[1] Univ Cape Town, Dept Phys, Cape Town, South Africa
[2] Univ Birmingham, Positron Imaging Ctr, Birmingham, W Midlands, England
[3] Univ Birmingham, Sch Chem Engn, Birmingham, W Midlands, England
[4] Univ Birmingham, Sch Phys & Astron, Birmingham, W Midlands, England