1. a Solid State Physics Laboratory , University of Groningen , 4 Nijenborgh NL 9747 AG, Groningen , The Netherlands
2. b Nuclear Geophysics Division , Kernfysisch Versneller Instituut , Zernikelaan 25 9747 AA, Groningen , The Netherlands
3. c Department of Geological Sciences , Florida State University , Tallahassee , Florida , 32306 , USA
4. d Department of Geology , University of Stellenbosch , Stellenbosch , R.S.A.