The generation, absorption and anisotropy of thick‐target bremsstrahlung and implications for quantitative energy dispersive analysis

Abstract

AbstractThe bremsstrahlung spectrum from a thick specimen bombarded with 20 ke V electrons is studied with emphasis on the shape rather than the absolute intensity in the energy range 1–10 keV. Monte Carlo calculations are described in detail and used to compare the absorption corrections for characteristic and continuous (bremsstrahlung) radiation from a thick specimen and to determine the extent to which anisotropy modifies the intensity distribution of the continuum. The absorption corrections are found to differ by roughly 5% for f(χ) = 0.5 and 10% for f(χ) = 0.2, but it is shown that the ratio of the corrections can be predicted by using the ratio of two Philibert‐type formulae. Anisotropy has little influence on the absorption correction but changes in atomic number or geometry may result in the intensity distribution being altered by typically 5‐10% across the energy range 1–10keV. Numerical integration gives values for the generated brems‐strahlung intensity which are used in constructing an expression to represent the functional form of the background. Although experimental spectra confirm this to be superior to the combination of Kramers' law and a characteristic absorption expression, several sources of error at low energies, including bremsstrahlung from the Be window, are pointed out and improvements suggested. In order to eliminate the principal uncertainty, that of the overall efficiency curve for a Si(Li) detector and pulse‐processor, it is recommended that a stored spectrum be used as a ‘background standard’ for explicit background corrections. Errors are then reduced to the 0.1% absolute concentration level, which is suitable for quantitative analysis.