This paper describes a circuit for identifying protons, deuterons, and tritons. The identification is accomplished by obtaining the product of the energy and the initial space rate of energy loss. The product is modified so that each type of particle gives a unique result in the form of an output pulse having an amplitude nearly proportional to the mass of the particle. The circuit as given is capable of handling input pulses as narrow as 1 &mgr;sec, although shorter pulses may be used if some test features are eliminated. The multiplying operation is performed by QK‐329 tubes, using the principle of difference of squares.