Measuring the Intensity of Molecular Beams

A Pirani gauge sensitive to pressure changes of the order of ${10}^{-8\mathrm{}}$ mm of mercury has been developed and used to measure the intensity of molecular beams defined by two circular openings, one 0.4 and the other 0.6 mm in diameter placed 25 mm apart. The gauge is situated 40 mm from the last opening. A beam of air gives a galvanometer deflection equivalent to 600 cm with a pressure of 0.5 mm of mercury behind the first opening. Under similar conditions ${\mathrm{H}}_2$ produces a deflection of 1800 cm. The galvanometer sensitivity is 11.6 mm per microvolt. The time lag of the gauge is less than the period of the galvanometer (7 seconds). The limits within which the beam-forming system may be expected to give a Maxwellian distribution of velocities are discussed. The final pressure in the gauge due to a given beam depends upon the dimensions of the gauge opening as kinetic theory predicts. The reduction in intensity of a beam of ${\mathrm{H}}_2$ by collision with either air or ${\mathrm{H}}_2$ present in the experimental chamber at various pressures has been measured. The data show that the gauge may be used to measure mean free paths and to obtain more detailed information than has previously been possible as to the probability of scattering through various angles by collision.