The frequency variation of nuclear interactions under lead has been studied by comparing the rates of stars in Ilford G5 emulsions exposed above and below 30 cms. of lead. The size frequency distribution is the same for stars in both sets of plates. The attenuation lengths in lead are :-- (305±7) gm./cm.2 for all stars, (380±65) gm./cm.2 for penetrating showers (n s ⩾2), (405±31) gm.]cm.2 for events with charged relativistic primaries, (260±34) gm./cm.2 for events with uncharged primaries (excluding simple evaporatioD stars). The ratio of the attenuation length to the interaction mean free path for penetrating showers is (1·8±0·3). The percentage of all stars produced by charged relativistic primaries increases from (13·±·9) per cent for the unshielded plates to (I7·0±1·0) per cent for the plates under lead due to the formation of stars by π mesons generated in lead. The ratio of charged to uncharged penetrating-shower primaries increases from (0·86±0·21) for the air plates to (1·94±0·32) for the plates exposed under 30 cms. of lead. It is suggested tentatively that the increase is due to the production of penetrating showers by fast π mesons created in the lead.
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Rosser et al. (1951) studied this question.
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