Saturation curves for free-air ionization chambers with electrode gap widths of 4.2, 8.4 and 18 mm were obtained for 10 and 15 keV undulator synchrotron radiation thinned with a 230 Hz rotating-disk chopper. Ion recombination in free-air ionization chambers was found to be inversely proportional to the applied electric field, and an expression that satisfactorily reproduced the ion-recombination rate is determined. A comparison of the expressions for continuous and pulsed X-rays revealed that chopped high-intensity X-rays require a higher voltage to attain saturation when the product of the pulse width and electric field exceeds a value that depends on the X-ray energy. This behaviour was observed explicitly for 10 keV X-rays in measurements with the ionization chamber placed before and after the chopper.
Keywords: X-ray chopper; ion recombination; ionization chamber; pulsed X-rays.