A useful observable for estimating keff in fast subcritical systems Paolo Saracco, Walter Borreani, Davide Chersola, Guglielmo Lomonaco, Gianni Ricco and Marco Ripani EPJ Web Conf., 153 (2017) 05017 Published online: 25 September 2017 DOI: 10.1051/epjconf/201715305017