of these counters are at 34 0 34 the positive pulse of the second monostable multivibrator 54 is thus used as the readout signal for the memories 21 and 22 during the time in which the output of the first monostable multivibrator 53 is at 34 1 34 time discrimination continues the trailing edge of the pulse at the output of the second monostable multivibrator 54 terminates the readout process of the memories 21 and 22 and simultaneously sets the inverting output 56 of the bistable flip stage 55 and the first input of the nand gate 57 to 34 1 34 if other input of the nand gate 57 which is connected with the inverting output of the comparator 7 is also at 34 1 34 the output of the nand gate 57 goes to 34 0 34 thus the bistable flip stages or memories 55 31 32 21 and 22 are reset to a predetermined starting position and enabled for a new measurement the linkage of the output 56 of the bistable flip stage or memory 55 with the inverting output of the comparator 7 assures that the reset pulse is given to all bistable flip stages only if it is certain that the threshold value s 1 of the comparator 7 has not been exceeded i e no detector signal 2 is on the line the pulse length of an alpha or beta pulse is about 150 ns the time required for a measurement is about 1 2 us due to the statistical distribution of the pulses one or several further pulses may occur during one measurement therefore as soon as a measuring process has been actuated by setting of the bistable flip stage or memory.