Zero‐shift thimble ionization chamber

Purpose: Verify experimentally the theoretical prediction of F. Tessier and I. Kawrakow [Med. Phys. 37, 96–107 (2010)] that it is possible to design a thimble ionization chamber with no shift in its effective point of measurement (EPOM), i.e., a chamber that provides a measure of the dose to the medium at the location of its central axis. Methods: Measure dose from a 25 MV photon beam incident on water with an Exradin A1SL ionization chamber inside a thin sleeve (as a means of effectively increasing the thimble wall thickness). The depth‐dose curve is compared to that obtained using a well‐characterized PTW Roos parallel‐plate chamber. Results: With an appropriate increase in thimble wall thickness, the EPOM shift of the Exradin A1SL vanishes. Further increase of the wall thickness yields a chamber with a positive (downstream) shift in its point of measurement. Conclusions: It is possible to design a thimble ionization chamber with a zero EPOM shift by adjusting the wall thickness.