Laser-assisted proton radioactivity of spherical and deformed nuclei

Abstract

Particle emission in strong electromagnetic radiation is a topic that has received considerable attention in atomic physics (single and multi-electron ionization). However the problem has been far less debated in nuclear physics, due to the high laser intensities required to excite nuclear states or to remove nuclear particles (nucleons, clusters, etc). The advent of new powerful laser facilities around the globe provides a good opportunity to assess the main features of particle emission from nuclei exposed to ultra-intense coherent electromagnetic radiation. In the first part of this paper we review some of the most conspicuous laser field quantities employed in the diagnosis of atomic ionization processes and estimate them on a wide range of values of intensity and photon frequency. In this way we probe the threshold from which relativistic effects start to be important, but also establish when multiphoton effects are important, or when perturbation theory is no longer legitimate. We then focus on the case of proton emission from the ground state of a spherical nucleus (171Au) and the deformed nuclei 141Ho and 145Tm. By solving the time-dependent Schrödinger equation we provide estimates of the proton decay-rate for long or short laser pulses as a function of intensity and photon frequency. As time-independent potential we use the Woods–Saxon form of the nuclear potential with parameters adapted for the proton-decay of 109I and the Coulomb potential produced by the uniformly distributed charge of the daughter nucleus inside the nuclear surface. We solve the time-dependent Schrödinger equation in cylindrical coordinates (two dimensions) using the Crank–Nicolson method. This method allows us to follow the tunneling dynamics in ultra-intense laser fields within a large spatial region compared to the nuclear volume. Application of a continuous laser field of Ti:sapphire type of very high intensity or of x-ray type with corresponding lower intensity induces only more or less pronounced oscillations around the field-free decay rate values. On the other hand, short laser pulses of rectangular shape with an odd number of half-cycles yield an enhancement of up to three orders of magnitude of the decay rate.