TU‐FF‐A2‐01: Unexpected Doserate Dependencies of Diodes for Beam Scanning

Abstract

Purpose: The doserate dependence in electron beams of four types of scanning diode were investigated — Scanditronix‐Wellhöfer EFD and SFD, PTW60012 and Sun Nuclear QED 1113000‐1. Method and Materials: The detectors were positioned at a fixed source‐detector distance in an uncollimated 20 MeV electron beam and the doserate varied by varying the incident beam intensity. There were therefore no changes in field size, scatter contribution etc. The dose per pulse was in the range 0–0.1 cGy, which is typical of clinical linacs. Measurements were also made in high gradient electron fields, e.g. depth‐dose curves and beam profiles, for a range of electron energies from 6–22 MeV. Results: Two of the diodes — the SFD and QED‐ showed little or no doserate dependence. The EFD showed a variation that just exceeded the manufacturer's specification and the PTW60012 showed a very large effect with a 15% change in sensitivity over the range of doserates investigated. These differences in response were also seen in the comparison of beam profiles and depth‐dose curves. The EFD and PTW diodes gave a difference in R50 of approximately 1 mm, which is equivalent to a 4% error for 6 MeV. Similar discrepancies were seen for the measurement of field size. Doserate corrections were obtained and corrected dose distributions generally gave agreement between all detectors within the measurement uncertainties. Conclusion: These results would indicate that users should not simply rely on manufacturers' specifications and should characterize any new diode for doserate dependence before using the device for beam measurements.