Mason Inman - science journalist

17 January 2005, for the CERN Bulletin

A new study of decaying carbon nuclei, in part by researchers at CERN, could help resolve decades-old questions about the element, with a wide variety of implications for astrophysics.

We owe our lives to stardust. The big bang produced the lightest elements, but the blazing cores of stars and explosive supernovae built up all the heavier atoms - including carbon, the backbone of life as we know it. Although this was figured out in the 1950s, since then physicists have found difficulties in pinning down key properties of carbon nuclei that determine how quickly it forms under varied conditions.

Now, as reported on 13 January in Nature, new measurements of unstable nuclei caught in the act of decay have gone a long way towards resolving long-standing questions about the primary reaction that creates carbon, known as the triple alpha process. In this reaction, two helium-4 nuclei (also known as alpha particles) collide and form beryllium-8, which then combines with a third alpha particle to make carbon-12.

Rather than recreate the scorching conditions inside stars, the researchers watched the reaction unfold in reverse, as nuclei of the isotope carbon-12 broke into three alpha particles. The collaboration - involving more than 30 people at eight European universities and institutes, including CERN - created short-lived isotopes of the elements that flank carbon on the periodic table, boron-12 and nitrogen-12. The boron-12 was produced at CERN's ISOLDE facility while the nitrogen-12 was created at the IGISOL facility in Finland. These unstable nuclei then transformed into carbon-12 through beta decay, in which a proton changes into a neutron or vice versa. Finally, in the reverse of the triple alpha process, the carbon broke into three helium-4 nuclei.

By measuring precisely the timing and energies of alpha particles shooting from the samples, the researchers were able to infer the states of the carbon nuclei just before decay. Atomic nuclei can exist in quantum mechanical states with discrete energy states, or resonances, analogous to the energy levels that electrons inhabit around a nucleus. And as the electrons' levels determine how atoms interact chemically, so the nuclear resonances influence nuclear reactions.

In the recent experiments, the collaboration was able finally to pin down the quantum mechanical properties - spin and parity - of a resonance first observed in 1958. With this information in hand, they were able to deduce the properties of another, higher-energy resonance. Both states are crucial in determining the rate of the triple alpha process over a wide temperature range.

For the conditions prevalent in most stars, the team's calculated rates for the triple alpha process agree with previous calculations. However, their findings suggest the triple alpha rate at the relatively low temperatures of the Universe's first stars, which began without carbon, was much faster.

Also, the group has calculated that at high temperatures, the triple alpha process would be significantly slower than previous estimates. Given the new rate, supernova explosions - which are a major source of the heaviest elements, those more massive than iron - might produce smaller amounts of heavy elements than previously thought.

Clearly, despite being the fourth most common element in the universe, carbon still holds some secrets. By watching these nuclei fall apart, researchers are discovering how the Universe assembled itself in the first place.