Roentgen
Non-SI unit · quantity: exposure to X-rays and gamma rays
01 · Definition
The roentgen is the amount of X- or gamma radiation that produces, in one kilogram of dry air, a charge of 2.58·10⁻⁴ coulomb of either sign. The unit describes neither a body nor a substance, but the ability of the radiation to ionise air.
Air was not chosen by accident: in 1928 the ionisation of a gas could be measured, whereas the energy deposited in tissue could not. The charge is collected on electrodes and counted exactly, so the quantity was reproducible from the start. The price is a narrow range of validity: the roentgen is defined only for X- and gamma radiation, only in air, and only up to energies of about three megaelectronvolts. For alpha and beta particles it has no meaning at all, and to obtain the dose in a person the reading has to be multiplied by a conversion factor: one roentgen gives 8.76 milligray in air and about 9.6 in soft tissue.
Soft tissue absorbs X-rays a little more readily than air, so at the same exposure it receives roughly ten percent more energy. For bone the gap reaches a factor of four at low energies — there calcium with its high atomic number does the work. Hence the rule of dosimetry: an exposure measured in air becomes an organ dose only through a calculation that accounts for the material, the spectrum of the radiation and the geometry of the irradiation. It was precisely this extra step that eventually forced the move from the roentgen to the gray, which measures the deposited energy directly in the substance at issue.
02 · Conversion
Enter a dose — the sheet will convert it for air and for tissue
The link to the coulomb per kilogram is exact by definition. Everything else is a transition from air to a material, and that depends on which material is meant.
The roentgen has been formally withdrawn: in Russia it was removed from the permitted units back in the eighties, with a transition period, while the international system replaced it with the coulomb per kilogram, which never caught on — nobody uses it. Yet the microroentgen per hour survived both reforms and appears on the scale of almost every consumer dosimeter, because all the Soviet standards and all the familiar reference points are written in these units. The one thing to remember when reading such an instrument: a hundred microroentgens per hour is roughly one microsievert per hour.
| Unit | Name | Value | Where it occurs |
|---|---|---|---|
| R | roentgen, exposure | 1 | instruments and old standards |
| C/kg | coulomb per kilogram, the SI unit | 0 | the definition, but not the practice |
| µC/kg | microcoulomb per kilogram | 258 | a convenient way to write it |
| Gy in air | absorbed dose in air | 0.009 | instrument calibration |
| mGy in tissue | absorbed dose in soft tissue | 9.6 | medicine and radiation protection |
| mSv | equivalent dose for gamma rays | 9.6 | present-day regulation |
The tissue dose is computed with a conversion factor of 0.0096 gray per roentgen, valid for soft tissue and X-ray tube energies. The equivalent dose equals the absorbed one, because for X- and gamma radiation the weighting factor is one
03 · Orders of magnitude
dose rate: from background to a wrecked reactorTen microroentgens per hour: natural background
04 · Measuring instruments
What roentgens are measured with
The primary standard: the beam passes between the plates while the collecting electrode outlines a volume of air known from the drawing to within micrometres. The instrument needs no calibration against another instrument — it reproduces the definition of the unit literally, which is why it stands in national metrology institutes.
The working instrument of radiotherapy: a pea-sized cavity inside a wall of air-equivalent material. It is calibrated against the standard chamber and inserted straight into a phantom, to measure the dose at the point where the tumour will be.
A portable civil-defence instrument with a scale up to two hundred roentgens per hour and a remote probe. Instruments of exactly this kind were on hand at the Chernobyl plant in the first hours of the accident: their limit of 3.6 roentgens per hour meant not safety but merely that the needle had hit the end of the scale.
A Geiger counter counts pulses but reports them in microroentgens per hour, because that is what the buyer is used to. The instrument is calibrated with caesium-137, and at other photon energies its readings drift by whole factors — it will understate the background in a radon-filled cellar and overstate it beside an X-ray machine.
05 · Writing rules
A capital letter, and no inflection
The symbol is a capital R with no full stop. The unit is named after a person, so the name is written in lower case in running text while the symbol keeps the capital: one hundred roentgens, symbol 100 R.
The middle line confuses a dose with its rate: the microroentgen is accumulated exposure, while the instrument shows microroentgens per hour, that is the speed at which it accumulates. The bottom equality holds only approximately and only for soft tissue and gamma rays: one roentgen gives about 0.96 rad there, so approximately — but not exactly — one rem. From this follows the main limitation of the unit, worth keeping in mind when reading old documents: exposure describes the radiation field at a point, not what the person standing there received, and without knowing their position, shielding and time of stay one cannot be converted into the other.
06 · Neighbouring units
The roentgen is the first step in a chain of four quantities: the activity of the source, the ionisation of air, the absorbed energy and the expected harm. Each further step calls for a new calculation.
the same quantity in SI
ionisation of air
energy in matter
Of the Simetrium passports nearby stand rad and rem — the next two steps of the same chain, gray and sievert, which replaced them, coulomb from the modern definition, and becquerel s curie for the activity of the source.
07 · Historical part
Dose was measured by burns and by the colour of a tablet
The first dosimeter was a tablet of barium platinocyanide that changed colour from green to orange. The physician placed it beside the patient and stopped the session when the shade matched the sample on a card. The accuracy depended on the lighting in the room and on the physician’s eye.
Another measure was the erythema dose — the one that produced a lasting reddening of the skin. It depended on the person, on the part of the body and on the length of the exposure, differing twofold between patients and fivefold between clinics.
The Second International Congress of Radiology adopted the roentgen: the charge produced in one cubic centimetre of dry air under normal conditions. The quantity could now be measured with an instrument and reproduced in any laboratory in the world.
The definition was restated in coulombs and kilograms, and then the system of units moved to the gray, which measures energy directly in matter. The roentgen fell out of use, surviving only on instrument scales and in accounts of old accidents, where every figure is written in it.
The only quantity defined through an unrelated substance
The roentgen is a strange thing to a metrologist: it describes radiation through the response of air, which has nothing to do with the case. The reason is historical — in 1928 the ionisation of a gas could be measured to within a fraction of a percent, while the energy deposited in tissue could not be measured at all. The price of that precision was twofold. First, the unit applies only to X- and gamma radiation and only up to energies of about three megaelectronvolts: above that the secondary electrons escape the measured volume and the equilibrium on which the definition rests breaks down. Second, between exposure and the dose in a person stands a calculation, and it depends on the spectrum of the radiation and on which organ is considered: for soft tissue the conversion gives a factor of about 1.1, for bone at low energies up to four. It was this extra step that decided the fate of the unit: the gray and the sievert measure what the physician cares about directly. Yet the roentgen keeps an advantage its successors lack — the free-air chamber reproduces its definition literally, out of geometry and charge, which is why national dosimetry standards are still built around that design and only then converted into grays.
Catalogue · units of measurement
A sheet for every quantity
Seven SI base units, twenty-two derived ones with special names, and those non-SI units that neither engineering nor daily life can do without. Each has its own sheet: definition, conversion, instruments, writing rules. In 36 languages.
Non-SI units
the roentgen sheet is openSI base units
in ochre — those the coulomb per kilogram is built fromThe four steps of radiation quantities
source · air · matter · organismPassport language
36 languages. The symbol R is internationally recognised; what is translated is the names of instruments and quantities and the explanations of the scales.