Resolution limits of a low-pressure ionography chamber

The sources of resolution loss are presented for an ionography chamber operated at low gas pressure (1-2*105 Pa). The alignment tolerances imposed by the spherical geometry have been calculated and are shown to be easily attainable. Monte Carlo simulations, calculating the blurring effect of photoelectron ranges, have been carried out for several diagnostic X-ray spectra and with Freon 13B1 as the chamber gas. The resulting line spread functions have been Fourier transformed to give the modulation transfer function. It is seen that for all of the X-ray beam qualities presented in this paper, the limiting resolution is in excess of 2 cycles/mm. Finally, the effect of ante-charge collection upon the resolution is discussed and a technique is proposed whereby this effect may be reduced.