Becquerel to Gigabecquerel

A single banana contains about 15 Bq of potassium-40, which led to the informal "banana equivalent dose" — a tongue-in-cheek way to put radiation exposure in perspective. It caught on because it makes an invisible phenomenon suddenly tangible. But the comparison has limits: your body tightly regulates potassium levels, so eating more bananas does not actually increase your internal K-40 inventory. You just excrete the excess.

The 1975 General Conference on Weights and Measures adopted the becquerel as part of the push to make all scientific measurement coherent under the SI system. The curie was awkwardly large (3.7 × 10¹⁰ disintegrations per second) and defined by a specific material — radium-226 — rather than a fundamental quantity. One becquerel equals exactly one decay per second, which is conceptually cleaner even if impractically small for everyday use.

A typical human body carries about 7,000–8,000 Bq from naturally occurring potassium-40 and carbon-14. This sounds alarming until you realize that activity (how many atoms decay per second) is not the same as dose (how much energy those decays deposit in tissue). The radiation from K-40 delivers roughly 0.17 millisieverts per year — a tiny fraction of the 2.4 mSv annual background. Your cells repair low-level DNA damage constantly; it is the rate and type of damage that matters, not the raw count of decays.

Becquerels count events — how many atoms disintegrate per second in a source. Sieverts measure the biological consequence of radiation absorbed by a person. A million-becquerel source locked in a lead safe delivers essentially zero sieverts to someone standing outside. The same source ingested could deliver a significant dose. You need to know the isotope, the radiation type, and the exposure pathway to go from Bq to Sv.

Bq/kg tells regulators exactly how many radioactive decays are occurring per second in each kilogram of food, which can then be converted to an ingestion dose using well-established dose coefficients for each isotope. The EU limit for caesium-137 in food after a nuclear accident is 1,250 Bq/kg; Japan set a much stricter 100 Bq/kg post-Fukushima. The unit is universal, isotope-neutral, and directly measurable with a gamma spectrometer — no assumptions about the consumer needed.

The patient swallows a capsule containing 1–7 GBq of I-131. The thyroid gland concentrates iodine from the bloodstream — it cannot tell radioactive iodine from stable iodine — so the isotope accumulates right where you want it. I-131 emits beta particles with a range of about 2 mm in tissue, which destroy thyroid cells from the inside while sparing nearby structures. The gamma rays it also emits are used for imaging to verify uptake. Within weeks the targeted tissue is dead, no scalpel required.

At 3–7 GBq, a freshly treated thyroid cancer patient is a walking radiation source. They emit gamma rays and excrete I-131 in sweat, saliva, and urine for days. Regulations typically require isolation until the retained activity drops below 1.1 GBq or the dose rate at 1 meter falls below 25 µSv/hr. That usually means 2–5 days of sleeping alone, using a dedicated bathroom, and avoiding prolonged close contact — especially with children and pregnant women, who are more radiation-sensitive.

Brachytherapy places a sealed radioactive source directly inside or next to a tumor — "brachy" is Greek for "short distance." High-dose-rate (HDR) sources of iridium-192 at 100–370 GBq deliver an intense, highly localized dose in minutes. The inverse-square law means tissue just centimeters away receives dramatically less radiation. This precision is why brachytherapy can treat cervical, prostate, and breast cancers with fewer side effects than external beam radiation alone.

Industrial radiography sources (1–20 GBq of Ir-192 or Se-75) live inside heavy shielded containers called "cameras" or "projectors" made of depleted uranium or tungsten. The source is only pushed out through a guide tube during an exposure, and the area is roped off with radiation monitors. Strict transport regulations, tamper-proof locks, and regular inventory audits apply. When sources decay below useful activity, they are returned to the manufacturer. The IAEA maintains a database of lost or orphaned sources — the ones that slip through the system occasionally cause severe accidents.

Diagnostic procedures use just enough activity to produce a readable image — typically 50–800 MBq (0.05–0.8 GBq). The goal is information, not tissue destruction. Therapeutic procedures aim to kill cells, so they use 10 to 100 times more: 1–7 GBq for thyroid ablation, 100–370 GBq for HDR brachytherapy sources. The line between them is roughly 1 GBq. Below that, you are taking a picture; above it, you are prescribing a lethal dose to a very specific target.