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Radioactive Decay Anomaly Finally Explained (Maybe)

A long-standing puzzle over periodic oscillations in the decay of some elements may have finally been solved, say physicists

  • October 12, 2011

Back in 2007, physicists at the GSI heavy ion accelerator in Darmstadt, Germany, made a puzzling discovery.

These guys were measuring the radioactive decay rates of praseodymium and promethium nuclei that had been stripped of all but one or two of their electrons, leaving them with a charge in excess of +50.

In these conditions, nuclei are known to decay in strange ways. Bare nuclei, for example, cannot decay by electron capture. This forces them to decay in other, less common ways, vastly changing their decay rate.

More generally, physicists have known for some time that changes in temperature and pressure can influence decay rate by a per cent or so, probably because this also changes the electron density around nuclei by a small amount.

But what the GSI guys found was stranger still. They discovered that the normal exponential decay rate of praseodymium and promethium oscillated with a period of about 7 seconds. It was as if an oscillation had been superimposed on the normal exponential decay curve.

Their experiment is interesting because it is unique. These guys produce a handful of ions in a synchrotron and the measure each one decaying by the change it produces in the resonance of the ion beam as it circulates.

This gives an exact measurement of the lifetime of each ion, rather than an average measurement of the half life of a bulk material, as all other experiments do. It’s easy to see that this effect would be smeared out and invisible in these kinds of experiments

So many physicists believe that the GSI experiment was actually measuring the properties of radioactive decay in its purest form.

The big question, of course, is what causes the GSI anomaly, as it has become known? The first explanations focused on the possibility that neutrino oscillations could account for the effect.

Praseodymium and promethium nuclei decay via the weak force in two ways. A proton can capture an electron producing a neutron and an electron-neutrino. Or the proton can decay spontaneously into a neutron, a positron and an electron-neutrino.

The thinking was that the the neutrinos produced in this reaction might change into other types, thereby influencing the rate of decay by a small amount.

However, various physicists pointed out that the result cannot be explained by neutrino oscillations for the simple reason that these can only occur after the neutrino is produced and when it is a significant distance from the nuclei.

That’s left physicists at an embarrassing loss. In the absence of any reasonable explanation, the GSI anomaly has become an uncomfortable itch on the arse of nuclear physics.

Today, Francesco Giacosa at J.W. Goethe University in Frankfurt and Giuseppe Pagliara at the Universita di Ferrara in Italy, provide some welcome relief.

These guys say the GSI anomaly can be explained if the two decay mechanisms of praseodymium and promethium nuclei operate at slightly different energies. The decay rate of each mechanism alone would be a standard exponential curve, albeit at slightly different rates.

But here’s the key. When both mechanisms occur together, the system oscillates between them. So the periodic variations observed at GSI are simply the effect of the system jumping from one decay mechanism to another. Giacosa and Pagliara liken this to the well-known Rabi oscillations which occur in many quantum systems.

What’s most exciting is that this theory leads to an interesting prediction. Giacosa and Pagliara say that if the the GSI team can measure the the decay rate at intervals much shorter than 7 seconds, the decay rate should rapidly drop to zero. “If the experiment at GSI could measure a few points below 10 s, our interpretation could be easily rejected or approved,” they say.

These ideas have broader implications. Giacosa and Pagliara say this effect could explain other strange periodic variations in decay rates that physicists have observed over much longer time scales. We’ve looked at some of these here.

What’s more, the variations are clearly fundamental effects in quantum mechanics that could have profound implications for our understanding of the nuclear processes that go inside the stars.

And all this means that there’s more fun to be had with the GSI anomaly, whether this explanation turns out to be true or not.

Ref: arxiv.org/abs/1110.1669: Oscillations In The Decay Law: A Possible Quantum Mechanical Explanation Of The GSI Anomaly?

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