Measurement of field distortion in free-air ionization chambers by analog method

Abstract

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 r elated to the resistance b etween 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. Ther e is some evidence from m easurements on three dimensional ionization chambers t hat the top a nd 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.

Design of free-air ionization chambers

“…Figures 5 and 6 show the distortion for s In one case considered. Miller and Kennedy [4] predicted a distortion of about 1 percent whereas the ionization chamber actually indicated an error of 0.3 percent. However, when the third-dimensional effect was eliminated in the ionization chamber, the measured distortion error was 0.7 percent-in fair agreement with the predicted 1 percent.…”

“…These considerations have been used to deter¬ mine and correct for field distortion. This has been done with actual chambers by measuring ionization under the two conditions of box poten¬ tial [23] and has also been used in a resistance analog method [4] for determining the distortion in two dimensions for a parallel-plate system. Unfortunately, the two-dimensional results over¬ estimate the three-dimensional distortion,8 but they are the only extensive data available on the subject.…”

Measurement of cobalt-60 and cesium-137 gamma rays with a free-air chamber

“…Results obtained with the thread chamber for cesium-1 37 gamma rays at 6.7 atm are shown 111 ~gLlre 5 a nd for cobalt-50 gamma rays at 12 atm 111 figure 6. The solid curves give the ionization current to the outer collector ring for different positions of each Bakelite sheet with the other sheet pos.itioned as close as possible to the collecting regIOn ; the dotted curves are for the ionization curren t to the central collector.…”

X-Ray measurements and protection, 1913-1964

“…233), and of field distortion and free-air ionization chambers using analog methods by Miller and Kennedy (Ref. 279).…”