ON THE CALIBRATION OF PLANE-PARALLEL IONIZATION CHAMBERS FOR ELECTRON-BEAM DOSIMETRY

The procedure recommended by different dosimetry protocols for the determination of the absorbed dose to air chamber factor, N(D,pp), of plane-parallel chambers, comparing absorbed dose determinations in a high-energy electron beam with a reference cylindrical chamber having a known N(D,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, k(cel)p(cel) = p(cel-global)(IAEA) = 1.008, yields inconsistent results compared with those obtained from a fully homogeneous ionization chamber; for the N E 2571 chamber, a value k(cel)p(cel) = P(cel-global)(IAEA) congruent-to 1.0 has been obtained. Analytical calculations of k(m)k(att) for Farmer-type cylindrical chambers and experimental determinations of the product k(m)k(att)k(cel)p(cel) in electron beams agree within experimental uncertainties, with no evidence of statistical significance for the commonly used assumption p(cel) = 1, which yields a 0.8% correction (due to k(cel) only) for the effect of the NE 2571 aluminium electrode in electron beam dosimetry. The use of a 'NACP-chamber' specific factor (k(pp) or k(m)k(att)) to obtain N(D,pp) from N(K,pp) in NACP plane-parallel chambers has been found unsatisfactory, and direct experimental determinations of N(D,pp) are recommended instead. It is suggested that Standard Dosimetry Laboratories provide N(D,pp) calibration factors in Co-60 beams.