Photometric Determination of Ionization of Cloud-Chamber Tracks

A photoelectric method for measuring the specific ionization of tracks on cloud-chamber photographs is described. An important feature of the method is the use of a comparison track of known ionization in the same part of the chamber. A microphotometer slit is made to scan the track perpendicular to its direction, and a tracing of the transmission of the track relative to the background transmission is obtained. A parameter relating the transmission of the track in question to the transmission of the comparison track is obtained directly from the tracings. Identified π-μ decays are used to establish a calibration curve, and the method is applied to various unknown tracks. The error of measurement is of the order of 10%. A theoretical treatment of the problem is carried out to show the effects of various physical parameters not easily ascertained experimentally.