Cosmic ray measurements under thirty metres of clay

Abstract

The absorption of cosmic radiation can very conveniently be studied by making determinations of the zenith angle distribution of the radiation, since rays incident at different angles with the vertical have traversed different distances after entering the absorbing medium. By the permission of the London Passenger Transport Board, we have therefore carried out such measurements in Holborn Underground Station, in a position where the thickness of earth above the apparatus is 30.8 metres. The soil consists mostly of strata of different kinds of clay. We have also investigated the production of cosmic ray showers at this level. This particular station, though very convenient in most respects, has the disadvantage that it is surrounded by large buildings and that it lies beneath the Kingsway tram tunnel, which locally reduces the thickness of earth above the apparatus; this shows up clearly in some of the results obtained. WE used throughout this work the method of observing the rate of occurrence of coincident discharges of Geiger-Müller counters. Counters of various sizes were used in different parts of the work, but the recording apparatus was the same throughout. The Rossi type of circuit was used for detecting the coincidences; the valves amplifying the impulses from the individual counters were capacity coupled to a thyratron which operated an automatic counting device.