19-11-2012, 04:50 PM
Evaluation of non-Rutherford proton elastic scattering cross section for magnesium
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Abstract
The experimental data available for magnesium (p,p) elastic scattering cross section at angles
and energies suitable for Ion Beam Analysis have been evaluated using the theoretical model approach
together with additional measurements and benchmark experiments. The results obtained provide the
evaluated differential cross sections for magnesium (p,p) elastic scattering in the energy region up to
2.7 MeV.
Introduction
This article continues a series of papers devoted to the evaluation of non-Rutherford cross sections for Ion Beam
Analysis (IBA). The results achieved so far are summarized in [1]. It was demonstrated that the evaluation of the cross
sections by combining different sets of experimental data in the framework of a theoretical model makes it possible to
calculate the smooth curves of d&/dθ(E,θ) needed for simulation of IBA spectra with a reliability exceeding that of any
individual measurement.
The evaluation procedure consists of the following: Firstly, a search of the literature and of nuclear data bases is
made to compile and compare relevant experimental data. The apparently reliable experimental points are critically
selected. Free parameters of the theoretical model, which involve appropriate physics for the given scattering process, are
then fitted within the limits of reasonable physical constraints. Details of the physics are described elsewhere [2].
Additional experimental data can be incorporated a posteriori. If necessary, benchmark experiments are performed to
arbitrate discrepancies.
Magnesium is an important element. It is the crucial component of, for example, light strong metal alloys
important for aerospace structural materials and certain automotive components. In any application where thin film
coatings or tribological layers are investigated we may expect the ability to use IBA to be useful.
Magnesium diboride is also an interesting new superconductor with a critical temperature of 39K. Rutherford
backscattering (RBS) has been used to determine the elemental depth profile in ion beam synthesised MgB2 [3], but the
sensitivity to B is poor in RBS. An alternative approach is to use elastic (non-Rutherford) backscattering (EBS) where the
sensitivity to B is enhanced by an order of magnitude for a 2.6MeV beam. However, at this proton energy the elastic
scattering cross-section for Mg is also strongly non-Rutherford, and must be determined for EBS depth profiling to be used.
Evaluation
The differential proton elastic scattering cross sections for magnesium in the energy range from Coulomb
scattering to 2.5 MeV were found in four papers: Mooring et al (1951) [4], Rauhala et al (1988) [5], Zhang et al (2003)
[6], and Wang et al (1972) [7]. The reported data were measured at laboratory angles of 164.5° (Mooring), 170° (Rauhala),
140°, 150°, 160°, 170° (Zhang), and 130°,150° (Wang) in the energy range of 0.40-3.95, 0.8-2.7, 0.8-2.5, and 1.5-3.0 MeV
respectively. Natural magnesium (78.99% of 24Mg, 10.00% of 25Mg, and 11.01% of 26Mg) was used for manufacturing
targets in Rauhala and Zhang, the target material in Mooring was 24MgF2 enriched by the 24Mg isotope up to 99.50%, and
the target in Wang was also of high enrichment (~99%). The measurements reported in Mooring, Zhang and Wang were
made with thin targets prepared by evaporation of magnesium onto graphite backing and with a thick sample in Rauhala. A
computer fit using the simulation program GISA [8] and TRIM77 [9] stopping powers for Mg provided the cross sections in
the last case. The spectra of elastically scattered protons were measured by means of a magnetic analyzer (Mooring) and
with silicon surface barrier detectors for all the others. A large background scattering from the impurities contained in the
graphite backing was found in Mooring and the corresponding correction was made for the cross-section determination.
Conclusion
The proton elastic scattering from natural magnesium has been evaluated, and can now be reliably calculated for any
scattering angle in the energy range from Coulomb scattering up to 2.7 MeV. The uncertainty of SigmaCalc cross-sections
proved to be not worse than 2%.
It is shown that sharp strong resonances observed in the cross-section are also prominent in thick targets. For example, the
full structure of the strong resonance at 1483keV was not reproduced in any reported thin target measurement, but a correct
simulation using the theoretical cross-sections reproduced the data well.