Bremsstrahlung emission in collisions between charged nuclei is equivalent to nuclear gamma decay between continuum states. The way the continuum spectrum can be treated is not unique, and efficiency and accuracy of cross section calculations depend on the chosen method. In this work we describe, relate, and compare three different methods in practical calculations of inelastic cross sections, that is, by (i) treating the initial and final states as pure continuum states on the real energy axis, (ii) discretizing the continuum states on the real energy axis with a box boundary condition, and (iii) complex rotation of the hamiltonian (complex scaling method). The electric quadrupole transitions, 2 + → 0 + and 4 + → 2 + , in α + α scattering are taken as an illustration.Keywords First keyword · Second keyword · More
IntroductionThe emission of bremsstrahlung in a collision between two charged particles constitutes an important background effect in Coulomb deexcitation processes. In a classical picture, this phenomenon is understood as the energy radiated due to the deceleration of a charged particle when deflected by another charged particle. The radiated energy is just the kinetic energy lost in the deceleration process. In a quantum mechanical picture, the process can be seen as the γ-emission due to the decay of a two-body system from some two-body continuum state into another continuum state of lower energy. A detailed derivation of the cross section for this kind of processes can be found in [1].The fact that these transitions involve continuum structures, both in the initial and final states, introduce technical as well as conceptual difficulties in the calculations. First, the treatment of the continuum spectrum itself, which can be handled in different ways, but in numerical studies almost necessarily by some kind of discretization. Second, the unavoidable matrix elements between the continuum states involved in the calculation are diverging and thus not well defined without some regularization prescription. These two problems, how to treat the continuum and how to obtain converged results, are possible sources of uncertainty that deserves detailed investigations.Different methods were previously used to compute the bremsstrahlung cross sections for various combinations of nuclei. In [2] α − α collisions were investigated in connection with the resonant electric