Influence of environmental parameter variations on X-ray beam intensities: a time-dependent absorption correction

Essential to the quality of X-ray analysis in crystallography, such as diffractometry and spectrometry, is a stable and reproducible X-ray source. Commonly, different optical elements are utilized to provide a dedicated X-ray beam. The stable alignment of all these components is a prerequisite in order to reduce aberrations and to achieve high signal-to-noise ratios. Besides such aberrations and electronically induced variations of the X-ray primary beam intensity, the environmental conditions are of particular importance, most prominently the barometric pressure, humidity and temperature. In a qualitative as well as quantitative study, the influence of the environmental conditions on the primary beam intensity of a sealed tube with a Cu anode and their correlations are determined. For a common setup, utilizing a scintillation counter, laboratory as well as external conditions are monitored simultaneously for 28 d. Their individual influence on the X-ray intensity and their correlations are evaluated by statistical analysis including time lag. By this comprehensive study, experimental intensity variations of up to ΔI/I = 1.153 ± 0.001% are determined during density of air changes of Δρ/ρ = 3.7 ± 0.6%. This is interpreted in terms of air transmission variations of up to T_X-ray = 1.137 ± 0.001% for a typical X-ray analysis setup due to ambient barometric pressure, temperature and humidity changes for natural mid- and long-term variations. Significant correlations with respect to daily and weekly cycles and in particular with ambient conditions are determined. These results are used for a time-dependent absorption correction of the measured intensity, which reduces the standard error by about 25%.