This paper describes a new principle of charged-particle identification by plastic track detectors. We find that the rate of chemical etching (by a suitable reagent) along the track of a heavy ion in a plastic depends only on the primary ionization rate, which, as the particle slows down, increases at a rate that depends uniquely on atomic number Z and mass A. To identify the Z, A, and energy of a particle we can either measure the etching rate at a known residual range [by analogy with a (dE/dx) -E detector] or measure the etching rate at two known points along the trajectory in a stack of plastics. We present experimental data for B 10 , B 11 , C 12 , N 14 , and O 16 ions from the Yale accelerator, showing that masses differing by 9% can be resolved. Unique features of this method are its discrimination against intense background radiation and its combination of high resolution with large collecting area, which make it feasible to do certain cosmicray experiments, such as the detection of Be 10 nuclei. D IELECTRIC detectors, 1 in which nuclear particle tracks are made visible by a preferential chemical etch, are uniquely useful for certain studies in nuclear physics, geophysics, and astrophysics. One of their most valuable features is the existence of a critical primary ionization rate such that only particles with primary ionization rates above the critical rate will produce etchable tracks. 2 This property has been exploited as a means of setting lower limits on the masses of those particles that record tracks. Inorganic crystals have been used, for example, to measure formation cross sections and angular distributions of ternary fission fragments without any background from an impinging beam of argon ions, 3 and to measure the abundance of cosmic rays with Z>30, relative to the abundance of iron nuclei. 4,5 In this paper we show how dielectric detectors can be used to identify atomic numbers and to distinguish between isotopes of certain energetic nuclei. Although the data reported here apply only to one brand of cellulose nitrate, the principle is valid for other plastics and possibly for other dielectric solids. This discovery extends the usefulness of dielectric detectors and makes feasible the identification of certain energetic isotopes in solar and cosmic rays and of light nuclei emitted in asymmetric fission. The principle of charge resolution is based on our observation, to be described below, that the rate of chemical etching along a particle track in a plastic depends only on the primary ionization, which is a function of the velocity and the atomic number Z of the particle.
