Nuclear physicists at the University of Birmingham have painted the most precise picture yet of the somewhat perplexing ‘Hoyle state’, which affects the nucleosynthesis of carbon in stars and, of course, life itself. What is more, they have done so using a machine designed for industrial applications rather than research.
At a time when university physics labs are closing, with most such research being conducted on large, dedicated sites, Birmingham has shown that it’s possible to adapt existing equipment to achieve experimental scientific ‘firsts’.
‘The cyclotron accelerator we used for our experiments – we’re the only university in England that has one – is used five days a week for isotope production: we supply hospitals around the country with isotopes for lung imaging,’ explains Dr Carl Wheldon, Lecturer in Nuclear Physics. ‘So, we are up there with the international research big boys using a machine not even meant for research; it’s historically been more of an applications machine.
‘Having a cyclotron facility on site has given us the opportunity to obtain these and other high-profile results and push the frontiers of nuclear science. The cyclotron is a great facility, which is also employed for applied research, and it has the capacity to serve not just our own needs, but also those of external researchers who are very welcome to use this unique UK centre.’
What Carl and his fellow researchers, including Dr Tzany Kokalova, have done is to produce an experimental calculation that is the most specific to date – and significantly smaller than previous ones – of the reaction rate of the break-up, or decay, of an excited state of carbon-12, known as the Hoyle state, into three alpha particles.
The Hoyle state is the prediction made in 1954 by British astronomer Sir Fred Hoyle that the amount of carbon in the universe could be made in stars only if there was an excited state in carbon with a particular spin and parity. This is known in carbon-12, comprising six neutrons and six protons. However, it is not predicted by standard nuclear models and over the past 60 years, nuclear physicists have struggled to understand the nature of the Hoyle state.
The work of Carl, Tzany and their colleagues takes them significantly closer to doing so. Their observations – almost 100,000 of them, no less! – have explored how the break-up, or decay, of the Hoyle state actually happens: Nuclear models predict that the Hoyle state of carbon-12 can split into its three constituent alpha particles (each particle comprising a cluster of two neutrons and two protons) either in two steps or, less frequently, at the same time.
The groundbreaking results are explained in their paper ‘New Measurement of the Direct 3α Decay from the 12C Hoyle State’, which was published in Physical Review Letters and recently won the College of Engineering and Physical Sciences' Paper of the Month award.
‘Carbon-12 is constantly being made and falling apart: it’s in equilibrium. The route we’ve been looking at is where all the alpha particles come out at the same time: so, not a two-step process, but all three together,’ explains Tzany, a Senior Lecturer in Nuclear Physics.
In 1994, it was thought that this happened up to four per cent of the time. The percentage has been coming down since then, but only now can physicists says with any certainty that the limit is less than 0.047 per cent.
‘Even though we still don’t know the precise percentage, it’s already useful because of the previous, higher predictions,’ says Carl. ‘This is the first time this has been done with such precision (a second paper, published at the same time by a different group of researchers has produced a very similar result).’
In the lab, the researchers did the opposite of what happens within stars: ‘In stars, the carbon fuses together. We took stable carbon and smashed alpha particles into it, bashing it to give it the energy to fall apart, which is called a breakup reaction,’ explains Tzany. ‘The alpha particles came out of the accelerator in a beam, and we collected the debris. Using software we had written and using simple, fundamental physics – everything is derived from energy and momentum – we put those particles back together and looked where they had come from. A bit like doing a jigsaw.
‘If you know how these states fall apart, then you can really extract information about the structure of those states. The implications of that reach as far as how everything has been made in the stars. This experiment is the first time that we’ve been so precise that we’ve been gone beyond the existing model. And by doing so, we’ve shown there is a problem with that model.’
Carl adds: ‘These measurements have shown that the limit is smaller than we thought. Before our experiment, it had gone down to 0.2 per cent. We wanted to beat that – and in 2014 we set our sights on doing so. Now, having done so, it doesn’t match the prediction, so that’s already interesting: it demonstrates that we don’t understand everything.’