The entrapment in tungsten of helium positive ions of kinetic energies in the range 60 eV-1 keV

Monoenergetic helium ions produced within a specially constructed 180° magnetic deflection mass spectrometer are directed at normal incidence on to the clean surface of a well-aged polycrystalline tungsten ribbon in an ultra-high vacuum. The total ion flux used is not greater than 10 13 ion/cm 2 at energies between 60 eV and 1 keV. The gas trapped in the metal is then desorbed into the known isolated volume of the mass spectrometer by heating the tungsten ribbon to 2200°C. The mass spectrometer, tuned to helium at mass number 4, measures the increase in partial pressure due to desorbed helium. From the measured quantity of helium desorbed, ion sticking probabilities are determined to be from 0·03 at 60 eV increasing to 0·69 at 1 keV.