Measurement of absolute differential collision cross sections of Co 60 photons for the Compton effect

Abstract

Absolute values of the differential collision cross sections of Co 60 photons (1.17 and 1.33 MeV) have been measured in the angular range of 13° to 140°. A uniform spectral sensitivity photon counter developed in the laboratory has been used in the measurement of relative values of the cross sections. Corrections have been made for the effects of self-absorption in the source, small deviations of the detector efficiency from uniformity, etc. Detailed analysis has been made of the multiple scattering and absorption phenomena occurring in the cylindrical copper scatters used in the experiment and an empirical procedure has been developed to eliminate errors caused by it. Accurate relative values obtained in this manner have been converted into absolute differential cross sections through an integral measurement in which the effects of photons scattered by angles smaller than 13° are automatically eliminated. The root mean square deviation of the experimental values from the corresponding Klein-Nishina cross section is 1.5% which is contained within the r.m.s. spread 3.5% of the probable error of the experimental points themselves. Experimental points completely rule out the distribution formulae of Breit, Gordon and Dirac.