Stopper foil

When a stopper foil of thickness 8y(atom/ cm ) is used, the hydrogen energy observed at the detector is not actually E, but a lower value, where... 

The final term of Equation (6) corrects for the distortion of the ERS spectrum caused by velocity-dependent energy losses as the H ions pass through the stopper foil. 

Eor analysis of emitted particles, solid state surface barrier detectors (SBD) are used inside the scattering chamber to measure the number and energy of the reaction products. Stopper foils are used to prevent scattered projectiles from reaching the detector. Depth profiles can be obtained from the energy spectra, because reaction products emitted in deeper layers have less energy than reaction products emitted from the surface. The concentration in the corresponding layer can be determined from the intensity of reaction products with a certain energy. 

Because RBS is rather insensitive to light elements and unable to detect hydrogen, one can make use of the complementary technique Elastic Recoil Detection (ERD) when sensitivity for light elements is required. In this case, recoiled particles are detected instead of the back scattered particles. The incident beam usually consists of heavier ions, e.g. 2 Si (4He is sufficient when one is interested in H and D only), and a stopper foil prevents backscattered particles from entering the detector, whereas the lighter recoiled particles are transmitted [32]. 

Figure 23.32 Deuterium depth resolution map for 1.3 MeV He in polystyrene for (a) 3.0p.m-and (b) 4.5 pm-thick Mylar stopper foils. The contour values represent the depth resolution in Angstroms. Adapted with permission from Ref [126] 1994, Elsevier.

In the second report, Oslanec et al. investigated the interaction between bromi-nated polystyrene and an inorganic substrate, using LE-FRES with a 1.7 MeV He+ ion beam at 15° incident and exit angles, and 6(im-thick Mylar stopper foil. The achieved depth resolution was 55 nm at the surface, and 65 nm at 200 nm beneath the surface. As a polymer model system, blends of d-PBro.oeS with PS (miscible at 190 °C) were used, with SiO and SiH as substrates [170]. An interfacial excess of d-PBro.oeS was observed on both investigated surfaces, in contradiction to previous reports (Figure 23.38) [171-173]. Unfortunately, to the best of our knowledge, this discrepancy was not further discussed in literature and remains unexplained. 

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