SU‐E‐T‐156: Can Sr‐90 Check Sources Replace Co‐60 Measurements for Monitoring of Reference Chamber Stability?

Abstract

Purpose:To determine the ultimate precision of a system for monitoring reference‐class ion chamber stability using a commercial Sr‐90 check source.Methods:A detailed investigation of a commercial Sr‐90 check source (PTW48002) was carried out using a series of Farmer‐type ionization chambers. Investigations included: positioning repeatability (angular variation as chamber is rotated in source, variation in ionization current with vertical alignment); chamber settling; short and long term repeatabilityResults:i) Measurement precision – the ionization current was typically 10 pA, and therefore a high‐precision electrometer is required to prevent electrometer noise/resolution/leakage biaising the results. ii) Chamber settling ‐ the chamber response stabilizes after approximately 10 minutes, which is longer than reported for linac beams and is likely due to the low doserate of the source.iii) The measured response depended at the 1 % level on the orientation of the chamber with respect to the source. However, consistent positioning resulted in repeatability at the 0.05 % level. Care was also required to ensure that the chamber was consistently positioned vertically with respect to the source. The sensitivity to vertical position was found to be > 1 % per mm.iv)With a uniform procedure the long‐term (> 6 month) repeatability was found to be better than 0.1 % for multiple chamber types and potentially a precision of 0.05 % is achievable.Conclusion:A Sr‐90 check source is easy to use and is a viable alternative to Co‐60 for monitoring reference chamber stability.