1. A system for detecting the presence of a pure signal in a discrete noisy signal measured at a constant average noise level, comprising: in series a converter for converting radiation received thereby into electrical pulses, a shaping means, a binary counting scale, a processing means, and a signalling means; said processing means comprising a processor, at least one memory, and an input-output circuit, said processor comprising means for measuring a counting rate N during successive equal time periods, means for estimating a value of an average noise level M, means for calculating a false detection probability PF as a function of said counting rate N and of the estimated average noise level M, and means for comparing said false detection probability PF with a predetermined false detection level TF, to thereby ascertain that a pure signal has been detected when said false detection probability PF is below said false detection level.