Angular distribution of scattered electrons associated with collimated bremsstrahlung and the tagging technique

We investigate the angular correlation between a bremsstrahlung photon and its corresponding post-bremsstrahlung electron within the context of a magnetic tagging spectrometer with the aim of improving the instrument's efficiency. Our results are given in terms of angular distributions of the post-bremsstrahlung electron associated with photons that pass through a circular collimator centered in the forward direction. We start from the fully differential Bethe-Heitler (first Born approximation) cross-section, including the Molière screening correction, which is then integrated over the photon azimuthal angle and over the photon polar angle defined by the collimator. These integrations are performed analytically, making no high energy or small angle approximations. To obtain numerical values from the results of these integrations a multiprecision program is used to avoid severe problems of cancellations, especially at high energies (above 1 GeV). Making use of the angular correlation between the electron and the photon, we show that it is possible to increase the usable photon flux if we avoid the detection of electrons with large angles that have no photon partner passing the collimator. This can be accomplished by limiting the size of the electron detectors in the plane perpendicular to the bend plane.