[1] SERC, Daresbury Lab, Magnet Spect Grp, Warrington WA4 4AD, Cheshire, England
[2] Univ York, York YO1 5DD, N Yorkshire, England
[3] Univ Amsterdam, NL-1081 XE Amsterdam, Netherlands
[4] CEA Grenoble, Serv Phys Mat & Microstruct, F-38054 Grenoble 9, France