Operational Quantities for External Radiation Exposure

Draft document: Operational Quantities for External Radiation Exposure
Submitted by Frank Busch, German Individual Monitoring Services
Commenting on behalf of the organisation

Comment on behalf of the four German Individual Monitoring Services, representing 350,000 occupational exposed individuals


The draft claims to overcome shortcomings of the current operational quantities (Hp(10) personal dose equivalent at depth 10 mm, H*(10) ambient dose equivalent, Hp(0.07) personal dose equivalent at depth 0,07 mm and H’(0.07, Ω) directional dose equivalent) and to provide a coherent concept that has a direct connection to the protection quantities. While not being measurable they should be estimated as best as possible, but conservative by the corresponding operative quantities. This claim is certainly well met at a conceptual level. On the one hand, the significant overestimation of the effective dose in the range of low X-ray energies is avoided and, on the other hand, the underestimation in the range of very high photon energies (> 5 MeV) is corrected.


From the user perspective we would like to criticize the chosen approach as well as the possible consequences for the practice.

The chosen approach contains two major changes that would have a significant impact.


First, the definition of the operational quantities is no longer based on the absorbed dose within ICRU tissue, but the maximum value of the effective dose. On the one hand, this achieves the desired good estimate of the effective dose - on the other hand, this is done by merely demanding this match and not by proposing a corresponding measurement concept, as is the case with the current quantities. As a consequence, this means a shift in problem solving from the definition of operational variables to the development of suitable measuring instruments, a topic that the draft does not address. The statement, that “Modifications of existing instrumentation might be required” (line 1286 of the ICRU / ICRP draft) is, in our view, severely understated. If such a grave paradigm shift is made, there is also a much greater need for discussion, involving in particular the stakeholders who have to deal the potential impact and need to finance it. We criticize the submitted draft for the fact that the practical consequences were not examined and thus an essential aspect for the discussion with relevant interest groups can not yet take place.

The second major change is the consistent use of particle flux as basic quantity for the derivation of the operational quantities. This is certainly a simplification in the presentation, but in practice without further advantages.
From the perspective of the official German dosimetry services, which currently monitor some 350,000 exposed persons, two aspects of the proposed operational quantites must be considered:


1. The availability of appropriate dosimeters and

2. The impact of such new dosimeters in practice, if they were available.


1. The availability of appropriate dosimeters

The draft does not, in any way, display how the proposed quantities could be measured and assumes that modifications to existing measuring instruments are sufficient. This gives the impression that this would be possible with only additional filtering or recalibration of the current dosimeters. This is certainly not the case. In contrast to the depth-personal dose equivalent, which is defined as the absorbed dose at 10 mm depth in the ICRU tissue, this tissue depth would vary energy-dependent according to the proposed definition from 60 mm in the X-ray range up to 160 mm for high-energy photons. This illustrates very clearly that the simple and inexpensive passive dosimeters used worldwide for individual monitoring can no longer be used in the future. A recalibration is certainly not enough. On the contrary, it is even questionable whether it will be possible to develop appropriate personal dosimeters for the entire required energy range. The construction of a simple dosimeter with only a single detector is certainly no longer possible. Whether it is possible to meet all the requirements of the relevant standards (e.g. IEC 62387) with a multi-detector system is questionable, since the angular dependence also becomes more complicated. In the worst case, there might not be a suitable dosimeter for certain applications. These questions must be clarified before a recommendation for new quantities is given.


Two examples of the previous redefinition of the operational variables 30 years ago may make this clear:

(1) For example, in Germany a limited transitional authorization to use area dosimeters approved for the previous quantities is still in force, as the availability of appropriate H*(10) dosimeters for certain X-ray examinations is not sufficiently guaranteed. This is expressly not a technical but an economic problem of the implementation.

(2) In contrast, the use of passive Hp(10) dosimeters worldwide is already very largely standard. Nevertheless, for example, in international intercomparisons by EURADOS, Hp(10) dosimeters in some countries still encounter difficulties and the implementation of Hp(10) can by no means be considered complete. Support from a scientific and technical point of view is still needed to achieve the required knowledge and infrastructure.


