Further measurements of the cosmic-ray energy spectrum

Abstract The work already described (Blackett and Brode 1936) of determining the energy spectrum of cosmic-ray particles, by measuring the curvature of their tracks with a counter-controlled cloud chamber in a strong magnetic field, has been continued. A new set of 641 tracks in a magnetic field of 12,000-14,000 gauss have been measured, making with the 188 already reported a total of 829 tracks. An expansion ratio of about 1·08 was used, with an argon-oxygen mixture for the gas and with an ethyl alcohol and water mixture for the liquid. In order to avoid the inconveniently large distortions of the lenses previously used, Messrs Taylor, Taylor and Hobson supplied a new pair of 35 mm. f/2 ciné lenses, which were especially adjusted to have a very small distortion, when used with a 1·2 cm. glass plate as the roof of the chamber: fig. 1 shows the distortion curves, that is, the image curvature c as a function of the displacement x, both for the normal and for these special lenses. The initial slope of the curve for the new lens is about one-fifth of that for the normal lens, while the deviation from linearity is only reduced in much less degree. This confirms the statement (Blackett and Brode 1936, p. 578) of Mr A. Warmisham of Taylor, Taylor and Hobson, Ltd., that the coefficient a1 of the linear term, which gives the initial slope of the curve c = f(x), is easily under the designer’s control, whereas the coefficient a2 of the cubic term, which governs the departure from linearity, is roughly a constant for a given type of lens. The use of these lenses has greatly facilitated the work.