The pair conversion decay of the Hoyle state

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 T_1/2=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. 

Searching for the decay of the X17 neutral boson with a magnetic pair spectrometer

Research fields:

• Nuclear physics
• Particle physics

Project Details:

Searching for new particles, which could help to find physics beyond the Standard Model is one of the highest priorities in fundamental science. A recent observation of an anomaly in the energy and angular correlation of electron-positron pairs of high-energy transitions in ⁴He and ⁸Be attracted a significant attention worldwide (see The Conversation, 10 December 2019) as it may signal the discovery of a new particle with a mass of 17 MeV. The particle is being called the X17. Electron-positron pair emission is a third-order electromagnetic transition in atomic nuclei. The reported observations of the X17 in a low energy nuclear physics laboratory used an array of scintillator detectors, which also detect the high energy gamma-rays as a possible background. This project is aiming to adapt the ANU pair spectrometer to check the X17 measurement. Regular nuclear pair transitions are emitted at relatively small separation angles between electrons and positrons. The observation of the pair decay of the hypothetical X17 particle requires the detection of electrons and positrons at much larger separation angles. The project will involve numerical simulations to examine various detector geometries to achieve optimal sensitivity for X17 detection.

Project Suitability:

• ASC (2^nd or 3^rd Year)
• Honours student
• PhD or Masters

Contacts: Dr. Tibor Kibédi, Prof. Andrew E. Stuchbery