On the calibration of plane-parallel ionization chambers for electron beam dosimetry

Abstract

The procedure recommended by different dosimetry protocols for the determination of the absorbed dose to air chamber factor, ND,pp, of plane-parallel chambers, comparing absorbed dose determinations in a high-energy electron beam with a reference cylindrical chamber having a known ND,cyl factor, has been investigated. Attention has been focused on the case that the chamber serving as reference has a solid aluminium central electrode. It has been found that using a wide spread Farmer-type chamber (NE 2571), together with recommendations which specifically take into account central electrode corrections for electron beam dosimetry, kcelpcel = pcel-global(IAEA) = 1.008, yields inconsistent results compared with those obtained from a fully homogeneous ionization chamber; for the NE 2571 chamber, a value kcelpcel = pcel-global(IAEA) congruent to 1.0 has been obtained. Analytical calculations of kmkatt for Farmer-type cylindrical chambers and experimental determinations of the product kmkattkcelpcel in electron beams agree within experimental uncertainties, with no evidence of statistical significance for the commonly used assumption pcel = 1, which yields a 0.8% correction (due to kcel only) for the effect of the NE 2571 aluminium electrode in electron beam dosimetry. The use of a 'NACP-chamber' specific factor (kpp or kmkatt) to obtain ND,pp from NK,pp in NACP plane-parallel chambers has been found unsatisfactory, and direct experimental determinations of ND,pp are recommended instead. It is suggested that Standard Dosimetry Laboratories provide ND,pp calibration factors in 60Co beams.

A new approach to electron‐beam reference dosimetry

“…One drawback of the system proposed is that some planeparallel chambers appear to have variable dosimetric properties which suggest that 60 Co calibrations may not provide accurate indications of behavior in electron beams. 19,20 To the extent that this is the case, this suggests the need for using electron-beam calibration factors which can easily be introduced following the procedure outlined here ͑Eq. ͑18͒͒.…”

The Status of High-Energy Photon and Electron Beam Dosimetry Five Years After the Implementation of the Iaea Code of Practice in the Nordic Countries

“…Some results in this study, however, appear to have been classified incorrectly, as differences close to 5% found by Laitano et al (107) for in-air and in-phantom configurations do not appear in a table which classifies both types of configurations under a common heading. Similarly, discrepancies in the in-phantom (PMMA) determinations of k,,(NACP) by (107) and (95,106) have not been discussed though they were close to 1.5%. An additional problem is that for a given make of chamber, sample-to-sample variations have been reported (95,106,109) which of course cannot be predicted by Monte-Carlo or any other calculations.…”

“…Similarly, discrepancies in the in-phantom (PMMA) determinations of k,,(NACP) by (107) and (95,106) have not been discussed though they were close to 1.5%. An additional problem is that for a given make of chamber, sample-to-sample variations have been reported (95,106,109) which of course cannot be predicted by Monte-Carlo or any other calculations. This variation calls into question the validity of a 'universal' factor valid for all chambers from a given manufacture (24,101), and some authors ( 105, 106) therefore recommend individual calibrations in electron beams for every chamber.…”

“…Correction for this effect might be important for aluminum electrodes, mainly when determinations of absorbed dose in electron beams are performed. An overview of existing data for in-air and in-phantom determinations has been given (106). It will be discussed here primarily in connection with the calibration of plane-parallel chambers in high-energy electron beams.…”

“…Problems associated with the correction for the influence of the electrode in this procedure have been reported (106).…”