Angular correlation spectrometer
The Bristol 2D-ACAR Spectrometer for Fermi surface studies
At the University, positrons are used to probe the structure and electronic structure of materials. The Bristol Two-Dimensional Angular Correlation of Annihilation Radiation (or 2D-ACAR) spectrometer is a unique instrument for probing the Fermi surfaces of metallic systems.
When a positron enters the sample under study, it quickly comes into thermal equilibrium (typically within a few picoseconds) and then, on a timescale of order of hundreds of picoseconds, it annihilates with an electron resulting predominantly in two gamma-rays travelling in (almost) opposite directions (conserving the energy and momentum of the annihilated electron-positron pair).
The gamma rays are detected by a pair of position-sensitive detectors, known as HIDACS (high-density avalanche chambers) operating in coincidence which allows Dr. Dugdale and Prof. Alam of the CES Group to measure the occupied momentum states and hence the Fermi surface of the material under study.
Measuring Fermi surfaces
Understanding the Fermi surface is vital for understanding the different behaviours of metals.