Abstract
The use of a definition of an operational quantity based on a value of a radiometric or dosimetric quantity at a point and a conversion coefficient to a protection quantity has been investigated previously. This approach is now considered acceptable as the International Commission on Radiological Protection (ICRP) has defined effective dose in reference computational phantoms that have published conversion coefficients from particle fluence to this quantity.
Effective dose is a universal risk-related quantity for control and optimization of exposures. It is applicable to both external and internal exposure and to all types of ionizing radiations. The drawback is that effective dose cannot be measured as it is defined as a weighted average over radiation types and organs and tissues occupying the volume of the human body. Further protection quantities are introduced for the assessment of dose to specific organs, the lens of the eye, extremities and local skin, and possible targets of deterministic effects of ionizing radiation.
The operational quantities are measurable quantities for the determination of ionizing radiation defined at a point in space. The recommendations in this Report on operational quantities are defined in terms of conversion coefficients to personal dose, personal absorbed dose to the lens of the eye, and personal absorbed dose to local skin; ambient dose, directional absorbed dose in the lens of the eye, and directional absorbed dose in local skin; that are related directly to the values of the protection quantities, and effective dose and absorbed dose in the lens of the eye and local skin. This is a significant change from the ICRU Report 39/51, where numerical values of the operational quantities are based on dose equivalent at a fixed depth in simple phantoms or the ICRU 4-element sphere, and will give a better estimate of the protection quantities. The system of protection and operational quantities is simplified and assists in the comprehension and consistency of radiation protection quantities by users.
The numerical values of conversion coefficients for ambient dose and personal dose for photons of energies from 70 keV to 2 MeV are within about 15 % of those given for the corresponding quantities defined in ICRU Report 51. The values of these quantities from 3 to 10 MeV, in conditions with full charged-particle equilibrium, are also within 18 % of those calculated with the kerma-approximation method in ICRU Report 57 (1998) and ICRP Publication 74 (1996). The only changes required for these photon energy ranges for the calibration of area monitors and personal dosimeters are changes to the instrument constants (ICRU Report 76, 2006). Conversion coefficients are also presented that have been calculated using the kerma-approximation for the calibration of photon area monitoring instruments and personal dosimeters that are routinely used under conditions of charged-particle equilibrium.
Increased accuracy in the assessment of absorbed dose to the lens of the eye can be of particular concern in nuclear medicine using beta radiation.
For neutrons, the recommended quantities for ambient dose and personal dose are a considerable improvement on the ICRU Report 39/51 quantity values for the determination of the protection quantities. For area monitoring instruments based on the moderation of the neutron fluence, no changes are required initially for workplace fields within the currently accepted degree of accuracy.
The recommended operational quantities will provide a solution to the protection problems in radiation fields of high-energy photons, neutrons, and electrons and for other particle types.
Modifications in the design or algorithm of existing instrumentation will be required at some stage, at least for some applications:
for photon energies below 70 keV, to correct for the large overestimation of effective dose by area monitoring instruments and personal dosimeters based on the ICRU Report 39/51 definitions;
to improve the estimation of photons and electrons of absorbed dose in the lens of the eye;
to determine ambient and personal dose for photons with energies between 3 MeV and 10 MeV if charged-particle equilibrium is not present, but this will depend on the detector system;
to improve the performance of area monitoring instruments and personal dosimeters for the changes in the conversion coefficients from particle fluence to H* and Hp compared with H*(10) and Hp(10), for the lower values for neutron energies from thermal to 2 MeV and for the higher values above neutron energies of 50 MeV.