Becquerel
SI derived unit · quantity: activity of a radionuclide
01 · Definition
Becquerel left a uranium salt in a drawer with photographic plates and a few days later found them fogged: the substance radiated by itself, without any external source. The unit is named after that accident of 1896, and the name was given to the SI only in 1975.
One becquerel is one decay per second. The unit is extremely small: a human body runs at about seven thousand decays a second, mostly from potassium-40 and carbon-14, and that is the norm, not a hazard. Because of this smallness the prefixes are in constant use: kilo-, mega-, giga- and tera-becquerels fill medicine and industry, while plain becquerels are left to food norms and to background measurements.
The dimension is the same — inverse second — but the meaning differs. The hertz describes regular cycles, the becquerel random events: a hundred decays per second does not mean one every hundredth of a second, only that on average there are a hundred. Precisely to keep the two apart the SI gave the same dimension two names.
02 · Conversion
Enter an activity — the sheet will convert it
The curie survives in medicine and industry to this day, although it was removed from the SI in 1975. The rutherford, the mache and the eman are entirely historical: they were tied either to a gram of radium or to the concentration of radon in water, and they disappeared along with the practices that needed them.
The curie was tied to a gram of radium-226, and its value was revised three times. In 1953 the number was fixed at exactly 3.7·10¹⁰ decays per second — a round figure rather than a measured one, so that laboratories would stop arguing about the third digit.
| Unit | Name | Value | Where it is met |
|---|---|---|---|
| Bq | becquerel, the SI unit | 3.7·10⁸ | one decay per second |
| kBq | kilobecquerel | 370 000 | household sources, food |
| MBq | megabecquerel | 370 | nuclear medicine |
| GBq | gigabecquerel | 0.37 | therapy, industrial sources |
| TBq | terabecquerel | 3.7·10⁻⁴ | flaw detection, sterilisation |
| decays/min | decays per minute | 2.22·10¹⁰ | the scale of laboratory counters |
| Ci | curie — the activity of a gram of radium | 0.01 | until 1975, still used in the USA |
| mCi | millicurie | 10 | medical doses in older records |
| µCi | microcurie | 10 000 | laboratory preparations |
| Rd | rutherford — a million decays | 370 | the 1930s, never caught on |
| decays/min | decays per minute | 2.22·10¹⁰ | counters count per minute |
| Bq | becquerel | 3.7·10⁸ | the present-day unit |
Activity says nothing about danger: that is set by the type of radiation and the route into the body, and those belong to the gray and the sievert.
03 · Orders of magnitude
from a banana to a reactor coreDose for a PET scan — 370 MBq
04 · Measuring instruments
What becquerels are measured with
Every particle triggers a discharge in the tube — a click. It counts events but does not tell them apart by energy: activity yes, nuclide no.
A germanium detector distinguishes energies and identifies which nuclide is present and how much of it. That is how caesium in soil and potassium-40 in food are measured.
The sample is dissolved in a scintillator: weak beta emitters such as tritium and carbon-14 cannot be caught any other way.
The detector surrounds the source from every side and counts every decay. The primary method: activity comes out without any reference source, from the counting geometry alone.
05 · Writing rules
Bq is activity, Sv is danger
Becquerels count decays in a source, sieverts describe the effect on a person. One cannot be obtained from the other without knowing the type of radiation, the energy, the distance and the route into the body. The same activity from an alpha emitter swallowed with food and from a gamma source behind a wall differ in consequence by orders of magnitude — which is why a radiation report always carries both figures.
Food norms are set in becquerels per kilogram: 100 Bq/kg of caesium-137 for milk, for example. That is a legal threshold, not a boundary of danger — it is set with a wide margin, and exceeding it means the product is withdrawn rather than that it is poison.
06 · Neighbouring units
The becquerel opens the radiological trio: it counts decays, the gray measures absorbed energy, the sievert the biological effect. Only the first is purely physical; the other two carry coefficients that depend on the tissue and on the type of radiation.
decays / s
J / kg
J / kg
The shorter the half-life, the higher the activity for the same number of atoms: a milligram of technetium-99m is a hundred million times more active than a milligram of uranium-238. Activity measures not how much substance there is but how fast it falls apart.
07 · Historical section
What radioactivity was counted in before the becquerel
The activity of a gram of radium-226. Removed from the SI, but alive in medicine and in industrial flaw detection.
A million decays per second — an attempt to introduce a convenient decimal unit. It never took hold and fell out of use by the 1950s.
The concentration of radon in water: used at spas with radon springs right up to the 1970s.
Also about radon in water and air: old radon surveys are marked out in emans.
The unit is so small that in its terms any food looks radioactive
The becquerel is one decay per second, and natural background in these units gives impressive figures: a kilogram of granite is about a thousand becquerels, a human body some seven thousand, a banana around fifteen. None of this is dangerous — the numbers are large simply because the unit is tiny. That is the trap of the becquerel: a headline saying «thousands of becquerels found» says nothing at all until it is compared with the background and with the norms.
Catalogue · the International System of Units
A passport for every quantity
Seven base units, twenty-two derived ones with their own names, and an archive of systems that have gone out of use. Each has its own sheet: definition, dimension, conversion, instruments, rules of writing. In 14 languages.