Kilobecquerel to Gigabecquerel

A standard ionisation smoke detector contains about 1 kBq (roughly 0.9 microcuries) of americium-241, an alpha emitter. That tiny speck of material ionizes air inside the detection chamber; when smoke particles disrupt the ion current, the alarm triggers. The alpha particles cannot penetrate the plastic casing, so the external dose is essentially zero. You would have to physically open the sealed source and inhale the material to face any health risk — which is why proper disposal matters but daily proximity does not.

German wild boar still exceed the 600 Bq/kg caesium limit 40 years after Chernobyl because of a phenomenon called the "wild boar paradox." The animals root in forest soil for deer truffles — underground fungi that concentrate Cs-137 from the soil far more efficiently than surface plants. Forest floors recycle caesium in a closed loop: leaves fall, decompose, fungi absorb the caesium, boar eat the fungi, boar excrete it back into the soil. Unlike farmland, which was plowed and diluted, forest ecosystems locked the caesium in a tight cycle. Hunters in Bavaria must still test every carcass before sale.

The US measures radon in picocuries per liter (pCi/L) because the curie was the dominant unit when the EPA set its action levels in the 1980s. Most of the rest of the world uses becquerels per cubic meter (Bq/m³) because they adopted SI units. The EPA action level of 4 pCi/L equals about 148 Bq/m³; the WHO recommends action above 100 Bq/m³. Same phenomenon, different yardsticks — and a perpetual source of confusion when reading international radon guidelines.

Consumer Geiger counters can detect gross contamination — the kind where food is obviously dangerous — but they cannot identify specific isotopes or give reliable Bq/kg readings. Proper food monitoring requires a gamma spectrometer with a shielded sodium iodide or high-purity germanium detector, plus a sample prepared to known geometry and mass. After Fukushima, Japan deployed thousands of these in public food monitoring stations where citizens could bring their own produce for free testing.

Brazil nuts hold the record among common foods, with activity levels of 40–260 Bq/kg from radium-226 and radium-228 that the trees concentrate from soil. Lima beans and bananas follow at 170 and 130 Bq/kg respectively, mainly from potassium-40. None of these pose a health concern — the amounts are tiny compared to regulatory limits, and K-40 is self-regulating in the body. You would need to eat several hundred kilograms of brazil nuts daily before the radium intake became medically interesting.

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.