<title>Use of mixed media to improve the performance of kinestatic charge detectors</title>

Abstract

The kinestatic charge detector (KCD) is a digital radiographic detector that in brief consists of a scanning drift chamber whose velocity is synchronized to the drift velocity of the ions produced therein by detected x rays, so as to cause the moving (kinetic) ions to appear at rest (static) in the patient rest frame. The spatial resolution of the KCD is limited by mobility dispersion when the detector operates with noble gases such as xenon or krypton. The magnitude and dependence on drift distance of the peak widths of ionic signal pulses produced in the KCD provide a measure of mobility dispersion. These parameters have been measured in a KCD filled with krypton gas at several high pressures and in the same gas mixed with dopants (such as amines, alkanes, ethers, etc.). Considerable improvement and probably elimination of mobility dispersion is seen. The four amines tested (ammonia, monomethyl amine, dimethylamine and trimethyamine) and dimethyl ether were successful in reducing mobility dispersion whereas none of the alkanes tested (which include methane, ethane, propane, butane and cyclopropane in order of decreasing ionization potential) were successful. A closer look revealed that even though some of the alkanes (cyclopropane and butane) had the desired ionization potential, all of them had a zero or near-zero dipole moment. This suggests that both ionization potential and dipole moment are important parameters for an effective dopant. The details of this effect are given in an accompanying paper (Giakos et al.). In addition, electron attachment and ionic recombination in a kinestatic charge detector lead to a loss in useful signal from the detector and a consequent reduction in the detective quantum efficiency (DQE) because the pulse height distribution gets broadened. Extremely small amounts (1 ppm or more) of oxygen and other electronegative impurities can cause significant electron attachment and consequent recombination loss in the detector. The addition of a polyatomic gas such as carbon dioxide (according to our findings) and methane (according to other workers, at very low pressures however) has been found to reduce electron attachment to a considerable extent via the Ramsauer Effect. The consequent reduction in the signal loss due to recombination results inconsiderable improvement in the signal obtained from the KCD. It was shown that use of 0.1 percent (1000 ppm) of carbon dioxide along with a dopant and a parent gas resulted in considerable reduction in electron attachment.