Gray
SI derived unit · quantity: absorbed dose of ionising radiation
01 · Definition
One gray is a joule of ionising-radiation energy absorbed by a kilogram of matter. The quantity is purely energetic: it knows nothing about the type of radiation, nor whether it faces living tissue or a slab of lead.
There is astonishingly little energy here. A dose of four grays, lethal for a human, warms the body by about a thousandth of a degree — as much as a second of a light bulb. The danger comes not from the heat but from the way it is delivered: radiation tears molecules apart one at a time, and a broken strand of DNA does not mend itself just because the energy was small. That is why radiologists speak of grays and not of degrees.
A gamma quantum scatters its energy along a long path in sparse ionisations; an alpha particle unloads the same energy over a few micrometres. The gray counts only the total, which is why the same dose leaves entirely different damage — and why the sievert exists alongside it.
02 · Conversion
Enter a dose — the sheet will break it down by unit
The rad held on in medicine into the nineties and still turns up in American protocols. The roentgen measures not the dose in tissue but the ionisation of air, so the conversion is conditional: the coefficient depends on the energy of the radiation and on which tissue is irradiated.
The rad appeared in 1953 as the «radiation absorbed dose» and was set at a hundred ergs per gram — close to the dose delivered by one roentgen in soft tissue. When the gray arrived in 1975 the ratio came out awkward the other way: a hundred rads to the gray. American clinics took almost twenty years to change over, and old case notes carry both entries side by side.
| Unit | Name | Value | Where it is met |
|---|---|---|---|
| Gy | gray, the SI unit | 2 | radiotherapy, dosimetry |
| mGy | milligray | 2000 | diagnostics, tomography |
| µGy | microgray | 2 000 000 | X-rays, natural background |
| kGy | kilogray | 0.002 | sterilisation, food processing |
| J/kg | joule per kilogram | 2 | the definition of the unit |
| Gy·kg | energy over a 70 kg body | 140 | total absorbed energy |
| rad | radiation absorbed dose, 1953 | 200 | USA until the 1990s, old case notes |
| mrad | millirad | 200 000 | personnel dosimetry |
| erg/g | erg per gram | 20 000 | the original wording of the rad |
| R | roentgen in air, approximately | 227.27 | ionisation of air, not of tissue |
| rem | biological equivalent, at a factor of 1 | 200 | the analogue of the sievert in old norms |
| Gy | gray | 2 | the present-day unit |
The gray counts the energy given to matter. Harm to health is already the sievert: the same dimension, but with a correction for the type of radiation.
03 · Orders of magnitude
from a dental X-ray to an industrial irradiatorTwo grays: a single fraction of radiotherapy
04 · Measuring instruments
What absorbed dose is measured with
The only instrument that measures dose directly from the definition: an irradiated block of graphite warms up, and a thermometer catches the rise in microkelvins. The primary standard of national laboratories, needing more protection from draughts than from radiation.
The working instrument of the clinic: a thimble chamber in a water phantom collects the charge created by the radiation and computes the dose from it. Its readings are tied to the calorimeter through a chain of calibrations, and the walls are chosen so that the chamber responds like water.
A crystal of lithium fluoride remembers the dose, holding knocked-out electrons in defects of the lattice. Heated to three hundred degrees it gives them back as light, whose brightness is proportional to the dose. Such tablets are worn on the coat by staff of clinics and nuclear plants.
The gel polymerises wherever the dose arrived, and the irradiated volume is then read out by a scanner: the result is a three-dimensional map. Radiochromic film does the same in a plane, darkening straight under the beam without any developing — that is how complex treatment plans are checked.
05 · Writing rules
Gy is energy, Sv is risk
The symbol is written with a capital G and a lower-case y. A dose in grays always refers to a particular organ or volume: «two thousand grays to the tumour» means nothing without naming the organ. In radiotherapy the dose is written together with the number of fractions.
The sievert and the gray share one dimension, so they can be swapped without any arithmetical error — and that is precisely why the SI brochure allowed both quantities their own names. The rule is simple: energy given to matter is grays; harm to health, weighted by the type of radiation, is sieverts. Dose rate is written as Gy/s or Gy/min.
06 · Neighbouring units
The radiological trio goes in a chain: the becquerel counts decays in the source, the gray the energy that reached the matter, the sievert the expected harm. Each next step needs data that the previous one does not carry.
The weighting factor is one for X-rays, gamma radiation and electrons, twenty for alpha particles, and from five to twenty for neutrons depending on energy. Among the Simetrium sheets the gray adjoins the becquerel, the sievert, the joule and the kilogram it is built from, and the curie the Simetrium passports adjoining the gray are becquerel, sievert, joule and kilogram, from which it is built, and curie with the roentgen in the historical section.
07 · Historical section
How we learned to measure what cannot be seen
The first measure of exposure was the dose that produced visible reddening of the skin. The doctor irradiated the patient until erythema appeared, and that counted as a unit. The threshold depended on the person, on the energy of the tube, and on how long the doctor watched.
The first physical unit: the amount of ionisation produced by radiation in a cubic centimetre of air. Objective and reproducible — but measuring the air rather than the patient, and working only for X-rays and gamma radiation.
The commission on radiological units introduced the rad — a hundred ergs per gram. The quantity at last described what happens in tissue, and it was chosen close to the dose from one roentgen so that doctors would not have to relearn their numbers.
The General Conference named the unit after the British radiobiologist who worked on the action of radiation on tissue and on neutron therapy. The gray is the same rad, multiplied by a hundred up to a whole joule per kilogram.
The unit almost nobody measures directly
The definition of the gray calls for measuring a rise in temperature, but that rise is vanishingly small: a therapeutic two grays warm tissue by half a thousandth of a degree. The calorimeter manages only because it sits inside a vacuum jacket and catches microkelvins, and the whole chain of clinical dosimetry is tied back to that one instrument.
Catalogue · units of measurement
A passport for every quantity
Seven SI base units, twenty-two derived ones with names of their own, and the non-SI units that neither engineering nor daily life can do without. Each gets its own sheet: definition, conversion, instruments, writing rules. In 14 languages.