Abstract
IN order to find whether showers are produced by the penetrating cosmic rays which reach considerable depths below ground-level, we have performed experiments in Holborn Underground station (by permission of the London Passenger Transport Board) at a depth corresponding to 60 m. of water. The vertical intensity of the radiation at this depth, as measured by the rate of occurrence of coincidences between three counters in a vertical plane, is about one-fifteenth that at ground-level. To count showers we used five counters, arranged in a pentagon formation, so that at least three particles are needed to discharge the five counters simultaneously. The occurrence of coincidences is therefore in itself definite proof of the presence of showers. The quintuple coincidence method is preferable to the triple coincidence method as usually used hitherto, as in the latter case a coincidence can be produced by two particles. The presence of showers can then be proved only by showing that there is an increase in the rate of occurrence of coincidences when a sheet of, say, lead is placed over the counters, the dimensions and disposition of the lead being such that coincidences due to particles originating in it must be due to at least three particles. The detection of showers then entails the establishment of a difference between rates with and without the lead, and these may be nearly equal.
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References
Auger et Bertein, J. Phys., 6, 253; 1935.
Pickering, Phys. Rev., 47, 423; 1935.
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FOLLETT, D., CRAWSHAW, J. Production of Cosmic Ray Showers at a Considerable Depth below Ground-Level. Nature 136, 1026 (1935). https://doi.org/10.1038/1361026b0
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DOI: https://doi.org/10.1038/1361026b0
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