Abstract: It comprises an analog-digital converter receiving the pulse signal and supplying, for each pulse, a first code representing the amplitude of said pulse, said first code being able to assume N values, a group of n scalers, in which n<N, each corresponding to an energy window and a coding means receiving said first code and supplying a second code for addressing one of the counting scales, whereby said second code can assume at least n values, the content of the addressed scaler being incremented, said coding means comprising a memory and a binary decoder, the memory having N storage cells and receiving the first code on its address input, the binary decoder being connected by its input to the data output of the memory and supplying the second code.