Space charge effects in a kinestatic charge detector.
The effects of space charge in a kinestatic charge detector (KCD) were examined using computer solutions to Poisson's equation. The KCD is a strip-beam parallel-plate drift chamber used for digital radiography. It was assumed that there is negligible electron attachment, i.e. there are no negative ions formed. The ionization rate per mA as a function of x-ray interaction depth was calculated for a detector filled with xenon at 25.3 x 10(5) Pa. Solution of Thomson's equations gave the positive ion density at the cathode, also as a function of depth. Water filtration values ranging from 0 to 30 cm were used in order to estimate the range of ion density values expected in a clinical KCD. The case of steady-state x-ray illumination was simulated for ionization rates less than the zero field limit (above which space charge changes the polarity of the electric field). Line spread responses were found for varying ionization rates to show the effect of space charge due to electric field distortion on the spatial resolution performance in the drift direction. The effect of imaging ideal edges with a KCD was calculated and the expected output signal was plotted for densities up to the zero field limit. Space charge dependence on the selection of KCD design and operating parameters is discussed. Because of the dependence of the KCD drift-direction spatial resolution on the uniformity of the electric field, space charge effects impose an upper limit on the detector entrance exposure and define the dynamic range of the device.
Wagenaar
D J
DJ
Curriculum in Biomedical Engineering, University of North Carolina, Chapel Hill 27599.
DiBianca
F A
FA
Tenney
C R
CR
Fritsch
D
D
D006801:Humans; D011856:Radiographic Image Enhancement / Q000295:instrumentation*; D011874:Radiometry / Q000295:instrumentation*; D013679:Technology, Radiologic