On the photon component of cosmic radiation and its absorption coefficient

Abstract

It has been established that the soft component of the cosmic radiation consists of electrons and photons. Much experimental data on the electrons forming the soft component are available and they are known to form a fraction of about 25-30% of the whole beam of ionizing particles at sea level, excluding particles below 10 7 eV (e.g. Rossi 1933; Nielsen and Morgan 1938). The energy spectrum of the electrons is known roughly from the work of Blackett (1938), Wilson (1939) and others. The energy loss of electrons in metal plates has been investigated by Anderson and Neddermeyer (1934), Blackett and Wilson (1937), Williams (1939), Wilson (1938, 1939), showing that the experimental values of the energy loss are in agreement with the prediction of the quantum theory (Bethe and Heitler 1934). Much less is known about the photon component of cosmic radiation, as comparatively few experiments have been carried out to investigate their properties. Further the results of the investigations available are partly contradictory. The theory of the absorption of high energy photons has been worked out to the same extent as for electrons (Bethe and Heitler). Owing to the lack of experimental material, the theory could be tested only up to energies of about five million volts (McMillan 1934; Gentner 1935). The success of the theory of cascade showers due to Bhabha and Heitler (1937) and Carlson and Oppenheimer (1937), based on the Bethe-Heitler theory of electrons and photons, provides however an indirect test for the validity of the absorption formula for high energy photons. The lack of experimental data on high energy photons is due to the difficulties in the method of observation; photons unlike electrons cannot be observed directly. In the present paper a simple method for investigating cosmic-ray photons is described. Using this method, data about the number, energy distribution and absorption of cosmic-ray photons have been obtained.