PALS Experiments Using a Pulsed Positron Beam
- Chief Supervisor
- Dr. James Sullivan
- Supervisors
- Professor Stephen Buckman
- Centre Research Nodes
- ANU
- Collaborators
- Tohoku University, Japan
Positrons are a useful probe of certain material properties, such as open volume and porosity, in a range that is difficult to measure using more conventional techniques. This project will focus on construction and characterization of the new positron materials beamline and to then use this beamline to investigate properties of a wide variety of materials.
At APBF, high energy positrons are supplied by a 22Na source, which then are guided through a frozen neon moderator to make a low energy beam. Using a series of electrostatic wells in a strong magnetic field, the low energy positrons are cooled to thermal speeds by collisions with nitrogen gas. From this we may extract a variable energy, pulsed positron beam. This beam will be compressed temporally to form sub-nanosecond pulses which will be injected into the chosen material at energies up to 20keV. The resultant annihilation gamma rays will be detected as a function of time, and the lifetime of the positrons inside the material can be related to the open volume structure. By varying the energy of implantation, the depth of the positrons inside the material can be varied.
This project will commence with finishing the construction of the beamline and characterising the operation of the trapping and beam formation process. The pulse compression scheme will then have to be implemented to achieve the timing resolution required, this will be done in conjunction with the development of fast correlated gamma ray detection. In addition, detectors that will measure the Doppler shift of the annihilating positrons will be incorporated into the apparatus, allowing an investigation of the chemical environment in which the positron annihilates. Once the characterisation of the system is complete, experiments will commence on a range of interesting materials systems, with possible targets ranging from silicon wafers to polymers and layered materials.
The student undertaking this project will gain experience in a wide range of areas, such as design and construction of a scientific apparatus, including vacuum and gas handling techniques, digital data acquisition, computer programming and electronics - in particular the handling of high voltages. New analysis techniques will have to be learned, as well as the development of data analysis and data processing skills. Travel to overseas laboratories collaborating on this project may also be required.
