Disintegrations per second to Kilocurie

Because dps is literally what the instrument measures — a detector counts individual nuclear decay events over time. Calling it "dps" keeps the language grounded in what physically happened. Calling it "Bq" applies an SI label to the same number. Old lab protocols, standard operating procedures written before 1975, and some US-centric equipment manuals still use dps because nobody rewrote the paperwork. Numerically, 1 dps = 1 Bq, so the conversion is trivially multiplying by one.

Counts per second (cps) is what the detector actually registers; disintegrations per second (dps) is how many decays actually occurred. No detector catches every decay — some radiation misses the detector, some is absorbed before reaching it, and some types of radiation are invisible to certain detectors. The ratio of cps to dps is the detection efficiency, which can range from under 1% (for low-energy beta emitters in a Geiger tube) to over 90% (for gamma emitters in a well counter). Getting from cps to dps requires careful calibration.

The sample is dissolved in a scintillation cocktail — a solvent containing fluorescent molecules. Each beta particle or electron excites the cocktail, producing a flash of light detected by photomultiplier tubes. But chemical impurities in the sample absorb some of that light (a phenomenon called quenching), so the counter sees fewer flashes than decays. The instrument runs an internal or external standard to measure the quench level, then applies a correction curve to convert raw cpm to true dpm, which you divide by 60 to get dps.

Not numerically — they are identical. But contextually, "dps" emphasizes the physical measurement process and appears in lab protocols where you are calculating detector efficiency: "the source emits 10,000 dps and the detector reads 3,200 cps, so efficiency is 32%." Writing that sentence with Bq would be technically correct but odd, like referring to your morning coffee temperature in kelvin. The unit name signals what kind of work you are doing.

Before the becquerel was adopted in 1975, there was no named SI unit for radioactivity — scientists just said "disintegrations per second" or used the curie. The CGPM gave the name "becquerel" to one disintegration per second to honor Henri Becquerel and to bring radioactivity into the SI naming system alongside the gray and sievert. The dps description never went away; it just lost its status as the primary label. Think of it like saying "cycles per second" instead of "hertz" — correct, but dated.

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.