Becquerel to Rutherford

A single banana contains about 15 Bq of potassium-40, which led to the informal "banana equivalent dose" — a tongue-in-cheek way to put radiation exposure in perspective. It caught on because it makes an invisible phenomenon suddenly tangible. But the comparison has limits: your body tightly regulates potassium levels, so eating more bananas does not actually increase your internal K-40 inventory. You just excrete the excess.

The 1975 General Conference on Weights and Measures adopted the becquerel as part of the push to make all scientific measurement coherent under the SI system. The curie was awkwardly large (3.7 × 10¹⁰ disintegrations per second) and defined by a specific material — radium-226 — rather than a fundamental quantity. One becquerel equals exactly one decay per second, which is conceptually cleaner even if impractically small for everyday use.

A typical human body carries about 7,000–8,000 Bq from naturally occurring potassium-40 and carbon-14. This sounds alarming until you realize that activity (how many atoms decay per second) is not the same as dose (how much energy those decays deposit in tissue). The radiation from K-40 delivers roughly 0.17 millisieverts per year — a tiny fraction of the 2.4 mSv annual background. Your cells repair low-level DNA damage constantly; it is the rate and type of damage that matters, not the raw count of decays.

Becquerels count events — how many atoms disintegrate per second in a source. Sieverts measure the biological consequence of radiation absorbed by a person. A million-becquerel source locked in a lead safe delivers essentially zero sieverts to someone standing outside. The same source ingested could deliver a significant dose. You need to know the isotope, the radiation type, and the exposure pathway to go from Bq to Sv.

Bq/kg tells regulators exactly how many radioactive decays are occurring per second in each kilogram of food, which can then be converted to an ingestion dose using well-established dose coefficients for each isotope. The EU limit for caesium-137 in food after a nuclear accident is 1,250 Bq/kg; Japan set a much stricter 100 Bq/kg post-Fukushima. The unit is universal, isotope-neutral, and directly measurable with a gamma spectrometer — no assumptions about the consumer needed.

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.