ESR study of atomic hydrogen and tritium in solid T$_{2}$ and T$_{2}$:H$_{2}$ matrices below 1K

TL;DR

This study uses ESR to investigate atomic hydrogen and tritium in solid T2 and T2:H2 matrices below 1K, revealing high atom concentrations, isotopic exchange, and diffusion mechanisms at ultra-low temperatures.

Contribution

First ESR analysis of atomic hydrogen and tritium in solid T2 and T2:H2 matrices at millikelvin temperatures, highlighting diffusion and recombination processes.

Findings

Achieved record-high T atom concentrations (~2×10^{20}cm^{-3}) in pure T2.

Revealed maximum H atom concentrations (~1×10^{20}cm^{-3}) in T2:H2 mixtures due to isotopic exchange.

Identified physical diffusion via tunneling or hopping as the main migration mechanism.

Abstract

We report on the first ESR study of atomic hydrogen and tritium stabilized in a solid T$_{2}$ and T$_{2}$:H$_{2}$ matrices down to 70$\,$mK. The concentrations of T atoms in pure T$_{2}$ approached $2\times10^{20}$cm$^{-3}$ and record-high concentrations of H atoms $\sim1\times10^{20}$cm$^{-3}$ were reached in T$_{2}$:H$_{2}$ solid mixtures where a fraction of T atoms became converted into H due to the isotopic exchange reaction T+H$_2\rightarrow$TH+H. The maximum concentrations of unpaired T and H atoms was limited by their recombination which becomes enforced by efficient atomic diffusion due to a presence of a large number of vacancies and phonons generated in the matrices by $\beta$-particles. Recombination also appeared in an explosive manner both being stimulated and spontaneously in thick films where sample cooling was insufficient. We suggest that the main mechanism for H and T migration is physical diffusion related to tunneling or hopping to vacant sites in contrast to isotopic chemical reactions which govern diffusion of H and D atoms created in H$_{2}$ and D$_{2}$ matrices by other methods.