X-ray tomography study on the structure of the granular random loose packing *Project supported by the Thirteenth Shanghai Jiao Tong University Students Innovation Practice Plan, China (Grant No. IPP13086), the National Natural Science Foundation of China (Grant Nos. 11175121, 11675110, and U1432111), and the Specialized Research Fund for the Doctoral Program of Higher Education of China (Grant No. 20110073120073). (August 2017)