Isotope-selective laser photoionization of tin in supersonic atomic beam

Abstract

The isotope ¹²⁴Sn enriched to better than 50% is needed in substantial quantity as detector material for the proposed neutrinoless double beta decay (0νββ) experiment in India (TIN.TIN). Present work investigates isotope-selective laser photoionization scheme for ¹²⁴Sn. Transition to the first excited state from the ground state lies at 286.3 nm having short lifetime (~ 5 ns) and isotope shift (IS), Δν(124–120) = 441 MHz. However, the availability of high repetition rate (kHz), narrowband tunable pulsed laser in UV with low temporal jitter is a technological impediment for achieving isotopic selectivity in this transition. An alternate three-color photoionization scheme is proposed for separation of ¹²⁴Sn. Laser vaporization of tin followed by supersonic expansion in a molecular beam apparatus provided the tin atomic beam. Tin atoms were first excited to the 5p6s,³P₁ state from the 5p²,³P₀ ground state by a broadband pulsed dye laser (λ₁ = 286.3 nm). Isotopic selectivity was achieved in the second excitation step at λ₂ = 855.2 nm by a narrowband continuous wave laser. Subsequently, resonant photoionization at λ₃ = 694.7 nm to a newly observed autoionizing state at 60,992.9 cm⁻¹ provided efficient photoionization. The IS for all even and hyperfine splitting for odd isotopes have been measured for the 855.2 nm transition. A selectivity factor of 24 is achieved for ¹²⁴Sn isotope. The absorption cross sections of the three excitation steps is reasonably high to have an efficient photoionization scheme.