-type crystals, irradiated by super dense subthreshold electron pulses at 287 K, were investigated by the positron annihilation method for the first time. We used Si samples (6K6D-500) with <111> axis orientation, of 1 5 mm diameter and 0.8 mm width. The oxygen concentration was 5 1 . 2 x 1 0~~ cm-3.Positron lifetime measurements w e r e carried out by a set with 300 ps time resolution. The treatment of positron lifetime spectra was carried out according to a standard "Positron fit" programme /3/. The annihilation photon angle distribution was measured in the long-slit geometry /4/ with angular resolution of 0.4 mrad (the temperature duiing the experiment was kept at 287 K).A standard positron source 22Na( $ : r ) on a g l a s s substrate was used. Its activity was 1 0 mCi. The spatial distribution of electron and positron energies absorbed by the matter was evaluated by the multi-step method, detailed in /5/. 1 ) P.O. Box 25, 634050 Tomsk, USSR.

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