PURPOSE: To make a titled instrument simpler in constitution, lower in cost and smaller in size by providing thin films and secondary electron generating plate on an orbit of neutral particles.

CONSTITUTION: The neutral particles collide against the inlet thin film 3 by passing a drift pipe 101 from a neutral particle source 2 made of a neutral particle generator 1. Only the neutral particle of the energy larger than the blocking power of the thin film 3 passes the film 3 and part thereof are ionized at a certain ratio. The ions are bent in the orbit by the next thin film electrode 4 and are measured by a charge particle detector 7 and a circuit 8 for the detector. The remaining particles collide against the electrode 4. Only the neutral particles of the energy larger than the blocking power of the electrode 4 pass the electrode 4 but again the neutral particles are ionized at a certain ratio. The neutral particles are bent in the orbit by the next electrode 4 and is measured by the detector 7 and the circuit 8. The neutral particles past the final electrode 4 collide against the secondary electron generating plate 5 to release the secondary electrons. Since the secondary electrons are captured by a secondary electron capturing electrode 6, the current flows between the plate 5 and the electrode 6. The analysis of the neutron energy is thus made possible.