What is the difference between sievert, roentgen, and gray? Why is radiation measured with different units? What other units are used to measure radiation?
In: Physics
Roentgen measures exposure, the strength of radiation at a certain point.
Gray measures the absorbed dose, the amount taken by an object.
Sievert measures the equivalent dose, the biological effects caused by radiation.
Different names are used to distinguish the 3 radiation quantities exposure, absorbed dose and equivalent dose.
Roentgen (or “R”) is a really old unit that is more “what could you measure 100 years ago” than it is “what do you WANT to measure”. Roentgen is technically a measure of how much free electric charges are created in a small volume of air at room temp, pressure, and (I think) zero humidity. It’s easy to measure that using a voltage source and current meter connected to a couple of metal plates, so Tesla or Edison could probably build those.
What you WANT to measure is typically either “how much energy is radiation dumping into this object” (measured in rad or gray) or “how bad is this for health” (rem or seivert). Those are harder to measure, especially the health one. The energy one (rad, gray) is good for questions like “when will the radiation make the steel reactor vessel get brittle?”
So for a very long time, there were instruments that could measure Roentgen, and the radiation level in rad or rem (literally Roentgen Equivalent in Man) were calculated using the Roentgen measurement and a fudge factor.
It’s no coincidence that in the old units of R, rad, rem, the most common radiation type you’re likely to run into has 1R =1 rad =1 rem, because that’s kind of how they got defined.
Later instruments could read rad or gray in a “standard” sensor material like sodium iodide or polyethylene plastic directly. Particularly for plastic, that makes the range of situations where 1rad=1rem broader.
I used to use devices that read in Roentgen or sometimes rad, and had a chart on the side that helped convert to rem if you knew exactly what kind of radiation you had.
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