Watching the Hoyle State Fall Apart

W e are used to picturing atomic nuclei as smooth and spherical distributions of neutrons and protons. But the reality is often very different, and the carbon-12 nucleus provides the perfect case in point. In its ground state and some of its excited states, carbon’s six neutrons and six protons are thought to segregate into three clusters of two neutrons and two protons, otherwise known as helium nuclei or alpha particles. Two experimental teams have now performed measurements that will help explore key details of this alpha-cluster model. Robin Smith and colleagues at the University of Birmingham, UK [1], and Daniele Dell’Aquila of the University of Naples Federico II, Italy, and colleagues [2] analyzed the breakup of an excited state of carbon-12, known as the Hoyle state, into three alpha particles. The teams’ measurements are sufficiently sensitive that they might be used to confirm recent model predictions about the Hoyle state, which is relevant to the nucleosynthesis of carbon in stars. Their results could also be used to probe certain parameters of the alpha-cluster model.