Sievert
SI derived unit · quantity: equivalent and effective dose
01 · Definition
The sievert is the absorbed dose multiplied by factors that account for the type of radiation and the sensitivity of the organs exposed. The dimension is the same as the gray’s, but the sievert answers not «how much energy» but «how dangerous is it».
The average inhabitant of the Earth receives about two and a half millisieverts a year, and most of it comes not from technology but from radon seeping out of the ground into basements and ground floors. Medical diagnostics adds roughly as much again in countries with developed health care. Accidents, nuclear power and weapons tests together give fractions of a per cent — yet it is they that shape how the unit is perceived.
Weighting factors do not follow from equations: the International Commission on Radiological Protection assigns them, generalising data on the survivors of Hiroshima, on uranium miners and on laboratory animals. The commission revises them every ten to twenty years, and the numbers change: neutrons were once given a stepped scale, now a smooth curve with a maximum near one megaelectronvolt. This means that a dose in sieverts computed in 1990 and the same dose under today’s rules are different numbers for one and the same exposure.
02 · Conversion
Enter a dose — the sheet will break it down by unit
The rem held on in Soviet and American documents into the nineties, and one rem equals a hundredth of a sievert — the same proportion as the rad to the gray. Converting from grays requires knowing the type of radiation: for photons and electrons the numbers coincide, for alpha particles they differ twentyfold.
The rem stands for «roentgen equivalent man» and appeared in the forties, when it was already clear that the same ionisation harms in different ways. The sievert took its place in 1979, making the unit a hundred times larger — and that brought the same confusion as with the rad and the gray: a figure from an old protocol read as a modern one understates the dose a hundredfold. In Russian radiation-safety norms the rem was formally permitted into the 2000s, and it still turns up in the documentation of older facilities.
| Unit | Name | Value | Where it is met |
|---|---|---|---|
| Sv | sievert, the SI unit | 0.0024 | accidents, acute doses |
| mSv | millisievert | 2.4 | norms, medicine, yearly doses |
| µSv | microsievert | 2400 | X-rays, flights, background |
| nSv | nanosievert | 2 400 000 | dose rate, sensitive instruments |
| Gy | gray at a factor of 1 | 0.0024 | photons and electrons only |
| years | years of world-average background | 1 | how many years of background it costs |
| rem | roentgen equivalent man, 1940s | 0.24 | USSR and USA until the 1990s |
| mrem | millirem | 240 | personnel dosimetry |
| R | roentgen in air, approximately | 0.27273 | ionisation of air, not of tissue |
| rad | rad at a factor of 1 | 0.24 | absorbed dose, not equivalent |
| Sv | sievert | 0.0024 | the present-day unit |
The line with grays is valid only for X-rays and gamma radiation, where the weighting factor equals one. The flight hours are computed for a cruising level of eleven kilometres at middle latitudes.
03 · Orders of magnitude
from a banana to an emergency shiftTwo and a half millisieverts: the world-average year
04 · Measuring instruments
What sieverts are measured with
A silicon detector in a coat pocket: it counts pulses, accumulates dose and warns with a beep when the rate threshold is passed. It shows not effective dose but an operational quantity — the thing an instrument is actually able to measure.
A polyethylene sphere a quarter of a metre across with a helium counter inside. The shell is chosen so that the instrument’s sensitivity follows the curve of the neutron weighting factor — the device reads out sieverts directly rather than counts.
Thermoluminescent tablets under windows of aluminium, copper and plastic. From the difference in darkening under the filters one recovers the energy and the type of radiation, and hence the factor — without which counts cannot be turned into sieverts.
A chair in a room walled with old steel and lead, where a detector catches the gamma radiation of isotopes that have got inside the body. It counts caesium and potassium in a person directly and converts them into the dose expected over the next fifty years.
05 · Writing rules
The sievert always comes with whose dose it is
The symbol is written with a capital S and a lower-case v. In everyday use the dose is almost always given in millisieverts and microsieverts: a whole sievert means severe injury, and in normal circumstances such a number does not occur. Dose rate is written in microsieverts per hour, and it must never be quoted without the time of exposure.
Effective dose was devised for managing protection, not for judging harm to a particular person: it averages organ sensitivity over a notional human of middle age and of both sexes at once. Applying it after the fact to someone who was exposed is a common mistake; for that one computes the dose to the specific organ. And one more rule: effective doses from different sources may be added together, but comparing them with the reading of a household dosimeter is almost never right.
06 · Neighbouring units
The sievert closes the radiological chain: the becquerel counts decays, the gray the energy that reached the matter, the sievert the expected harm. Each step adds information that the previous unit does not carry.
Tissue factors are distributed unevenly: the red bone marrow, the lungs, the stomach and the breast get twelve hundredths each, the skin and the bones one hundredth. Among the Simetrium sheets the sievert adjoins the gray, the becquerel, the curie, and the joule with the kilogram the Simetrium passports adjoining the sievert are gray, becquerel, curie and joule s the kilogram at the base of the dimension.
07 · Historical section
How energy was learned to be turned into risk
Rolf Sievert headed the physics department of Radiumhemmet and became one of the founders of the international commission on radiation protection. He also built the first instruments for monitoring patient doses and insisted that a limit should be a number, not a doctor’s opinion.
Work with reactors and neutrons showed that the same ionisation harms in different ways. The rem appeared — a dose equivalent in effect to one roentgen — and with it the notion of relative biological effectiveness.
The General Conference on Weights and Measures introduced the sievert four years after the gray, acknowledging that two quantities with the same dimension need different names. Otherwise it was impossible to tell from a document which of them was meant.
The commission replaced the stepped scale for neutrons with a smooth curve, changed the weights of the breast and the gonads, and introduced a separate factor for protons. Every dose computed under the 1990 rules had to be declared incomparable with the new ones.
The only SI unit that cannot be measured
No instrument measures effective dose, and none can in principle: it is defined as a sum over the organs of a living person, weighted by factors that a commission assigns for reasons of protection. To keep dosimetry workable, operational quantities were invented — ambient dose equivalent and personal dose equivalent. They are computed not for a person but for a notional sphere of tissue-equivalent material thirty centimetres across, and those can already be reproduced by a standard and written into an instrument’s certificate. Instruments are calibrated against these quantities, and the commission separately checks that they leave a margin on the safe side relative to effective dose. What results is a three-storey construction: a measurable quantity, a computed quantity, and factors joining them by agreement rather than by an equation. That is exactly why the figure from a household dosimeter and the figure in a yearly-dose report are not the same number, even though the unit on both is identical.
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