The voltage output of each preamplifier (FIG. 2) due to the instantaneous transfer of charge QO at position xO is: ##EQU6## where Cf is the feedback capacitance of a preamplifier in farads, L is a givenn linear dimension of the detector, ??D is the time constant of the detector (i.e. ??D = 2RD CD), xo defines the position of interaction and m is a summation variable.