Science & Technology

Supercomputer used to solve the mystery of Beta Decay

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Researchers have used a supercomputer solved a puzzle in the process of beta decay which has lingered for half a century, capturing a glimpse of physics beyond the standard model. 

Beta decay is the process that powers massive stellar explosions synthesizing heavier elements such as platinum and gold. But 50 years it has harboured a secret, one which researchers at Oak Ridge National Laboratory believe they have unlocked.

When an atomic nucleus contains either too many protons or neutrons — collectively known as nucleons —  it is unstable. To restore stability a proton transforms into a neutron or a neutron transforms into a proton. Elements are determined by their proton count — thus this is how an atom of platinum-198 can become an atom of gold-199. This is the process of beta decay.  

The problem nuclear physicists have had is that whilst calculations of the beta decay rate for a single neutron seem to meet observations when this is scaled up to the atom as a whole — it appears to be proceeding too rapidly. Why can researchers calculate the decay rate of a single neutron but not scale this up? 

Thomas Papenbrock, professor of theoretical nuclear physics at the University of Tennessee, Knoxville explains that the adjustment for this discrepancy comes in the form of “quenching” — reducing a fundamental coupling constant between decay rates for a free neutron and the decay rate scientists observed in an atomic nucleus. 

The puzzle was that there was no first-principles theoretical understanding for why quenching worked.

Papenbrock, the co-author of the paper detailing these findings published in Nature Physics, says: “Our work shows that the beta decay of a nucleus is more complicated. Even if we think of it as a decay of a neutron to a proton inside the nucleus, this decay is influenced by processes where two neutrons interact and transition into a proton and a neutron.

“Taking these effects into account, and using state-of-the-art nuclear models and supercomputers, we were able to solve the puzzle of ‘quenched’ beta decays.”

Using the Cray XK7 Titan Supercomputer at ORNL, the researchers simulated the decay of tin-100 into indium-100. Tin-100 is known as a “doubly magic” nucleus: it has 50 protons and 50 neutrons which are in complete shells, making it strongly-bound. 

The relative simplicity of its structure makes tin-100 an ideal candidate for the sort of large-scale calculations where scientists need to understand the forces between the protons and neutrons in an atomic nucleus.

The results of this study are consistent with experimental data and could also inform predictions for the matrix element that governs the hypothetical process — neutrino-less double beta decay — where two neutrons decay into protons at the same time, but without emitting neutrinos. 

Papenbrock and his colleagues point out that neutrino-less double beta decay has its own puzzle in terms of understanding the scale of mass in neutrinos, which up until 1998 were thought to have no mass at all.

Original research available at http://dx.doi.org/10.1038/s41567-019-0450-7

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