Computation of the electron beam quality kQ,Q0 factors for the NE2571, NE2571A and NE2581A thimble ionization chambers using PENELOPE

The quality correction factor kQ,Q0 for electron beams was calculated for three thimble ionization chambers, namely, NE2571, NE2571A and NE2581A. The Monte Carlo code PENELOPE was used to estimate the overall correction factor fc,Q of these chambers for electron beams with nominal energies ranging between 6 and 22 MeV, corresponding to a Varian Clinac 2100 C/D. A 60Co beam was used as reference quality Q0. Also eight monoenergetic electron beams reproducing the quality index R50 of the Clinac beams were considered. The kQ,Q0 factors were calculated as the ratio between fc,Q and fc,Q0. Those obtained for the NE2571 ionization chamber show a nice agreement with those calculated by Muir and Rogers with EGSnrc. As it occurred to other ionization chambers analyzed in previous works, the kQ,Q0 factors found for the monoenergetic beams are larger (smaller) than those corresponding to the Clinac beams at low (high) R50 values, the differences being slightly above 0.5%. Finally, the kQ,Q0 factors obtained in the case of the NE2571A chamber are systematically ∼0.5% below those of its predecessor chamber, the NE2571.