Disintegrations per second to Microcurie

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.

Because microcuries are the threshold where regulatory accountability begins for most isotopes. A lab ordering 250 µCi of P-32 must log the receipt, track usage, survey for contamination weekly, monitor personnel doses, and account for every fraction disposed of or decayed. Multiply that by dozens of labs across a campus, each using different isotopes with different rules, and you get a full-time radiation safety program. The obsession is not about the hazard of any single vial — it is about preventing the slow accumulation of untracked material that eventually leads to a contamination incident or regulatory violation.

It depends on what the isotope emits. A 100 µCi tritium source needs no shielding at all — the beta particles cannot penetrate a sheet of paper. A 100 µCi phosphorus-32 source (high-energy beta) needs about 1 cm of acrylic to stop the betas, but acrylic is preferred over lead because lead produces bremsstrahlung X-rays from energetic betas. A 100 µCi caesium-137 source (gamma emitter) needs a thin lead container. At microcurie levels the shielding is lightweight and portable — nothing like the heavy lead pigs used for millicurie medical sources.

Most check sources contain 0.1–1 µCi of caesium-137, chosen because Cs-137 has a convenient 662 keV gamma ray and a 30-year half-life — long enough that the source maintains predictable activity for decades without frequent recalibration. The activity is high enough to produce a clear above-background reading (several hundred counts per minute) but low enough to be exempt from most transport regulations. Technicians hold the check source near the detector before each use to verify the instrument is responding. If the reading is off by more than 10–20% from the expected value, the instrument goes back for calibration.

Not from external exposure — the dose rates are far too low. At 1 meter from a 500 µCi unshielded Cs-137 source, the dose rate is about 1.6 µSv/hr, which is only a few times background. The danger from microcurie quantities comes from internal exposure: inhaling or ingesting even micrograms of an alpha emitter like polonium-210 or americium-241 can deliver a concentrated dose to lung or gut tissue. Alexander Litvinenko was killed by roughly 26 µCi of Po-210 dissolved in tea — a quantity invisible to the eye.

Autoradiography uses radioactive decay to make an image — you label DNA or protein with P-32, separate the molecules on a gel, press the gel against X-ray film or a phosphor screen, and the beta particles expose the film wherever your target molecule sits. A typical experiment uses 50–250 µCi, which gives a visible image in hours to overnight. P-32 is favored because its high-energy beta (1.7 MeV) produces sharp, high-contrast bands without the weeks-long exposure times that weaker emitters like S-35 or C-14 require.