Megabecquerel to Disintegrations per second

Diagnostic imaging doses fall neatly in the MBq range — a PET scan uses 185–370 MBq, a bone scan 500–800 MBq. Using becquerels would mean writing hundreds of millions; using gigabecquerels would mean awkward decimals like 0.37 GBq. MBq is the Goldilocks unit for the hospital pharmacy: large enough to avoid scientific notation, small enough to express a single patient dose as a tidy number on a syringe label.

That depends entirely on the isotope. Technetium-99m, the workhorse of diagnostic imaging, has a 6-hour half-life — so a 740 MBq injection drops to 370 MBq in 6 hours, 185 MBq in 12, and becomes negligible within 2 days. Fluorine-18 (used in PET) has a 110-minute half-life and is essentially gone in a day. Iodine-131 (used in therapy) lingers for about 8 days per half-life. Hospitals choose isotopes partly based on how fast they want the activity to vanish.

Most diagnostic isotopes (Tc-99m, F-18) have half-lives under a day, so hospitals simply store waste in shielded bins and let it decay. After 10 half-lives — about 3 days for Tc-99m — the activity is down to less than 0.1% of the original and can be disposed of as normal clinical waste. Longer-lived therapeutic isotopes like I-131 require weeks of decay storage. The vast majority of nuclear medicine waste is never shipped to a radioactive disposal site; it just sits in a locked closet until physics solves the problem.

A patient injected with 370 MBq of F-18 for a PET scan emits gamma rays at a dose rate of roughly 5–6 µSv/hr at one meter. That means sitting next to them for two hours gives you about 10–12 µSv — less than a chest X-ray. Staff handle dozens of patients daily so they follow time-and-distance protocols, but for family members the exposure from a single visit is trivially small. The activity halves every 110 minutes, so by evening the patient is barely distinguishable from background.

Molybdenum-99, which decays into the technetium-99m used in 30+ million scans per year worldwide, can only be produced in a handful of aging research reactors. It has a 66-hour half-life so it cannot be stockpiled — you have to make it, ship it, and use it within days. When a reactor goes down for maintenance (as happened in 2009 when both the Canadian NRU and Dutch HFR shut down simultaneously), hospitals worldwide face scan cancellations within a week. New production methods using particle accelerators and LEU targets are slowly diversifying supply.

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.