Bursts in Material of Low Atomic Number

A large ionization chamber has been used to obtain an integral burst size-frequency distribution by amplifying and recording the electron pulses produced by bursts. A comparison of the integrated burst rate from the upper hemisphere with that produced by primaries with a small zenith angle permitted an estimate of the burst rate resulting from interactions of the $N$ component. Calculations were made of the expected burst distributions arising from the electromagnetic interactions (bremsstrahlung, knock-on, and direct pair production) of spin 0 and spin ½ $\mu$-mesons. About $\frac{1}{3}$ of the bursts observed under 400 g ${\mathrm{cm}}^{-2\mathrm{}}$ of sand-lime brick and concrete at sea level were attributed to interactions of the $N$ component. The remaining burst rate agreed with the rate calculated for spin ½ to within the estimated uncertainty of the comparison—about 20 percent. Lack of quantitative information concerning the nuclear interactions of $\mu$-mesons prevents an estimation of the actual electromagnetic cross sections. The expected $Z$ dependence of $\mu$-meson bursts was roughly confirmed by a comparison with results obtained elsewhere under material of higher $Z$.