The Free Path of Slow Protons in Helium

Abstract

POSITIVELY charged hydrogen atoms with velocities acquired by falling through 300 to 900 volts have been found to possess an unexpected range in helium and other gases. With helium pressures so high as 0.5 mm. of mercury, the protons will complete a semicircular path 16 cm. in length and still appear as a positively charged bundle of rays. The magnetic deflexion shows also that they remain charged throughout their entire path. As the free path given by the kinetic theory of gases for a rapidly moving particle is 1.5 millimetres at this pressure, the protons must pass unaltered through more than one hundred helium atoms. The capture of electrons by α-rays of various velocities observed by Henderson and Rutherford, and Rüchardt's experiments with canal rays, would lead one to expect a rapid neutralisation of protons of this velocity. The neutralisation of the more rapidly moving particles observed in those experiments has been explained by the presence of a great number of electrons, due to ionisation, with which the particles may combine, and we may account for the absence of neutralisation in the present experiments by the assumption that no free electrons are produced by protons of the velocity used. In fact, energy considerations would suggest the improbability of an electron leaving a helium atom with an ionisation potential of 24.5 volts, in order to form a neutral hydrogen atom possessing a much smaller ionisation potential.