Measurement of Field Distortion in Free-Air Ionization Chambers

A two dimensional allaJog of a free-air ionization chamber for the measurement of fi eld distortion was obtained by painting conducting lines on resistance paper to represent cross sections of the electrodes. Guard wires were simulated by driving record p layer needles into the paper . The volume of the chamber from which charge was collected was related to the resistance between two of the lines on the paper, and small changes in this volume could be determined accurately by observing resistance changes. Grounded plates external to the chamber were simulated by additional lines painted on the paper, and the effect of t hese plates on the collecting volume was determined by measuring a change in res istance as the potential of these (simulated) external plates was changed. M easurements were also made for gnards in the form of double wires and strips. There is some evidence from m easurements on three dimensional ionization chambers that the top and bottom of t he surrounding grounded X-ray shield tends to decrease the effects of distortion caused by the ends of the X-ray shield, so that the data obtained here would represent an upper limit to the error for the indicated chamber cross section.