Thermionic emission of hydrogen isotopes (Mu, H, D and T) from W and interpretation of a role of the hot W as an atomic hydrogen source

Abstract

Resonant ionization was applied to learn about the production mechanism of thermal neutral hydrogen isotopes including muonium (Mu; μ +e − ) from a hot tungsten surface as reaction products with pulsed 500 MeV protons. The activation energy E a obtained in the temperature dependence of production of neutral muonium (1.72 (5) eV) as well as that of T atom (1.89 (1) eV), is consistent with the emission of the atoms directly from bulk sites in the solid rather than from the surface for which one should expect E a≈3 eV. Since hydrogen molecules are easily dissociated into hydrogen atoms interacting with a surface of the hot tungsten, even residual H 2 in the ultra high vacuum chamber of 3×10 −10 mbar at 1200 K was found to be easily detected and extracted by this method. From the results obtained through the temperature and pressure dependence measurements for D 2 and H 2, we propose a view of the role of hot W as a catalyzer to dissociate H 2 molecules and producing hydrogen atoms. After dissociation on the surface, the hydrogen atoms are dissolved into the bulk solid, and are thermionically emitted from the bulk to the vacuum at high temperatures.