The entrance probability and range of helium in W(100) for energies 8eV to 5000 eV as measured by thermal desorption spectrometry

Abstract

Abstract Measurements have been made using thermal desorption spectrometry of the entrance probability η of He+ ions incident on a W(100) surface at energies 8 eV to 5000 eV. For energies above 250 eV, η for normal incidence, ηn, is within 20 of unity, while for “random” incidence ηn = 0.6 ± 0.1. Below 250 eV, both decrease and become equal for E ≤ 25 eV. At 25 eV, η = 0.2 and at 8 eV, η ≈ 0.01. The measured values of η were used to obtain values for the average penetration d of the helium ions from previously measured values of the product ηd for the same two directions of incidence. Above ∼ 150 eV. the range for random incidence. was found to be 0.12 E0.9 A (E in eV) while for normal incidence, the range is larger by a factor of about 3. Below 150 eV. both decrease rapidly and become equal (≈ 1 A) at 25 eV. The fraction of the normally incident ions channeled fc was derived from the measured η r and η n . It was found to be > 0.8 for E ≥ 1000 eV, and <0.5 for E < 200 eV.