Computation of the electron beam quality k_{{rm Q,Q}₀} factors for the NE2571, NE2571A and NE2581A thimble ionization chambers using PENELOPE

The quality correction factor $k_{Q,Q_{0}}$ 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 $f_{\rm c,Q}$ of these chambers for electron beams with nominal energies ranging between 6 and 22 MeV, corresponding to a Varian Clinac 2100 C/D. A $^{60}$Co beam was used as reference quality ${\rm Q}_0$. Also eight monoenergetic electron beams reproducing the quality index $R_{50}$ of the Clinac beams were considered. The $k_{Q,Q_{0}}$ factors were calculated as the ratio between $f_{\rm c,Q}$ and $f_{{\rm c,Q}_0}$. 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 $k_{{\rm Q,Q}_0}$ factors found for the monoenergetic beams are larger (smaller) than those corresponding to the Clinac beams at low (high) $R_{50}$ values, the differences being slightly above $0.5\%$. Finally, the $k_{{\rm Q,Q}_0}$ factors obtained in the case of the NE2571A chamber are systematically ~0.5% below those of its predecessor chamber, the NE2571.