The becquerel measures how many atoms decay per second in a source, the gray measures energy absorbed per kilogram of tissue, and the sievert weights that absorbed energy by how biologically damaging the radiation type is. A dose figure is meaningless without its unit, its quantity type and, for a rate, its time basis: a dose is a total, a dose rate is how fast it accrues, and the two are not interchangeable.
Part of Radiation Safety
Radiation units answer different questions: how active is a source, how much energy did a body absorb, and how much harm should that energy be expected to do.
Three Units Answer Three Different Questions
The parent article, Nuclear Safety, defines three units:
| Unit | Quantity type | What one unit means | What it does not tell you |
|---|---|---|---|
| Becquerel (Bq) | Activity | One atomic decay per second in a source | How much dose anyone receives |
| Gray (Gy) | Absorbed dose | One joule of energy deposited per kilogram of tissue | How harmful that energy is, for any radiation type |
| Sievert (Sv) | Biologically weighted dose | Gray weighted for the damage the radiation type does | Anything about timing: a total, not a rate |
A source of 1,000,000 Bq may deliver almost nothing to someone far away and a great deal to someone who swallows it. Activity describes the source, not the exposure.
Sieverts Are Grays Multiplied By A Weighting
For gamma and beta radiation, the parent article says 1 gray is roughly 1 sievert. For alpha radiation, 1 gray equals roughly 20 sieverts, because alpha causes about 20 times as much damage per gray. The weighting corrects for long-term biological risk and is not a statement about how quickly a person falls ill. Neutron weighting depends on neutron energy, so no single factor is given here.
Acute radiation effects are described in grays of whole-body absorbed dose, but for gamma and beta exposure, the case almost every acute figure in this section concerns, gray and sievert are numerically close. The parent article’s acute-syndrome table is written in sieverts; see Dose Thresholds.
Dose And Dose Rate Are Different Quantities
Dose rate is dose per unit time, such as grays per hour. Dose is the total received:
- Dose = dose rate x time.
- Hypothetical: a reading of 2 units per hour for 3 hours gives 2 x 3 = 6 units; a reading of 0.5 units per hour for 12 hours also gives 0.5 x 12 = 6 units. The same total from very different fields.
A figure with no averaging time cannot be used. “A dose of 10” is a total over an unstated period; “a reading of 10” without “per hour” or “per second” could be either. Never compare a rate to a total, and never multiply a short reading across a long time without noting that the field itself changes as isotopes decay; see Half-Life.
How Little Energy A Lethal Dose Carries
A gray is one joule per kilogram. For a 70 kg adult, a whole-body dose of 1 Gy is 70 kg x 1 J/kg = 70 J. Water-like tissue has a specific heat near 4,180 J per kilogram per kelvin (a recalled value), so the temperature rise is 1 / 4,180 = 0.00024 degrees. The damage is ionisation of molecules, not heating.
Old Units You May Meet On Salvaged Instruments
Many surviving instruments and documents use older units. The rad and the rem are the older counterparts of the gray and the sievert; the roentgen is an older unit of exposure in air, and the curie is an older unit of activity. The standard conversions are 1 Gy = 100 rad, 1 Sv = 100 rem and 1 curie = 3.7 x 10^10 Bq. These conversions are recalled, not from the corpus. If a dial is marked in an unfamiliar unit, do not guess; find the printed scale legend.
Common Mistakes
- Confusing becquerels with dose. Activity does not give dose without isotope, distance and intake.
- Writing “sieverts” for a rate. Write “sieverts per hour”; the missing “per hour” has produced wrong decisions in real incidents.
- Mixing prefixes. A thousandth of a sievert (milli) is not a millionth (micro). A thousand-fold slip makes a lethal field look harmless.
Safety
A number without its unit is worse than no number
If you cannot state the unit, the quantity type and the time basis of a figure, do not use it to decide anything. No home-made device in this section reads in these units; see Improvised Detectors.
What’s Next
- Learn next: Dose Thresholds — what totals do to a person
- Combine with: Chronic Exposure — why small totals still count