Some variants of the Feynman alpha method in critical and accelerator driven sub critical systems

Some problems related with the Feynman variance to mean (v=m) method commonly used in reactor noise analysis are studied. These are: (i) an exact analysis of the e€ect of an extending (paralysable) dead time in the circuit; (ii) an approximate analysis of the same problem for non extending (non paralysable) dead time; (iii) e€ect of the non Poisson char- acter of the neutron source Ð a problem of current interest in the context of accelerator dri- ven systems. Our methods and results are di€erent from those obtained in some recent publications.