Ion recombination corrections for the NACP parallel-plate chamber in a pulsed electron beam

Abstract

The NACP electron chamber is one of three parallel-plate chambers recommended for use in the UK. Measurements with this chamber type have indicated a problem in determining the recombination correction. This is due to a variation of the ionization current I with polarizing voltage V which deviates from the accepted Boag theory. It is shown that there is a chamber-dependent threshold voltage below which the NACP chamber follows the Boag theory. Above this voltage the chamber should be used with caution, although it is still possible to correct for the dependence of the chamber response on the dose per pulse. The existence of such deviations from theory demonstrates the usefulness of the 1/I against 1/V plot and the limitations of the Boag two-voltage analysis. Values for the initial recombination and the coefficient of general recombination are measured for several NACP chambers. It is shown that from these one can derive a value for the effective plate separation and the collector radius of each chamber. Differences in the behaviour of NACP chambers manufactured by Scanditronix and Dosetek are discussed and the implications of free-electron collection are considered.

Ion recombination correction factors () for Varian TrueBeam high‐dose‐rate therapy beams

“…This correction has received theoretical attention 3 , 7 and has been studied for clinical radiotherapy scenarios 6 , 11 . Clinically, $P_{\text{ion}}$ is determined using the two‐voltage method (12) (using high voltage [${\mathrm{normalV}}_{\mathrm{normalH}}$] and low voltage [${\mathrm{normalV}}_{\mathrm{normalL}}$] that produce measured ionization readings ${\mathrm{normalM}}_{\mathrm{normalH}}$ and ${\mathrm{M}}_{\mathrm{L}}$, respectively).…”

Determination of the ion recombination correction factor for intraoperative electron beams

“…On the other hand, different absolute values of (k s ) jaff th could be the result of different measurement conditions, especially the electrode distance of the applied chamber and the dose-per-pulse value investigated [21]. As reported by Burns and McEwen, a difference in the parallel-plate separation of 0.16 mm causes changes in the ion recombination factor of up to 8% [4]. Furthermore, a dose-rate difference of 10 mGy/pulse results in a difference of up to 5% in the value of the ion recombination correction factor [10].…”

“…The dosimeter readings M were corrected for the polarity effect, temperature and pressure. On theoretical basis the plot of 1/M versus 1/V should be approximately linear, thus k s can be obtained by linear extrapolation to an infinite chamber voltage (1/V = 0) [4]. In the case of pulsed beams, as recommended in the TRS-398 protocol, the range of linearity of a chamber must be established over a range of polarizing voltage up to the manufacturer's recommended maximum.…”

“…This is mainly because the effect of the ion recombination correction factor k s will be more profound. This factor corrects the response of the ionization chamber for the ion loss due to the recombination of ions inside the chamber [4]. Based on current dosimetric protocols, the k s factor is mostly determined by Boag's two-voltage-analysis (TVA) method or the Jaffé method [5][6][7][8].…”

Radiation Dosimetry for Oncology

“…The theoretical aspects of ion recombination for pulsed and continuous radiation have been well discussed in the literature (37)(38)(39). However, in recent years a number of authors (40)(41)(42) have presented recombination data that do not agree with the standard theory.…”