Adose rate is quantity of radiation absorbed or delivered per unit time. It is often indicated in micrograys per hour (μGy/h)[1] or as an equivalent dose rate ḢT in rems per hour (rem/hr) or sieverts per hour (Sv/h).[2][3]
Dose and dose rate are used to measure different quantities[1] in the same way that distance and speed are used to measure different quantities. When considering stochastic radiation effects, only the total dose is relevant; each incremental unit of dose increases the probability that the stochastic effect happens.[4] When considering deterministic effects, the dose rate also matters. The total dose can be above the threshold for a deterministic effect, but if the dose is spread out over a long period of time, the effect is not observed. Consider the sunburn, a deterministic effect:[4] when exposed to bright sunlight for only ten minutes[5] at a high UV Index, that is to say a high average dose rate,[6] the skin can turn red and painful. The same total amount of energy from indirect sunlight spread out over several years - a low average dose rate - would not cause a sunburn at all, although it may still cause skin cancer.
On a Montreal-Fredericton flight in October, 1976, we measured the dose rate with a sensitive radiation monitor. The results are given in Figure 3.1. As you can see, the ground level dose rate was about 0.10 μGy/h; whereas, at the maximum flight altitude (8.8 krn or 29,000 ft), it was about 2.0 μGy/h. The total excess dose for the Montreal to Frederickton flight is only 0.7 μGy.
ICRP 60 uses the term "deterministic" to replace "non-stochastic". We'll stick with what we're used to from ICRP 26. Let's digress for a moment to give you a couple of everyday examples of non-stochastic and stochastic effects. Sunburn has a threshold; above this threshold exposure, the degree of sunburn becomes more and more severe with increasing exposure to the sun, and below the threshold no harm is done. Compare this with winning a million bucks in a lottery; this is pure chance - the probability depends on the exposure (the number of tickets you buy), but the magnitude of the effect doesn't change. You either win a megabuck or you don't.
A radiative transfer model determines the flux of UV radiation for a range of wavelengths. An action spectrum weights the response of the human skin to UV radiation at each wavelength. Once weighted, the flux values are integrated over the entire range, resulting in an erythemal dose rate — the instantaneous amount of skin-damaging radiation reaching the surface
