Some observations on cosmic rays using a large randomly operated cloud chamber

Abstract
Cloud-chamber photographs of cosmic rays obtained with a countercontrolled chamber do not represent the normal relative frequency of the different types of particles and of particles of different energy present in this radiation. There is, for instance, a strong bias in favour of "shower"’ electrons, while low-energy particles, owing to their limited effective range, are under represented. A knowledge of the true statistical distribution of events in cosmic rays is now becoming desirable, and this can only be obtained from photographs of tracks produced during random expansions. For this, and for certain other reasons, it was considered worth while to revert to the use of the randomly operated chamber for certain cosmic-ray investigations. The main objection to its use, viz. its very low efficiency, may be overcome by increasing the size of the chamber. The average number of cosmic-ray tracks recorded per expansion increases roughly as the fourth power of the linear dimensions of the chamber, and it was estimated that a chamber about a foot in diameter and a foot deep would on the average record one or two cosmic-ray tracks every photograph. A cloud chamber of these dimensions was constructed and was found to fulfil expectations. In this paper an account of it is given and of some results obtained by its use. These results are of a preliminary nature. More extensive observations on the various effects discussed will be made with a still larger chamber which is now under construction. The results given in this paper are mainly concerned with the relative number and the spectrum of cosmic-ray particles with energy below 10 8 V— the region of energy where a counter-controlled chamber exerts most strongly its selective action. The evidence for mesotrons (“heavy electrons”) obtained earlier by Williams and Pickup (1938) was obtained in experiments with the chamber described here. A fuller account of certain details connected with this work is given in the present paper, together with a discussion of the relative number of slow mesotrons in cosmic rays at sea-level. I wish to thank Mr Pickup for his permission to reproduce here the mesotron tracks observed in our work.

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