FIELD: detection of low fluxes of ionizing radiation from mobile and immobile, open and closed beta-, gamma- and neutron sources with use of ionization counters ( proportional or Geiger counters ) or scintillation detectors. SUBSTANCE: threshold q0 is determined before start of test of objects in correspondence with probability of false operations specified by operator by tables for normal distribution, then average rate b of background counting for time tb and average rate n of counting of additive mixture of signal and background obtained for time tn of object presence in field of vision of detector. Later parameter ? is found by processing of measurement results by formula which are compared with threshold q0 to make decision on result of detection. If ? > q0 then decision on detection is taken, otherwise decision on failure to detect is taken. EFFECT: minimal detected signal decreased to level close to theoretical limit, achievement of greatest probability of detection with preset probability of false alarm. 1 tbl.