Measurement of collision integral cross-sections of double-photon Compton effect using a single gamma ray detector: A response matrix approach M.B. Saddi, Bhajan Singh, B.S. Sandhu * Physics Department, Punjabi University, Patiala 147002, India Received 30 November 2007; received in revised form 18 March 2008 Available online 6 April 2008 Abstract The collision integral cross-sections of double-photon Compton process are measured experimentally for 662 keV incident gamma photons. The measurements are successfully carried out using a single gamma ray detector, and do not require the complicated slow-fast coincidence technique used till now for observing this higher order quantum electrodynamics (QED) process. The energy spectra of one of the two final photons, originating in this process, in direction of the gamma ray detector are observed as a long tail to the single-pho- ton Compton line on lower side of the full energy peak in the observed spectra. An inverse response matrix converts the observed pulse- height distribution of a NaI(Tl) scintillation detector to a true photon spectrum. This also results in extraction of events originating from double-photon Compton interactions. The present measured values of collision integral cross-section, although of same magnitude, devi- ate from the corresponding values obtained from the theory. In view of the magnitude of deviations, in addition to small value of prob- ability of occurrence of this process, the agreement of measured values with theory is reasonably acceptable. Ó 2008 Elsevier B.V. All rights reserved. PACS: 32.80.t; 13.60.r; 78.70.g Keywords: Doubly differential collision integral cross-section; Scintillation detector response unfolding; Energy spectra; False events and detector effic- iency 1. Introduction The double-photon Compton scattering is a quantum electrodynamics (QED) process in which the interaction of a gamma photon with an electron results in a final state consisting of two simultaneous degraded photons at the same time as the recoil-electron. This phenomenon needs to be investigated in detail because it is a major back- ground process to the study of another QED process namely photon splitting in the fields of heavy atoms, the first experimental confirmation of which has been reported by Akhmadaliev et al. [1]. The data analysis of their exper- iment, in the energy region of 120–450 MeV, results in about 400 photon-splitting events for 1.6  10 9 photons incident on the BGO target. The double-photon Compton scattering of the incident gamma photons by the atomic electrons (c +e ? c 1 + c 2 + e) is the significant back- ground contributing to the registered events. This process also provides a test of QED implicitly, a mechanism of photon multiplication along with bremsstrahlung in astro- physics and there is appreciable contribution to attenua- tion coefficients at higher incident photon energies where this process is more likely to occur. Mandl and Skyrme [2] using S-matrix formalism of quantum electrodynamics have provided an exact theory of this process, which has not been subjected to exten- sive experimental study although a number of measure- ments have been reported. This QED process has also 0168-583X/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2008.03.230 * Corresponding author. Tel.: + 91 0 98728 09265; fax: +91 175 2286412. E-mail address: balvir@pbi.ac.in (B.S. Sandhu). www.elsevier.com/locate/nimb Available online at www.sciencedirect.com Nuclear Instruments and Methods in Physics Research B 266 (2008) 3309–3318 NIM B Beam Interactions with Materials & Atoms