Neutrinos lay bare solar proton reactions

Set the controls for the heart of the Sun


A key prediction of solar physics, that its nuclear reactions should emit neutrinos with a particular signature, has been confirmed according to scientists at the University of Massachusetts Amherst.

Using the Borexino instrument in Italy’s Apennine Mountains, the research team detected electron-flavour neutrinos emitted by proton-proton (pp) fusion reactions in the Sun. The pp process, the university explains, is the starting point in a reaction sequence responsible for 99 per cent of the Sun's power.

Neutrinos are hard to detect: they interact through the nuclear weak force. Even though there's “as many as 420 billion” striking each square inch of Earth's surface per second, only a handful of them interact with matter on Earth each day – and it's also hard to distinguish between different types of neutrinos (for example, those produced by nuclear decays of Earth-bound elements).

“These pp neutrinos, emitted when two protons fuse forming a deuteron, are particularly hard to study. This is because they are low energy, in the range where natural radioactivity is very abundant and masks the signal from their interaction,” said Umass Amherst physicist Andrea Pocar, part of the team of 100 scientists involved in the work.

It takes thousands of years for light emitted in the reactions at the centre of the Sun to emerge at the surface and embark on the eight-minute journey to Earth, but the low-interaction neutrinos make their journey without so much impedance – providing something of a “time machine” view of the Sun.

“By comparing the two different types of solar energy radiated, as neutrinos and as surface light, we obtain experimental information about the Sun’s thermodynamic equilibrium over about a 100,000-year timescale”, Pocar said.

On their way to Earth, the electron neutrinos oscillate between the other two flavours, known as muon and tau neutrinos.

Arriving at the Borexino instrument, the neutrinos occasionally interact with scintillators filled with a benzine-like liquid described as among the oldest petroleum that exists: “We needed this because we want all the Carbon-14 to have decayed, or as much of it as possible, because carbon-14 beta decays cover the neutrino signals we want to detect. We know there is only three atoms of C14 for each billion, billion atoms in the scintillator, which shows how ridiculously clean it is”, Pocar explains.

On the rare occasions that two C14 atoms decay simultaneously, it produces a signature similar to a pp nuclear reaction, so team member Keith Otis worked on the statistical techniques to identify and remove these events. ®


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