Three Geiger-Müller counters were arranged vertically so that double coincidences were recorded by the counters 1 and 2 and triple coincidences by counters 1, 2 and 3 simultaneously. The third counter was permanently shielded by a lead cylinder of 2.5 cm thickness which is sufficient to absorb all the secondaries produced in the absorber. Counts were made under three conditions: The reduction in the number of counts due to the lead was thus slightly greater in position $B$ than in position $C$. That this difference can be ascribed to secondaries is shown by the fact that $\frac{B}{C}$ is less for the double coincidences than for the triple ones. The data can be interpreted by assuming that the coincidences are due chiefly to penetrating ionizing particles, some of which are stopped by the lead (for which $B$ should equal $C$), with the addition of a few coincidences in case $C$ due to secondaries produced in the lead by a penetrating ray associated with a penetrating particle traversing the shielded tube 3. It does not appear possible to account for these coincidences in terms of corpuscles excited by photons unless the photons either are absorbed in the atmosphere before reaching the apparatus, or are accompanied throughout their path by ionizing particles, which would practically mean treating the photons as ionizing particles. That is, the coincidences seem to be due to penetrating ionizing particles and not to secondary particles produced by photons which traverse the lead. This view is further supported by the close agreement of the absorption coefficients computed from this experiment ($\mu =2.2\mathrm{}\times {10}^{-3\mathrm{}}$ ${\mathrm{cm}}^{-1\mathrm{}}$) with those determined from ionization chamber measurements of the cosmic rays ($\mu =1.90\mathrm{}\times {10}^{-3\mathrm{}}$ ${\mathrm{cm}}^{-1\mathrm{}}$).

Close

PrevNext

View figure in article