1. Key Laboratory of Ion Beam Bioengineering, Hefei Institutes of Physical Science, Chinese Academy of Sciences, PO Box 1138, Hefei, Anhui 230031, PR China;
2. State Key Laboratory of Nuclear Physics and Technology, Peking University, Beijing 100871, PR China; and
3. Center for Radiological Research, College of Physicians and Surgeons, Columbia University, New York, New York 10032