1. Grupo de Física Nuclear, Dpto. de Física Atómica, Molecular y Nuclear, Universidad Complutense de Madrid, Campus de Excelencia Internacional Moncloa, Avenue Complutense S/N, Facultad de Ciencias Físicas, Madrid 28040, Spain
2. Department of Electrical and Computer Engineering, Tufts University, 161 College Avenue, Medford, Massachusetts 02155, USA
3. Department of Medical Physics and Biomedical Engineering, University College London, Gower Street, London, WC1E 6BT, United Kingdom