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Submitted by Dr A K Bakshi, Radiological Physics & Advisory Division, Bhabha Atomic Research Centre
   Commenting on behalf of the organisation
Document Operational Quantities for External Radiation Exposure
 

The changes proposed  are welcome since the new approach is much simpler. However the following points may be considered:


The value of effective dose given in ICRP-74 using the anthropomorphic phantom do not seem to be different from the value of effective dose given in ICRP-116 using the voxel phantom for photon energies between 200 keV to 1 MeV. However, the value of Hp(10) was chosen such that there would be an over-estimation of effective dose even in these energy ranges. Now, there is a sudden shift and it is required to measure effective dose  without any over-estimation.  Though the concept of personal dose is easier to understand and seems more appropriate, there should be more explanation for the reason for the shift, especially since it seems to mean a reduction in individual doses for a large majority.


The fact that Hp(10) overestimated photon doses for < 70 keV was well-known. However, the fact that the RBE for lower energy photons is higher has also been highlighted  and the overestimation of Hp(10)  was justified with (ICRP-103): "In this case of external exposure, however, the operational dose quantities H*(10) and Hp(10) are used for radiation protection monitoring and for assessing effective dose. For photons with energies between 10 keV and 40 keV and frontal irradiation (AP) of the body, H*(10), is up to a factor 6 higher than E and, for other directions of radiation incidence (PA, LAT, ROT, ISO), this conservatism is even greater" . It is not clear how the use of wR of 1 can be justified for lower RBE in the present case.


2 sets of conversion coefficients for photons (from air kerma) are given, one  with kerma- approximation and the 2nd set with (possibly)  full  transport calculations. As expected, these values are different for energies above 1 MeV and below 70 keV. Kerma approximation may be more suitable in practical situations which involve broad spectra and wide angular distributions and electron scattered from the air and surroundings (ICRU 43). This should be explained clearly.

The conversion coefficient (from air kerma) for 600 keV for personal dose is 1.02 while for eye lens, it is 1.17. For 1 MeV also the same level of difference is present. Though there is a difference in phantom for both, with this difference in conversion coefficients, it is most likely that for workers exposed to uniform fields of high energy photons, the dose to eye lens will exceed the personal dose. With the present dose limits for both being the same, this may create a situation where monitoring for eye lens becomes essential for all radiation workers.

The calibration of dosimeters continues to be under conditions of electronic equilibrium. However, there is no discussion on the validity of such conditions. It appears that this is being done just because the practice is already in place.


Table A.3.2  of Conversion coefficient from neutrons Fluence to Directional and Personal absorbed dose to lens of eye and the corresponding figures are not provided


Table A.4.2.1 of Conversion coefficient  from neutron Fluence to Directional and personal absorbed dose in local skin on the slab phantom  and the corresponding figures  are not provided


Line No. 495, For operational quantity The ambient dose, H*, the conversion coefficient h*Emax,i(Ep) used for ith particle of energy Ep and corresponding maximum value of effective dose should be calculated separately for adult male and adult female reference phantom and average of the two may be recommended for use to calculate operational quantity. This approach can be similar to the method by which wT values were estimated by averaging over male and female population.


 


Line No. 582, For operational quantity, The personal dose, Hp, the conversion coefficient hp,i(Ep,Ω) = used for ith particle of energy Ep incident in direction Ω, should be calculated separately for adult male and adult female reference phantom and average of the two may be recommended for use to calculate operational quantity. This approach can be similar to the method by which wT values were estimated by averaging over male and female population.