Answer to Question #8943 Submitted to "Ask the Experts"

w_R (National Council on Radiation Protection and Measurements [NCRP] Report No. 116) and Q (10 CFR 20 [U.S. Nuclear Regulatory Commission]) are meant to give us different radiobiological damage per absorbed dose for different types of radiation, so they have basically the same purpose. Then why are there such large differences in some of the numbers: w_R for neutrons ranges from 5 to 20 in NCRP 116, but Q in 10 CFR 20 ranges from 2 to 11 and w_R for protons is 2 in NCRP 116, but Q in 10 CFR 20 is 10? Is there a practical, real life situation where we would need to use w_R? Secondly, when a neutron rem meter is calibrated, does either w_R or Q somehow play a role in its calibration?

The reason they differ is that they apply to the radiation field in different ways. w_R applies to the incident radiation, before it enters the medium, such as the body, whereas Q applies at the point at which the dose is evaluated. This means that if you have a beam of 5 MeV neutrons incident on the body, you apply w_R for 5 MeV neutrons. However, you have to calculate the spectrum of secondary charged particles that are generated within the medium at the point of interest by the neutron spectrum, then calculate the LET (linear energy transfer) for each of these particles as they travel through the medium, and from that use the International Commission on Radiation Units & Measurements (ICRU) relationship to determine the Q value.

The idea is that it is easy for the health physicist to apply w_R to estimate the protection quantity, usually the equivalent dose to an organ, but it is very difficult to calculate the dose equivalent in the ICRU sphere, which is used to calibrate detectors such as area monitors. The net result of all of this is that the reading of the area monitor, in terms of say the ambient dose equivalent (calculated using Q), will give a conservative estimate of the protection quantity, say the effective dose. As for the rem meter, the gold standard is the rem ball, which has an energy response that parallels closely the neutron dose equivalent function. This is just a happy coincidence, but it is used in the design of these instruments. The exact shape of the response function of these instruments can be modified by slightly changing the diameter of the sphere and sometimes also by adding a very thin absorber layer within the sphere.