Becquerel, Gray and Sievert: Three Radiation Units That Are Not Interchangeable

Radiation reporting is frequently confused because three distinct quantities get described with the single word "radiation". They answer different questions and no simple factor converts between them.

Becquerel: how active is the source?

The becquerel (Bq) measures activity — one nuclear decay per second.

It says nothing about the energy of those decays or their effect on anything. A becquerel of a weak alpha emitter and a becquerel of a strong gamma emitter are the same activity and very different hazards.

The older unit is the curie (Ci), originally the activity of one gram of radium-226, now defined as exactly 3.7 × 10¹⁰ Bq. The becquerel is a tiny unit by comparison, so environmental readings often run to megabecquerels or gigabecquerels — producing alarming-looking numbers that convey little without context.

Gray: how much energy was absorbed?

The gray (Gy) measures absorbed dose — one joule of radiation energy deposited per kilogram of matter.

This is a physical quantity and applies to any material, not just tissue. The older unit is the rad, equal to 0.01 Gy.

Absorbed dose still does not tell you biological effect, because different radiation types deposit energy differently. One gray of alpha radiation does far more damage than one gray of gamma radiation, because alpha particles deposit their energy in a very short, dense track.

Sievert: how much biological harm?

The sievert (Sv) measures equivalent dose — absorbed dose weighted for the type of radiation, and in effective dose, for the sensitivity of the tissue exposed.

The weighting factors are approximately:

• X-rays, gamma, beta: 1
• Protons: about 2
• Neutrons: 5–20 depending on energy
• Alpha particles: 20

So one gray of alpha radiation to tissue is about 20 sieverts equivalent. The older unit is the rem, equal to 0.01 Sv.

The sievert is a large unit; practical doses are usually in millisieverts or microsieverts.

Reference doses

• Natural background: 2–3 mSv per year, varying by location and geology
• Chest X-ray: about 0.1 mSv
• Mammogram: about 0.4 mSv
• CT scan of the abdomen: 8–10 mSv
• Transatlantic flight: about 0.05 mSv
• Occupational annual limit (many jurisdictions): 20 mSv averaged over five years
• Acute radiation syndrome threshold: around 1 Sv
• Typically fatal without treatment: around 5 Sv acute

Note the factor of about 10,000 between a chest X-ray and the ARS threshold. Context matters enormously, and a dose figure without a comparison is hard to interpret.

Internal versus external exposure

An important distinction the units alone do not capture.

Alpha particles cannot penetrate skin, so an external alpha source is nearly harmless. Ingested or inhaled, the same material deposits its energy directly into sensitive tissue, and the alpha weighting factor of 20 applies in full.

This is why contamination and irradiation are different problems. An irradiated object is not radioactive; a contaminated one carries radioactive material and continues to emit.

Dose rate versus total dose

Sieverts per hour is a rate; sieverts is a total. Reports often quote one and imply the other.

A reading of 10 µSv/h sounds small, but sustained for a year it gives about 88 mSv — well above occupational limits. Conversely, a brief exposure to a high rate may total very little.

Reading the news critically

When a report cites a radiation figure, check:

1. Which unit — Bq, Gy or Sv? They answer different questions.
2. Rate or total?
3. Compared with what baseline?
4. External or internal exposure?

Our radiology converters cover activity, absorbed dose, dose equivalent and exposure as the separate quantities they are.

What different detectors actually measure

The instrument determines which quantity you get, and instruments are not interchangeable.

• Geiger-Müller counter — counts ionising events. It reports counts per second, which relate to activity but depend heavily on geometry, distance and radiation type. A GM tube calibrated for gamma will badly misreport a beta or alpha source.
• Scintillation detector — more sensitive, and energy-resolving versions can identify which isotope is present from its characteristic emission spectrum.
• Ion chamber — measures exposure or dose rate more directly, and is the usual choice for accurate dose measurement.
• Dosimeter badge — film or thermoluminescent badges integrate total dose over weeks, giving accumulated exposure rather than a rate.

A consumer Geiger counter reporting "µSv/h" is applying a conversion factor that assumes a particular radiation type and energy. For the caesium-137 gamma it was probably calibrated against, the figure is reasonable. For anything else it may be substantially wrong.

Why alpha, beta and gamma need different handling

The three common emission types differ enormously in penetration:

• Alpha — stopped by a sheet of paper or the dead outer layer of skin. Harmless externally, seriously damaging if inhaled or ingested.
• Beta — stopped by a few millimetres of aluminium or plastic. Can cause skin burns; a hazard internally too.
• Gamma — requires dense shielding such as lead or thick concrete. Penetrates the whole body, so external exposure matters.

One counter-intuitive detail: shielding beta radiation with a high-density material like lead can make things worse, because decelerating electrons produce X-rays — a process called bremsstrahlung. Beta sources are shielded with low-density plastic first, then lead if needed for any secondary X-rays.