Technical Note: Influence of Compton currents on profile measurements in small-volume ion chambers.

Purpose: This work is to evaluate the effects of Compton current generation in three small-volume ionization chambers on measured beam characteristics for electron fields.

Methods: Beam scans were performed using Exradin A16, A26, and PTW 31014 microchambers. Scans with varying chamber components shielded were performed. Static point measurements, output factors, and cable only irradiations were performed to determine the contribution of Compton currents to various components of the chamber. Monte Carlo simulations were performed to evaluate why one microchamber showed a significant reduction in Compton current generation.

Results: Beam profiles demonstrated significant distortion for two of the three chambers when scanned parallel to the chamber axis, produced by electron deposition within the wire. Measurements of ionization produced within the cable identified Compton current generation as the cause of these distortions. The size of the central collecting wire was found to have the greatest influence on the magnitude of Compton current generation.

Conclusions: Microchambers can demonstrate significant (>5%) deviations from properties as measured with larger volume chambers (0.125 cm(3) and above). These deviations can be substantially reduced by averaging measurements conducted at opposite polarities.