Gigabecquerel to Kilocurie

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.

In 1987, scrap metal scavengers in Goiânia, Brazil broke open an abandoned caesium-137 teletherapy source containing about 1,375 Ci (50.9 TBq). The glowing blue Cs-137 powder fascinated locals — they rubbed it on skin, gave it to children, and spread it across multiple homes. Four people died, 249 were contaminated, and the cleanup produced 3,500 m³ of radioactive waste. The incident became the textbook case for why sealed sources must be tracked and securely stored throughout their entire lifecycle, and why the IAEA created its Code of Conduct on the Safety and Security of Radioactive Sources.

Yes, multiple times. In Ciudad Juárez, Mexico (1983), a stolen Co-60 teletherapy source was sold as scrap and melted into rebar, contaminating 4,000 tonnes of steel and exposing thousands. In Samut Prakan, Thailand (2000), a junked Co-60 source killed three scrap workers who pried it open. In Yanango, Peru (1999), a welder pocketed an Ir-192 industrial radiography source and carried it in his pocket for hours — his leg was amputated. The IAEA documents over 30 serious radiation accidents involving orphaned or stolen sources since the 1960s, collectively killing dozens and injuring hundreds.

Cobalt-60 has a 5.27-year half-life, so a 500 kCi source drops to 250 kCi after five years and becomes too weak for industrial throughput after about 15–20 years. The spent source pencils are returned to the manufacturer (typically in Canada or Russia) for reprocessing or secure storage. Transport uses heavily shielded Type B casks certified to survive a 9-meter drop and 30-minute fire. The manufacturer often offers a swap program: deliver fresh sources and take back decayed ones in the same shipment, minimising the number of high-activity transports.

The Fukushima Daiichi disaster released an estimated 10–30 PBq (10,000–30,000 TBq) of caesium-137 directly into the Pacific Ocean between March and July 2011 — the largest single marine radioactive release in history. For comparison, the Sellafield reprocessing plant in the UK discharged about 40 PBq of Cs-137 into the Irish Sea over decades of operation (1952–2000). Soviet dumping of entire reactor compartments from nuclear submarines in the Arctic added further inventory. Despite these numbers, ocean dilution is vast: Pacific Cs-137 levels from Fukushima peaked at about 50 Bq/m³ near the plant and dropped below 2 Bq/m³ within a few hundred kilometers.

This is exactly why the IAEA, NRC, and national agencies track high-activity sources so aggressively. A kilocurie Cs-137 or Co-60 source dispersed by conventional explosives would contaminate a few city blocks — not causing acute radiation casualties (the blast itself is deadlier) but creating a costly, panic-inducing cleanup lasting months. The actual health risk to the public would be low, but the economic and psychological damage would be enormous. Post-9/11 programs like the US GTRI (now NNSA OSRP) have recovered or secured thousands of orphaned high-activity sources worldwide.