Abstract: Disclosed are methods for determining the energy of a particle beam, for example a proton beam, by measuring the ratio of the radioactivities associated with two radioisotopes that are simultaneously produced within a plurality of target foils versus the calculated beam energy drop through each individual foil. This method relies on the disparate production of related radioisotopes in a single material as a function of the beam energy. A calibration curve may be established by irradiating target metal foils of known thickness and measuring the relative radioactivities of at least two target radioisotopes resulting from that irradiation. In particular, the method can be used to determine beam energies in the 10 to 18 MeV range by measuring the relative production of 63Zn and 65Zn in natural Cu foils.