The statement below was submitted to ICRU directly in August 2016.
The technical Annex is the outcome of an ongoing discussion and was not submitted previously.
Statement of the PTB
on the ICRU’s proposed redefinition
of the operational quantities for external irradiation
We refer to the talk of Akira Endo (The Operational Quantities and New Approach by ICRU. 3rd International Symposium on the System of Radiological Protection, Seoul, October 2015). PTB appreciates the opportunity for an open discussion. In a thorough internal discussion PTB balanced the methodological approach of the proposal with practical experiences, and industrial and societal interests.
The academic improvement of the new concept for the redefinition of the operational quantities for external irradiation is obvious. However, some practical aspects lead to the following topics for discussion:
- New dosemeter developments would be necessary although the current practice of radiation protection is satisfying: Several radiation protection dosemeters would no longer have a suitable response in terms of the redefined quantities. This would lead to the need of expensive and time consuming developments of instruments while it is not seen that the practice of radiation protection would be improved. It has to be taken into account that the costs for radiation protection are often only a small part of the costs for general occupational safety (for example general safety precautions in mines compared to the radon issue of the mines). Replacement of dosemeters should only be performed if the safety can be improved significantly.
- The development of these new dosemeters could become impossible for physical reasons: The reason is that the redefinition would be based on a “mathematical construct” like the maximum effective dose, Emax, (with the maximum build over different angles of incidence) rather on a physical formulation of the definition. The current quantities are defined physically: the dose in the ICRU sphere made of ICRU 4‑element tissue at a certain depth or the dose in a person at a certain depth. For the calibration of personal dosemeters conversion coefficients are calculated in phantoms made of ICRU 4‑element tissue while (water filled) calibration phantoms of the same size are used during the calibration. Thus, in principle it is possible to build a dosemeter that represents the current definition (apart from the 4‑element tissue) but this could become impossible with the proposed redefinition in the future. If this is only due to a gap in the industrial research and development or indeed a principle problem should be addressed in a research work before the redefinition.
- The rationales for the redefinition are based on calculations only: The assumption that at high photon and neutron energies charged particle equilibrium is not achieved might be wrong for realistic workplace fields as radiation protection measurements are always performed behind protective shielding and not in vacuum. Reliable measurements concerning the amount of secondary charged particles should be undertaken before making such a far reaching change of the definitions.
- The redefined quantities could lose their conservativeness in the future: A future change of the values of protection quantities (e.g. due to a change of wR or wT) could result in the fact that the redefined operational quantities would not be conservative any more. Especially the radiobiological effects, like bystander effect, are actually under research. Despite that, the variation of the individual radio-sensitivity is still unclear.
- Redefined quantities could however solve existing inconsistencies
- In case of neutron radiation, H*(10) under- and overestimates the maximum effective dose, Emax, in some energy regions by more than a factor of two. These neutron energy regions are of relevance in practical fields. Overestimates of about a factor of two can be found for thermal neutrons (relevant for FRM-II) and for neutrons with energies of a few hundred keV (relevant for power reactors, casks with spent fuel). Underestimates of about a factor of two can be found in the energy region from 100 MeV to 200 MeV (relevant at high energy accelerators). Especially the underestimation of effective dose can be an issue for upcoming hadron therapy facilities and can be considered a safety issue which may need improvement.
- In neutron fields with unknown energy distribution of the neutrons, dosimetry is often performed with spectrometry using Bonner Spheres and applying energy dependent conversion factors. As mentioned in the previous point, the calculation of H*(10) from measured spectral distributions can lead to underestimations of the effective dose. The proposed new definition of the operational quantities would be more adequate for this issue.
- In case of external dosimetry for aircraft staff calculated estimates of “effective dose” are stored in the dose registers already now. For comparison with measured values, both “effective dose” and H*(10) need to be calculated. The proposed new definition can avoid the use of two quantities.
As outline above, PTB acknowledges the academic improvement of the new concept for the redefinition of the operational quantities for external irradiation.
Nevertheless we cannot see practical improvements in radiation protection, which could counterbalance the costs of implementing new quantities. At this time we are convinced, that this approach does not lead to a significant societal benefit, therefore the PTB does not support the proposed redefinition of the operational quantities for external irradiation.
Physikalisch-Technische Bundesanstalt
Division 6 Ionizing radiation
Dr. Jörn Stenger
2017-11-01
Technical Annex
Determination of the reference value for Hp in complex and workplace neutron fields:
We would like to point out two problems related to the determination of reference values for Hp in neutron fields with complex energy and angular distribution, e.g., workplace fields used for calibration/testing of personal dosimeters under realistic conditions.
a) The main problem is that it is not possible to calculate Hp for a spatially distributed neutron field in which the neutrons come from different angles and have different fluences and spectra (even if we had a perfect detector that gave us complete angular and spectral information or if the full information about the neutron field was available from e.g. a Monte Carlo simulation). The conversion coefficients tabulated in the draft are determined as maximum values of E from the left or right for angles from 0° to 90° in steps of 15°. But no guidance on what to do for other angles (say for neutrons incident from the top at an angle of 45°) or for neutrons coming from behind (between 90° and 180°) is given. Simply carrying out new calculations or interpolations (as has been done in previous cases by e.g. Siebert [1] following the procedure described in [2] for angles > 90° for the quantity Hp(10) ) will not work, because of the rule of taking the maximum value – how do we do it?
b) Another problem is that taking the maximum value will lead to an inconsistency if the Hp of an isotropic field is calculated in two different ways, either by using the ISO coefficients, or by adding all the partial doses corresponding to individual angles. The problem is that the doses for left and right (say at 15°) are typically different because of the location of organs in the body, but for the angular fluence-to-dose conversion coefficients always the maximum is taken while for the isotropic field a realistic calculation (which will average the left/right or top/down asymmetry of the human body) is carried out.
[1] F. d'Errico, V. Giusti, B.R.L. Siebert. A new neutron monitor and extended conversion coefficients for Hp(10), Radiat. Prot. Dosim. 125 (1), 345–348 (2007).
[2] B.R.L. Siebert, H. Schuhmacher. Quality factors, ambient and personal dose equivalent for neutrons, based on the new ICRU stopping power data for protons and alpha particles. Radiat. Prot. Dosim. 58 (3), 177–183 (1995).