Operational Quantities for External Radiation Exposure

Draft document: Operational Quantities for External Radiation Exposure
Submitted by Hannes Stadtmann, IMS, Seibersdorf Labor GmbH, Austria
Commenting on behalf of the organisation

Comments on the ICRU/ICRP draft report

Hannes Stadtmann, Monitoring Service, Seibersdorf Labor GmbH, Austria

1. Nov. 2017

The recommended operational quantities allow to assess the protection quantity effective dose, or selected organ doses in a much better way than the previous operational quantities. We acknowledge the big effort the committee has undertaken to prepare this draft, and including all data. The consequences of the change of the current operational quantities, however, now need to be discussed by all relevant stakeholders. Numerous details should be clarified before the report is published.

The proposed operational quantities, H* and Hp, are directly linked to the protection quantity effective dose, E. The current point quantity dose equivalent utilizing a quality factor, Q(LET), is no longer used in the new definitions.

The draft document changes the previous ICRU concept where measurable quantities are defined exactly and physically (simple geometrical calibration phantoms as well as the physically defined relation Q(LET).) ICRP now propose to use complex anthropomorphic phantoms and a priori defined tissue and radiation weighting factors (wT and wR) in the definition of those quantities. It is not verified, however, whether these new quantities can be measured (especially by simple cost effective dosemeter badges, both for personal and area monitoring) in practice. A possible change of the weighting factors in the future could lead to further changes in the definition of quantities (conversion factors). From the viewpoint of a manufacturer/supplier of dosemeters, exact, clear and stable definitions are required. Otherwise, a redesign and change of thousands or tens of thousands of dosemeters is not feasible nor economical.

The following comments are compiled mainly in consideration of the consequences of the proposed changes on personal/area monitoring in photon fields.

The new definitions dramatically change the energy and also the angular response of Hp and H* dosemeters. A factor of >5 for low-energy photons cannot be introduced by changing the dose calculation algorithm or the calibration (at least for our multi detector devices). A change of all dosemeter badges, including type tests and new accreditation or approval procedures are necessary. It is the question whether high costs are justified especially if the current dosemeters show a conservative estimate of E. A current perfect dosemeter for Hp(10) will no longer fulfil the requirements of IEC62387 for photon energies below 40 keV.  Most individual monitoring in Austria is performed for persons employed in the medical sector (90%), mainly in medical diagnostics.

The change from dose equivalent (Sv) for skin and eyes to absorbed dose (Gy) will require new ICRP dose limits. Or, if the dose limits for skin and eye will not change (e.g. 1Sv=1Gy), dose limits for all radiation types for which wR>1 implicitly will increase by a corresponding factor. Although this might not be an important issue for the dose to the local skin and possibly the dose to the lens from neutrons and protons, possible consequences on the dose limits need to be mentioned and discussed.

Since the new operational quantities are directly defined by conversion factors, these defined factors should be available for many more energies and angles (e.g. interpolated data available online or as CD). We think it is not sufficient to just recommend an interpolation procedure, as this interpolation is not unambiguous especially for two variables (energy and angle). 3rd order Lagrangian Interpolation does not lead to a continuous 1st derivative. Better methods could be considered and implemented (Spline etc.) in the definition.

Closer inspection of the somehow linked tables (e.g. Tab A5.4.1b and A4.1.1b) shows no good agreement between the values that are expected to be similar (conversion coefficients for low-energy photons with and without kerma approximation). It seems that the statistical uncertainties for calculations without kerma approximation are quite high (which would in fact be expected). This, however, is quite problematic since the dose quantities are defined by those factors and possible high level interpolation between noisy data is problematic. Unfortunately, no real uncertainties are stated for the conversion factors, which would be very helpful.

Since the new dose quantities are defined by conversion coefficients, it is essential that no errors (not uncertainties) are within the published data. If all recommended data were available in a convenient data format (text files), the community could help to check these enormous amounts of data for their consistency.

Please check the data, especially in the following tables:

Tab A5.4.1b 0.04 MeV 45deg

Tab A4.1.1b 0.04 MeV 45deg and eventually 0.1 MeV 75deg

Again, we thank the report committee for preparing this elaborate draft and hope that a discussion with stakeholders will improve this report and make it practically applicable.