Available student project - The pair conversion decay of the Hoyle state

Research fields

  • Astrophysics
  • Physics of the Nucleus
ANU magnetic pair spectrometer

Project details

Carbon, the fourth most abundant element in our universe, is produced in the triple-alpha process in helium-burning red-giant stars. In 1953 Fred Hoyle realized that the fact that there is carbon in the universe requires a resonant state in 12C very near 7.7 MeV energy. The subsequent observation of the state in 1953 is often cited as the beginning of experimental nuclear astrophysics. In the so-called triple-α process, the unstable 8Be nucleus, which decays back to two a particles with a half-life of T1/2=6.7 x 10-17 s, is combined with a third α-particle to form the 7.654 MeV resonant state in the stable nucleus 12C. However 99.96% of time the resonant state decays back to 8Be by alpha emission, producing no stable carbon nuclei. A small fraction of the time, the resonant state decays to lower states in the carbon nucleus, which remain stable against α-emission. These decay paths, the only source of carbon in the universe, proceed by the emission of a 3.215 MeV electric quadrupole (E2) and a 7.654 MeV electric monopole (E0) transition.

The sum of absolute E2 and E0 decay rates is known to only 12% accuracy, which has been identified as a major obstacle to improve current stellar models. We are planning to measure the relative E0 and E2 decay rates by observing the electron-positron pairs emitted in these high-energy nuclear transitions.

The Hoyle state can be excited in the laboratory by 10.5 MeV protons incident onto a 12C target. The main goal of the student project is to develop a magnetic pair spectrometer, based on a superconducting solenoid transporter combined with an array of semiconductor detectors. The project will involve instrumental developments and extensive Monte Carlo simulations to understand the spectrometer response to the high energy electron-positron pairs, as well as photons and other background radiations. The understanding of the energy and angular correlations of the pairs plays a crucial role in reaching the desired accuracy in the E0/E2 branching ratio. 

Project suitability

This research project can be tailored to suit students of the following type(s)
  • PhB (2nd or 3rd year)
  • Honours project
  • Phd or Masters

Contact supervisor

Kibedi, Tibor profile
Senior Fellow

Other supervisor(s)

Stuchbery, Andrew profile
Head of Department

Updated:  17 August 2017/ Responsible Officer:  Director, RSPE/ Page Contact:  Physics Webmaster