[en] A portable beta-gamma dose rate instrument capable of tissue-equivalent dose rate measurements in mixed beta-gamma fields has been developed and tested. The detector is a 5 mg/cm²-thick plastic scintillator with a 4 mg/cm²-window and a 1 cm-thick lucite light pipe to achieve the backscatter geometry required for this application. Pulse shape discrimination techniques are used to minimize contributions from Cerenkov events within the lucite light pipe, photomultiplier tube glass, and direct interaction within the photomultiplier tube dynode structure. The response of the prototype instrument has been demonstrated to be independent of beta end-point energy to a precision of +- 5% from 150 keV to 2.9 MeV and 1 mRem/h to 200 Rem/h. Response to gamma radiation in the energy range >400 keV is also accurate, but low energy photon response (<100 keV) is considerably lower than expected