Neutron-impact ionization of He

The double ionization of He by either photon impact or bare ion impact are simple examples of the quantal three-body Coulomb breakup problem. For example, the double photoionization of He near threshold produces two slow moving continuum electrons in the Coulomb field of an alpha particle, which is extremely difficult for perturbation theory to handle. On the other hand, various non-perturbative theoretical approaches, including R-matrix methods [1–5], the converged close-coupling method [6], the time-dependent close-coupling (TDCC) method [7], and the exterior complex scaling (ECS) method [8] have yielded double photoionization cross sections for He in good agreement with experiment [9]. Another simple example is found in neutron scattering from the nucleus of the He atom [10, 11]. Depending on the energy acquired by the target nucleus, the atomic electrons can either follow its movement and remain in bound states or be released to the continuum. When both electrons are released to the continuum, we again have to solve the three-body breakup problem. Recently a non-perturbative ECS method was used to calculate neutron-impact single and double ionization cross sections for He [12]. They found significant differences in the ratio of double to single ionization cross sections from those found in photon impact and bare ion impact processes. With almost two dozen neutron scattering facilities in the world [13], a better understanding of the motion of atomic electrons following neutron-impact could provide insight into many neutron research projects in not only physics, but astronomy, biology, chemistry, and engineering. For example, new detectors are currently being designed to search for weakly interacting massive particles (dark matter candidates) based on neutron scattering experiments [14]. In this paper we present a theoretical study of the single and double ionization of He over a broad energy range. The non-perturbative TDCC method allows us to incorporate the interaction of two continuum electrons in the presence of a positive ion field in an accurate manner. We present total cross sections for the single, single with excitation, and double ionization of He. We compare cross section ratios of double to single ionization with previous ECS results [12]. We also present energy and angle differential cross sections for the single and double ionization of He. The rest of the paper is organized as follows. In section 2 we present the non-perturbative TDCC method for neutron-impact ionization of atoms. In section 3 we present total and differential cross sections for the neutron-impact single and double ionization of He. In section 4 we conclude with a brief summary and future plans. Unless otherwise stated, we will use atomic units.