Answer to Question #13223 Submitted to "Ask the Experts"

I am studying for the Department of Energy (DOE) "core card" exam. In the section for radiation sickness (prodromal, latent, Illness, recovery, or death), the units for exposure and effect are in grays. A gray of fast neutron is worse than a gray of gamma. Why don't they use sieverts? Sieverts applies the radiation weighting factor (previously known as quality factor).

This question arises quite commonly, often out of confusion, regarding radiation effects and appropriate units to describe the effects. I shall attempt to clarify the rationale used for the instance you describe.

The sievert unit applies only to the specification of equivalent dose to a tissue and effective dose to the whole body. It was devised specifically as an aid to establish appropriate radiation protection standards. The risk factors involved are largely based on stochastic effects for which the probability of observing a particular effect, such as cancer or genetic effects, is proportional to the dose; the dose in such situations is modified from absorbed dose to account for the differences among radiation types in their ability to produce particular biological damage. Thus, high LET radiations such as fast neutrons (actually, the LET in this case is primarily the result of the fast protons set free by neutron collisions with hydrogen in the body) produce a great deal more ionization per unit pathlength traversed than do low LET radiations such as gamma rays (or electrons, which are the major causes of ionization from gamma ray interactions). This difference in ionization density results in differences in the degree of potential biological impact per unit of absorbed dose for the two types of radiation (e.g., the number of double strand DNA breaks per unit dose); the absorbed dose is therefore multiplied by a radiation weighting factor whose magnitude is associated with the LET differences—so a fast neutron might have a radiation weighting factor of 20 compared to 1 for gamma rays. This construct is used only to reflect the potential risk of developing disease or other possible stochastic effects in order to make judgments about the health significance of radiation exposures and to influence our decisions regarding radiation protection requirements. It is a major component of our radiation protection philosophy and methodologies used to control exposures to occupational workers and the public.

Quite different from these considerations is the notion of acute radiation effects typically associated with relatively large doses of radiation, generally considerably larger than those experienced among occupational radiation workers doing their usual jobs. The immediate concerns in such cases are not with long-term effects, such as cancer, but with health impacts that have short-term consequences—e.g., skin erythema, changes in blood cell populations, gastrointestinal tract effects, and others. Such effects, referred to as deterministic effects, are of a nonstochastic nature, with initiation typically being associated with respective threshold absorbed doses. In these cases, it is the absorbed dose that is of concern, since it is the amount of energy deposited per unit mass that determines the extent of immediate impact, the severity of the effect being associated with the magnitude of the absorbed dose. For stochastic effects mentioned above, the probability of observing a specific effect varies with the magnitude of the dose, but the magnitude of the specific effect does not—e.g., leukemia is leukemia, regardless of whether it was induced by one 0.1 Sv of effective dose or 1 Sv of effective dose, but blood cell depletion will be much more severe at a dose of 5 Gy compared to 1 Gy. Thus, we should evaluate the absorbed dose to judge the significance of a large acute radiation exposure (on the order of 0.1 Gy or more) in terms of possible short-term impact. Beyond that there may be an additional interest in assessing the dose from a radiation protection viewpoint associated with possible long-term stochastic effects. In such instances the radiation weighting factors would then be used to convert absorbed doses to equivalent doses to tissues and probably, ultimately, effective dose.

I hope this provides some clarification for you. Best wishes as you continue your studies with the DOE.

George Chabot, PhD