Ion recombination in parallel-plate free-air ionization chambers for synchrotron radiation

Abstract

The saturation characteristics of two sizes of parallel-plate free-air ionization chambers were investigated for synchrotron radiation at bending-magnet, wiggler and undulator beamlines of SPring-8. The gaps of the electrodes were 4.2 and 85 mm. The monoenergetic photon energies ranged from 10 to 115.56 keV and the air kerma rates from 0.2 mGy s-1 to 150 kGy s-1. Ion recombination at the high dose rate was found to be smaller than that predicted by Boag's expression, which was based on volume recombination, and the difference increased with an applied electric field. In the high dose rate region, the reciprocal of the current was linear to the reciprocal of the electric field near saturation, which represented the occurrence of initial recombination and diffusion loss. At the low electric field and the low dose rate, the reciprocal of the current was linear to the reciprocal of the square of the electric field. The reduction of total ion recombination was attributed to the shift of the contribution from volume recombination to initial recombination and diffusion loss.

Determination of X-ray flux using silicon pin diodes

“…A detailed description of the design and use of ion chambers is beyond the scope of this paper, but generally these devices consist of two metal plates on either side of a gas-filled box with a voltage applied across the plates. Voltages of order 100-2000 V cm À1 are required to avoid space charge saturation for highly brilliant X-ray beams (Wyckof, 1979;Nariyama, 2006). X-ray photons with the wavelengths used for MX interact with matter in one of three ways: photoelectric emission, elastic scattering or Compton scattering.…”

Current saturation in free-air ionization chambers with chopped synchrotron radiation

“…at high electric field), free electrons that do not attach to oxygen molecules play an important role, as has been shown via the pulse shape (Nariyama, 2012). Owing to this effect, a lower electric field than expected has been observed at saturation with high-intensity X-rays (Nariyama, 2006).…”

“…For 10 keV X-rays incident on an ionization chamber with a 4.2 mm gap, equation (10) reduces to the following simple relation, where 5 Â 10 13 V 2 A À1 m À1 has been substituted for (Niatel, 1967;Nariyama, 2006) and 3 Â 10 10 V m C À1 for (ICRU, 1982), Plots of K against E demonstrating that K is inversely proportional to E. The circles, squares and triangles denote IC1, IC2 and IC3, respectively. The open and closed marks are data for 10 and 15 keV X-rays, respectively.…”

A gas scintillator detector for 2D dose profile monitoring in pencil beam scanning and pulsed beam proton radiotherapy treatments

“…, averaged over their characteristic time response) during treatment can be up to tens of μA, two to three orders of magnitude larger than that with continuous beam accelerators. At the resulting high instantaneous dose delivery rates, the response of air ionization chambers conventionally used as treatment dosimeters becomes nonlinear, due to recombination of the produced ions (Almond 1984, Palmans et al 2006, Nariyama 2006). This effect makes it highly desirable to develop alternative real-time dosimeters that operate linearly up to much higher dose rates.…”