Experimental study on the influence of the central electrode in Farmer-type ionization chambers

In the IAEA TRS-381 protocol, k(cel) and p(cel) account for the central electrode perturbation during the air kerma chamber calibration and the in-phantom measurements. The values of these correction factors are based mainly on Monte Carlo simulations. In the present work experimental data on k(cel) and p(cel) for the NE-2571 chamber is provided, relative to a graphite electrode. In addition, the relative influence of the 3 mm diameter A-150 central electrode of the NE-2581 chamber is studied. The experimentally determined value of k(cel) for a 1 mm aluminium electrode is 1.008 +/- 0.2%, and of p(cel) in photon and electron beams 0.993 +/- 0.2% and 0.997 +/- 0.2% respectively. The experimental data and the Monte Carlo simulations agree to within 0.2%. No significant influence of the A-150 central electrode diameter on the absorbed dose determination is shown.