An experimental study of ion recombination in parallel-plate free-air ionization chambes

Abstract

An experimental study of the saturation conditions of parallel-plate free-air ionization chamhers is presented. The results may be explained by assuming that two different types of ion recombination (initial and volume recombination) take place simultaneously. Their relative values can be calculated from the results, and are found to agree with theories proposed hy Boag, by Scott and Greening, and by Kara-JIichailova and Lea. It was found that for the saturation current to be determined by extrapolation, the collecting field strength should he high enough for volume recombination t o be negligible compared with initial recombination. The ext,rapcllation is of the reciprocal of ionization current against the reciprocal of collecting field strength.

Saturation characteristics of the ionization chamber at a low dose rate

“…5. It can be seen that the initial recombination component is almost a linear function of 1=V ; which is consistent with the previous study by Niatel (1967). The line in Fig.…”

“…3, it can be seen that initial recombination is the dominant process for large values of 1=V : Niatel (1967) proposed an experimental method to determine the initial and general components of the recombination loss quantitatively. The fractional recombination loss at voltage V, pði sat ; V Þ; is defined by…”

“…The two voltage method only requires measurements of the ionization current i 1 at a nominal operating voltage, V 1 ; and i 2 at a much lower voltage, V 2 : However, an initial recombination can play a dominant role when the ionization current is measured at a low dose rate and the large-volume ionization chamber is used. Niatel (1967) carried out an experimental study of the ion recombination taking into account the initial and the general recombination. Piermattei et al (1996) obtained the fraction of the initial and general recombination based on the Niatel's approach, and he proposed the new two voltage method.…”

Ion recombination and polarity correction factors for a plane–parallel ionization chamber in a proton scanning beam

“…Based on the conclusion of Palmans et al., that a cyclotron proton beam should be treated as a purely continuous beam, and assuming that this also applies to a scanned beam, the k s value was calculated according to the formula derived from Boag's theoretical model:where is a parameter representing volume recombination, as given by Boutillon et al., is the electrode spacing, is the volume of the ionization chamber, is the ionization current, and is the chamber voltage. The parameter is a property of the gas in the cavity and it value depends on the recombination coefficient and on the mobility of negative and positive ions in the gas.…”

Note: Voltage and intensity dependence of the saturation curves of free-air ionization chambers irradiated with chopped synchrotron radiation beams

“…For freeair ionization chambers, the expression for continuous x rays has been given and confirmed experimentally. [2][3][4] However, the experiments with pulsed photon beams have not been performed because the available pulsed photon beams have been bremsstrahlung in high energy electron linac accelerators. X ray free-electron lasers, however, are available at the linac-based accelerator facilities of the Linac Coherent Light Source and SPring-8 Angstrom Compact Free Electron Laser (SACLA).…”