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The measurement of small quantities of hydrogen (a few ppm) present in the bulk of materials by elastic recoil detection analysis is very difficult as the sensitivity is limited by a high background, almost continuous in level, in the low energy region, i.e. bulk region of the spectrum. We have quantified and successfully assigned the different contributions to the background to three main effects in the measurement of hydrogen in the bulk of Cu, Ge, Ag and Au samples. (A) The contribution from recoil hydrogen from the sample holder or the walls of the scattering chamber due to impact of ions scattered from the entrance slit. It is reduced by the placement of an entrance collimator which reduces the background by approximately 40%. (B) The second contribution is due to recoil hydrogen from the foil in front of the detector. Our study shows that the replacement of a mylar stopping foil by an Al foil reduces the apparent content of bulk hydrogen by more than a factor of 10. (C) The contribution of recoil hydrogen from the sample surface reaching the detector due to double scattering. This effect has been investigated by reducing the amount of surface hydrogen by Ar sputtering and studying its effect on the apparent hydrogen concentration in the bulk. It is found that a reduction of the surface hydrogen by a factor of 10 by sputtering results in a similar 10-fold reduction of the apparent bulk concentration. It is further shown that by consequently removing these contributions, it is possible to measure bulk concentrations of hydrogen well below 1 at ppm in single crystalline samples. The bulk concentrations measured for the samples of Ag, Au, Ge and Cu are found to be 36.8, 2.5, 1.6 and 0.7 at ppm. |
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