24 June 2009 by Justin Mullins
It’s 1986, and there’s a puzzle on Dave Alburger’s desk. Not Ernö Rubik’s latest toy, but the data from a four-year experiment to measure the half-life of the rare radioactive isotope silicon-32. On one level, the numbers fit together just fine, adding up to a half-life of 172 years, in keeping with previous estimates.
There’s a devil in the detail, however. The sample’s radioactivity has not been dropping steadily over time, as the textbooks demand. It has fallen, to be sure, but superimposed on that decline is an odd, periodic wobble that seems to follow the seasons. Each year, the decay rate is at its greatest around February and reaches a minimum in August.
If we know anything about radioactivity, it’s that this kind of thing just doesn’t happen. Radioactivity decreases predictably over time. That’s why we can tell the age of rocks, fossils and prehistoric artefacts by the activity of radioactive atoms within them, and why nuclear waste becomes less toxic over time.
The fault was surely in some detail of the experimental set-up. Yet try as they might,Alburger and his colleagues at the Brookhaven National Laboratory on Long Island, New York – all nuclear physicists highly versed in this kind of painstaking measurement – couldn’t find it…..
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