Megabecquerel to Rutherford

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.

The rutherford was proposed in the 1940s when the curie was the only game in town and was inconveniently large for many lab measurements. At 10⁶ dps (1 MBq), the rutherford sat in a useful range. But the 1975 SI reform chose the becquerel (1 dps) as the base unit with standard SI prefixes — kBq, MBq, GBq — which covered every scale. Having both the rutherford and the megabecquerel for the same quantity was redundant. The scientific community picked one, and the rutherford quietly disappeared from everything except unit conversion tables and physics trivia.

Rutherford discovered the atomic nucleus by firing alpha particles at gold foil (1911), identified alpha and beta radiation as distinct particle types, and performed the first artificial nuclear transmutation — turning nitrogen into oxygen — in 1917. He won the Nobel Prize in Chemistry in 1908, which famously annoyed him because he considered himself a physicist. His students went on to split the atom (Cockcroft and Walton) and discover the neutron (Chadwick). Nearly every branch of nuclear science traces back to his Manchester and Cambridge laboratories.

Several. The stat (1 disintegration per second, identical to the becquerel but proposed earlier), the eman (used for radon concentration in water, equal to 10⁻¹⁰ Ci/L), and the mache unit (another radon measure used in Austrian and German spa water literature) are all effectively extinct. The curie itself is technically obsolete under SI but persists through sheer institutional momentum in the US. The pattern is typical of measurement science: every era invents its own units, and standardisation eventually consolidates them.

Unlikely in practice because the rutherford disappeared from active use by the 1970s, before the megabecquerel entered common parlance in the 1980s. You would only encounter the rutherford in papers from roughly 1946–1970, primarily in European nuclear physics journals. If you see "Rd" in a modern unit conversion tool, it is there for completeness and historical interest, not because anyone is publishing in rutherfords. The real risk of confusion in old literature is between the curie and the becquerel, where a missing prefix can mean a billionfold error.

The "sunshine unit" — officially the strontium unit — was coined by the US Atomic Energy Commission in the 1950s to describe strontium-90 concentration in bones and milk during nuclear weapons testing. One sunshine unit equalled 1 picocurie of Sr-90 per gram of calcium. The name was a deliberate PR choice to make fallout contamination sound cheerful and harmless. It backfired spectacularly when journalists mocked it as Orwellian doublespeak, and the term was quietly dropped in favor of pCi/g Ca. It remains a cautionary tale about naming units for political rather than scientific reasons.