There is certainly agreement that radiation protection worldwide has already reached a good level. Not at least, this is a result of the easy availability of simple and inexpensive passive dosimeter systems, e.g. based on TLD or OSL. Therefore, a new concept of operational quantities must necessarily be valued by how it can be put into practice and what impact it will have. With the last redefinition 30 years ago it was clear in advance that most passive personal dosimeters could be reused for nearly the same energy range after appropriate recalibration. Therefore, it must be emphasized very clearly that the change to the new operational parameters proposed here represents a much larger step than the previous one.


In any case, significant costs will arise for new development, new procurement, new approval, and recalibration of very many Dosimeters. For Germany alone, we estimate the costs to be more than 20 million Euros. In our view, it is therefore essential to first investigate the feasibility of existing or newly developed dosimeters, estimate the costs appropriately and perform a cost-benefit analysis. The feasibility assessment must be based on the requirements of IEC 62387. We strongly urge you to conduct these analyses and conduct the discussion with all relevant stakeholders before recommending new operational quantities through the ICRU and the ICRP.


2. The impact of such new dosimeters in practice, if they were available

For the estimation of the effective dose with Hp or H* as new operative quantities, two energy ranges are to be considered in particular where significant changes occur. These are the low X-ray energies below 50 keV and the high photon energies above 5 MeV (see Figure 4.2 in the ICRU/ICRP draft).


Without question, there is a need for action in the field of high energies. An underestimation of the effective dose is not acceptable. Also, the use of accelerators in medicine and the ever higher acceleration energies have led to more workers in this area. However, it must also be taken into account that the number of persons subject to radiation monitoring in this area is still very low compared to the vast majority of X-ray monitorings. Unfortunately, there are no current, reliable data on the number of monitorings in these high-energy applications; we estimate the fraction in the low single-digit percentage range. Therefore, we consider transitional arrangements to be useful and feasible for these few workers. Thus, e.g. correction factors based on the current operational quantities might be used, up to specific measurement quantities and measuring devices especially for these applications.


Of far more significance, on the other hand, is the X-ray area, as the vast majority of persons under surveillance are registered here, especially in the field of medicine. Also, the activities in the X-ray fields are very different from those at high photon energies. In the X-ray region, very direct access to the primary radiation field is often possible, and the proportion of significant scattered radiation is also relatively high. The exposure risk is thus higher in this area than, for example, in workspaces with high photon energies. Overestimation by up to a factor of 5 was deliberately accepted in the past and, in our opinion, has also been a positive impetus for practical radiation protection in the sense of the ALARA principle. Even more, one must now deal with possible consequences of a reduction of the measured dose values by a factor of 5. The comparatively high degree of conservatism has led to raised alertness in this field of application, which results in a high degree of radiation protection optimization. Monthly doses of the order of 1 mSv are meanwhile rarely. We therefore recommend not abandoning this conservatism, as it is an essential component of radiation protection when working in X-ray fields. Especially in the X-ray area, the location of the dosimeter is crucial for dose measurement. In practice, when the dosimeter is worn on the chest, as usual, a movement of the person in the radiation field (irradiation from the side or from the back) will quickly “use up” conservatism. This non-conservative behavior is well covered by the current quantities and their overestimation of the effective dose, but in the case of the proposed new quantities, such behavior leads to an underestimation of the exposure in practice in any case. With the introduction of the proposed quantities, the standards achieved would therefore have to be secured by accompanying measures, for example by correspondingly lowering limits for device shielding and controlled area limits. At this point we would like to explicitly warn against a possible deterioration of the existing radiation protection standards, which would also be difficult to convey to the public.


In conclusion, our urgent recommendation to the ICRP is not to publish this draft as a recommendation, as long as key points for practical implementation are not clarified. The draft comprehensively explains how dosimeters can be calibrated. However, it is not really explained how the practical radiation protection dosimetry could be implemented and the essential questions of the reusability of existing dosimeters as well as the extent and effort required for new developments are not clarified. The practical consequences for radiation protection must be discussed with all stakeholders worldwide, and no hasty facts should be created here.

Therefore, our vote on the present draft is a clear NO.