Reduced Radioactivity of Tritium in Small Titanium Particles

Concerning the PdH-system, the processes at the surface (H2-dissociation, surface diffusion) have been described in [2] in two chapters, p. 84 ff. and 134ff. At about the same time, Minnerup [3] showed that the entrance of the H atoms into the Pd-lattice does not start before the external surface is covered by a hydro­ gen monolayer. Minnerup further showed that the H-atoms between the surface layer and the bulk phase have to pass a "sub-surface state" [4]. This state, inter­ mediate between adsorption on the surface and ab­ sorption in the bulk, was detected a few years earlier with LEED-investigations by Christmann et al. [5]. The enthalpy of desorption of hydrogen from the sur­ face is much higher than the enthalpy of the decompo­ sition of the PdH compound in the bulk; the potential wells of the H atoms at the surface are correspond­ ingly deeper, and the vibration frequencies of the H atoms are higher. This is the reason why the surface will be covered before the H atoms enter the interior of the lattice. Concerning the TiH-system, the differences be­ tween the surface and the bulk are similar. The mea­ surements of the local modes hco by Hempelmann et al. [6] and Ross et al. [7] where performed by neu­ tron scattering (as in the PdH-system) and are there­ fore not sensitive to the surface states but represent only the vibration modes of the hydrogen in the bulk.