Attachment of mobile particles to non-saturable traps

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Abstract
Thermal desorption spectra have been measured for helium attached to xenon atoms in a tungsten crystal. The xenon was implanted at 1.5 keV and annealed to 2150 K, and the helium then injected at 250 eV along the direction normal to the crystal surface. The binding energy of the helium was found to increase monotonically with the number of helium atoms occupying the trap, from 1.2 eV for a single atom to ≈2.1 eV for 7 to 10 atoms. At still higher injected doses, the attachment of up to 100 helium atoms to each trap has been observed. The binding energy continues to increase to ≈4 eV for 100 atoms, and the apparent trapping radius increases by a factor of four from the empty trap value of 3.3 Å.