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Operational Quantities for External Radiation Exposure


Draft document: Operational Quantities for External Radiation Exposure
Submitted by TADA, Jun Ichiro, N. P. O. Radiation Safety Forum (RSF)
Commenting on behalf of the organisation

I’d like to appreciate the endeavour of the report committee to simplify and rationalise the system of radiation dose for protection purposes. Although the change has been recognised necessary for more than a decade, the committee might have overcome many obstacles, since no matter how irrational it is not easy to break down the established custom. Therefore this is indeed a good work.

The followings are my suggestions and comments that would make the report more comprehensible.

 General:

  • The proposed operational quantities, ambient and personal dose, are nothing but the effective doses at specified exposure geometries. Therefore, it is not adequate to call them by special quantity names. It is sufficient to specify them simply as the maximum effective dose, EMax, (for environmental monitoring) and the effective dose of unidirectional incidence, E(Ω), (for individual monitoring). We should face up to the reality that the introduction of the sphere based operational quantities, which are mare common procedure for approximating the effective dose (equivalent) at all, have led misunderstanding on the system of protection dose for more than 30 years.
  • Various graphs relating to conversion coefficients are shown in the draft without explaining their characteristic feature. It is important to show the physical reason of their peaks, valleys, changes in gradient and so on to foster understanding the nature of conversion coefficients. For this purpose, it is necessary to show the graphs of the conversion coefficients themselves instead of the ratio of new and old ones.

    •  

    Line 300

    The statement “… effective dose is considered to provide a single risk-related quantity for stochastic radiation effects, valid for all adult persons exposed under the same conditions…” is wrong. The effective dose is age-independent concept used for the management (dose limitation and optimisation) of radiation protection, and is valid for all ages.

    Lines 313~314

    Although it is stated that “To assess a more precise value and its uncertainty as doses approach the constraints or investigation levels.”, the nominal increase in cancer induction below 100 mSv is at most 0.5%, which does not need “precise dose assessment” at all. A tolerance of ±3dB in dose estimation and measurement would be reasonable for doses that cannot cause tissue reactions.

    Lines 371~374

    It would be desirable to show figures of each exposure geometry.

    Line 474

    Average of 100 cm2 of skin dose had been used for long years. Although it is stated in the margin of Table 6 in Publication 103 that skin dose is averaged over 1cm2, there was no explanation on the change.

    Line 518

    Although the stylised eye phantom seems to be well designed, are the results simulated with this phantom valid for the people wearing eyeglass or contact lens?

    Lines 550~553, and 611~627

    The explanation of the 2mm thick skin layer should be stated in the beginning. The reader will understand before seeing the statement that the average absorbed dose should be taken in the region between the depths 50 and 100μm in the slab of density 1 g /cm3, and in the pillar or rod of density 1.11 g /cm3. It is also desirable to show the sketches of each phantom.

    Lines 580~583, and 660~661

    The statement “For a given Ω, the maximum value of the effective dose is taken for radiation incident from left or right” is not comprehensible, since none can assign left or right for an arbitral direction Ω. If the statement assumes the situation where radiation horizontally incident on the standing reference person as shown in the figure of Appendix A (lines 386~395), and if the commission does not have plans to provide conversion coefficients for radiation incidence with elevation or depression angle, using directional angle α instead of Ω is preferable. It is also desirable to show the figure here.

    Line 592

    The expression “at a point on the head or body” makes sense only for homogeneous whole-body irradiation, however, such situation seldom holds for β-ray exposure. Although Hp(3) is often monitored by dosemeter worn on the body in the practical dose monitoring of occupational radiation workers, is it a recommendable habit?.

    Lines 647~648

    Although what the draft wants to say can be imagined, the expression “the direction of incidence between the direction of the radiation field, Ω, and the direction, Ω0, that is antero-posterior to the phantom, is α” does not seem to be polished.

    Lines 691~693

    It is not reasonable to define such a detailed interpolation procedure in assessing doses below 100 mSv.

    Table A.1.3, Figure A.1.3, Table A.2.3 and Figure A.2.3

    The CSDA range of electrons in water is shorter than 50μm below kinetic energy 55 keV (ICRU Report 37), while non-zero values of conversion coefficients of the ambient and personal dose are shown. The reason could be understood as the limitation of the voxel phantom. It would be reasonable to eliminate data corresponding to kinetic energies that electrons cannot reach the basal cell layer of skin. Similar changes are also necessary for other charged particles.

     

    Trivial matter

    Line 498

    “protons” may be missing

    Lines 498, 652, 657 and so on

    “He ions” should be specified as “He2+ ions.”

    Line 544

    The suffix “i” of the particle fluence is not necessary.

    Line 581

    A suffix “i” of the effective dose of particle energy Ep and incident direction Ω is necessary.

    Lines 585 and 586

    An exponent “2” is missing in the differential of particle fluence.
















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