Ionization of short-lived isotopes in a hot cavity – Numerical simulations

• The model of ionization of stable and short-lived nuclides in a hot cavity ion source is presented.
• Influence of radioactive decay, sticking to hot surfaces and diffusion on ion yield are studied.
• Current–voltage curves and changes of ion source efficiency due to ionizer length are calculated.
• Calculated changes of ion source efficiency with temperature match experimental results.
• Release curves were obtained for various sticking and diffusion time-scales.

An extended Monte Carlo method based numerical model of the hot cavity ion source is presented. Not only the radioactive decay and delays due to the sticking of atoms to hot walls are taken into account, but also (i) delays due to the diffusion of nuclides, (ii) contributions from electron impact ionization, (iii) calculations of ion release curves and (iv) the case of hemispherical cavities are implemented. The code enables calculations of ionization efficiency for a broader range of parameters including ionizer material, size, geometry and temperature; extraction voltage; timescales characterizing radioactive decay, particle sticking, out-diffusion; and electron impact ionization cross-sections. The dependences of ion source efficiency on decay half-life, sticking time and diffusion timescale are shown and discussed. Two different schemes of radioactive nuclide generation are introduced and compared. Influence of ionizer length and extraction voltage is extensively studied. The importance of the electron ionization for short-lived isotope ion production is for the first time demonstrated. Two new analytical models of ionization in the case short-lived nuclides are introduced and compared to simulation results. Good agreement of experimental and simulated data (efficiency vs. temperature) is shown. New features enabling simulations of ion release curves are demonstrated.