Dal Granville and Dal Granville

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Thursday, February 26, 2015

Time:   3:30 - 5:00 pm

Location:   Carleton University - Herzberg Building - Room HP4351

1. "LET measurements in therapeutic proton beams"
Dal Granville - Carleton University
Abstract: The biological response resulting from proton radiotherapy depends on both the absorbed dose in the irradiated tissue and the linear energy transfer (LET) of the beam. While absorbed dose is readily measured using a variety of detectors, there is no device available for the routine measurement and verification of LET. This work aims to further develop the optically stimulated luminescence (OSL) technique to allow for routine measurements of LET in therapeutic proton beams. This presentation will focus on the LET dependence of OSL detectors, and detail the progress made in using them to verify LET in proton therapy treatment plans.

2. "Investigating the anomalous response of the NE2575 ionization chamber, and ideal chamber shapes"
Frédéric Tessier - National Research Council Canada
Abstract: In 1993, upon acquiring a Cs-137 irradiator, physicists in Ionizing Radiation Standards at the NRC noticed that measurements with a large volume 600cc ionization chamber model NE2575 showed an unexpected deviation from the inverse square law, with a discrepancy of up to 4% at 8 meters from the source. Although this anomaly was confirmed experimentally and was well documented, a definitive explanation remained elusive. Twenty years later, we revisit this problem using EGSnrc Monte Carlo simulations to discern the contribution of each chamber component to the anomaly. We show that the observed deviation arises mostly from long photon attenuation paths inside the chamber cylindrical side wall. We propose an empirical correction to address the issue in practice, but also uncover an  optimal chamber angle at which the expected behaviour is recovered. Finally, we expand the question and consider the ideal shape of an ionization chamber to minimize deviations from the inverse square law.