OMPI Seminar and tour of NRC facilities: Alexandra Bourgouin and Bryan Muir

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Thursday, October 20, 2016

Time: 3:30 - 5:00 pm. Refreshments start at 3:15 pm.

Location: NRC - 1200 Montreal Road, Building M-36 - Kelvin Room (please check in at the front desk).

TOUR: There will be a tour of NRC facilities right after the seminars, starting around 5 pm. NRC staff would like to know the number of people interested in the tour in order to organize the logistics. If you're interested please email Dr. Malcolm McEwen (Malcolm [dot] McEwenatnrc-cnrc [dot] gc [dot] ca (Malcolm [dot] McEwenatnrc-cnrc [dot] gc [dot] ca)).


1) "Ion chamber dose measurements - problems with the temperature-pressure correction factor"
By Alexandra Bourgouin, PhD student, Carleton University and NRC
Supervisor: Malcolm McEwen

Purpose: To investigate the behavior of ionization chambers over a wide pressure range. Methods: Three cylindrical and two parallel-plate designs of ion chamber were investigated. The ion chambers were placed in vessel where the pressure was varied from atmospheric (101 kPa) down to 5 kPa. Measurements were made using Co-60 and high-energy electron beams. The pressure was measured to better than 0.1% and multiple data sets were obtained for each chamber at both polarities to investigate pressure cycling and dependency on the sign of the charge collected.

Results: For all types of chamber, the ionization current, corrected using the standard PTP, showed a similar behaviour. Deviations from the standard theory were generally small for Co-60 but very significant for electron beams, up to 20 % below P = 10 kPa. The effect was found to be always larger when collecting negative charge, suggesting a dependence on free-electron collection. The most likely source of such electrons is low-energy electrons emitted from the electrodes. This signal would be independent of air pressure within the chamber cavity. The data was analyzed to extract this signal and it was found to be a non-negligible component of the ionization current at atmospheric pressure. In the case of the parallel plate chambers, the effect was approximately 0.25 %. For the cylindrical chambers the effect was larger - up to 1.2 % - and dependent on the chamber type, which would be consistent with electron emission from different wall materials. For the electron beams, the correction factor was dependent on the electron energy and approximately double that observed in Co-60.

Conclusion: Measurements have indicated significant deviations of the standard pressure correction that are consistent with electron emission from chamber electrodes. This has
implications for both primary standard and reference ion chamber-based dosimetry.

2) "Progress toward updating the TG-51 protocol for electron beam reference dosimetry"
By Bryan Muir, PhD
National Research Council Canada

The addendum to the TG-51 protocol for photon dosimetry was published in 2014 and included refinements to the original protocol. The revision for electron beam dosimetry will require more extensive changes. In this talk, I will discuss progress toward updating the protocol for electron beam dosimetry including: (i) updated Monte Carlo calculations of electron beam quality conversion factors, (ii) gradient corrections and optimal shifts for accurate electron beam dosimetry, (iii) the use of cylindrical chambers in electron beams, and (iv) electron beam primary standard water calorimetry to obtain measured beam quality conversion factors at the National Research Council.