Abstract: Near−surface light atom detection in metals by a proton backscattering technique

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Abstract
Recent interest in the behavior of helium and hydrogen isotopes in first wall CTR candidates has stimulated efforts to develop a technique capable of profiling light atom species in the near−surface region of metals. The ion elastic backscattering technique is essentially nondestructive and would be ideal for such determinations, except that the distribution of low Z elements in metals is difficult to observe by this technique because the elemental Rutherford scattering cross section at a given energy decreases as the square of the decreasing atomic number. For this reason, detection of helium and hydrogen isotopes in heavier host materials is especially difficult and, in fact, has been considered beyond the capabilities of the Rutherford backscattering technique. 1 However, it has recently been shown that a specialized form of proton backscattering is capable of detecting not only the above mentioned elements, but also all light element impurities (except 1H), which may be present in thin film or foil substrates up to 10 μ in thickness. 2,3 The data obtained are straightforward to interpret and exhibit simultaneously the depth distribution of all impurities lower in Z than flourine in volume elements as small as 10−9 cm3. Depth resolution and detection are 400 Å and 0.5 at.% for 4He in copper and 690 Å and 7 at.% for deuterium in titanium. It has more recently been demonstrated that the enhancement in elastic scattering cross sections of D and 4He for 2.5 MeV protons is sufficient to use proton backscattering to measure the depth distribution of these elements in thick targets as well as in foils. 4 Alternatively, the presence of helium which has been implanted in metals can be observed as a notch in the backscattering peak of the host material and the depth distribution deduced therefrom. 5 Details of these developments as well as a review of the basic technique will be presented and examples of its application in fields such as range energy measurements and fusion reactor materials research will be discussed.

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