Experimental measurements of doubly and singly differential collision cross-sections of double-photon compton scattering - 071

The quantum electrodynamics (QED) process in which interaction of an incident photon with an electron gives rise to emission of two simultaneously degraded gamma quanta at the same time as the recoil electron is known as double-photon Compton scattering. Mandl and Skyrme using S-matrix formalism of QED have provided the exact theory of this process, which so far has not been subjected to extensive experimental study. This phenomenon requires investigation in detail because it is a major background process to the study of another non-linear QED process namely photon splitting in the fields of heavy atoms, the first experimental confirmation of which has recently been reported by Akhmadaliev et al. In the data analysis of their experiment, the significant background contributing to the registered events being from double-photon Compton scattering of the incident photons by the atomic electrons. In the present work, the doubly and singly differential collision cross-sections of this process are measured experimentally for 0.662 MeV incident gamma photons obtained from 6 Ci ¹³⁷Cs source. The measurements are carried out using a single gamma detector, a technique avoiding the use of the complicated slow-fast coincidence set-up used till now for observing this process. The energy spectra of detected photons are observed as a long tail to the single-photon Compton line on the lower side of the full energy peak in the recorded scattered energy spectrum. The detected signal events recorded by the gamma detector are a composite of double-photon Compton scattering (DPCS) events, Bremsstrahlung, multiple Compton scattering and single-photon Compton scattering (SPCS). The counts due to DPCS events are extracted from the observed events. The agreement of present measured values of doubly and singly differential collision cross-sections, which are first of its kind, with theory appears to be quite satisfactory. The error estimated in the present measurements comes to be 6-16%. (authors)