1. Department of Engineering Physics, Key Laboratory of Particle and Radiation Imaging under Ministry of Education, Tsinghua University, Beijing 100084, China
2. Center for Neutron Research, National Institute of Standards and Technology, Gaithersburg, Maryland 20899, USA
3. Department of Chemical and Biomolecular Engineering, University of Delaware, Newark, Delaware 19716, USA
4. Department of Physics and Astronomy, University of Delaware, Newark, Delaware 19716, USA