Operational Quantities for External Radiation Exposure

Draft document: Operational Quantities for External Radiation Exposure
Submitted by Giuseppe Schettino, National Physical Laboratory
Commenting on behalf of the organisation

The redefinition of the operational quantities in terms of what is essentially an envelope function for the protection quantities certainly removes some of the problems that exist with the current operational quantities. It will remove the need for investigations, which have been extensively undertaken in the past by a number of bodies, of the degree to which the operational quantities under- or over-estimate the protection quantities, in particular in real workplace fields. This is definitely a step in the right direction, and it is unfortunate that this approach was not taken years ago when the operational quantities were first introduced. The changes do, however, introduce some problems of their own and fail to address some existing problems in particular in the area of calibration of personal dosemeters.

 The changes in the operational quantities may mean that instruments and dosemeters which satisfied international standards, those of the IEC for example, for the old operational quantities no longer satisfy the standards if the new quantities are used. This in itself is no reason not to introduce the new quantities. The fact that a device had a good response to a quantity that was not optional in terms of estimating a risk is not a good reason for not changing to a quantity that is a better estimate of the risk. The consequences of the changes should, however, be treated with common sense. If an instrument now over-reads by a factor of two in a field where no one receives a significant dose the need to improve that instrument response, although present, is not urgent.

 An aspect that needs to be considered before introducing any changes is the effect they will have on the views of the communities where radiation protection measurements are made, the nuclear industry in particular, of radiation protection quantities and the institutions that recommend them. The definitions of the quantities are not simple, and the majority of the people wearing dosemeters, making area survey measurements, and even those responsible for dosimetry, do not understand them. Constant changes, to the quantities and their names, undermine confidence in whether their risks are being correctly monitored. They also cause confusion. Members of the working group my find it easy to recognise the difference between H*(10) and H* but the users of the quantities may not. Any changes need to be well founded, not only in terms of making an improvement, but also of the improvement being sufficiently large to warrant the change at this time with everything it entails for dosemeter and survey instrument manufacturers. For calibration laboratories the change does not introduce any new problems although it may result in a lot of work.

 One change which will probably cause some consternation is the way the directional dependence of personal dose is presented. Quantities hp(±15º), hp(±15º), etc., are defined (the nomenclature is assumed as it is not given in the document) which are the maximum, for right and left angles, of effective dose; but values for effective dose at these angles have not been published by ICRP/ICRU, although they must have been calculated for this document. In essence this is no different to the position with the old operational quantities where hp(10,a) was given at specific angles, a, but E was only defined for A-P, P-A, ROT, etc. fields, but in the new incarnation this seems clumsy and something that will be hard to explain to people.

 A consequence of the direct link between the protection and operational quantities is that a change in the protection quantities, a re-evaluation of wr and wT values for example, should result in a change in the operational quantities. This may or may not be a good thing!

 As a calibration lab one problem that the proposals do not address is that of determining personal dose in the field from a point source when calibrating personal dosemeters on phantom. The new quantity is defined as a point quantity which means it can be defined anywhere, but this does not resolve the problem of where this point should be for the calibration situation of a dosemeter on the front face of a slab phantom. For the plane parallel beam used in calculating the quantity there is of course no problem, but for the divergent field of a point source the point at which to determine the fluence or kerma, which will then be multiplied by the appropriate conversion coefficient to derive the personal dose, is still unclear.