Abstract: A system and method are disclosed for post-exposure radiation biodosimetry on subjects using electron paramagnetic resonance (EPR) spectroscopy of nail clippings from the subjects. Basis spectra averaged from a plurality of nail clipping measurements are used to spectrally decompose an EPR-measured signal and identify a radiation-induced signal (RIS). The RIS is used to determine an exposure dose from a standard curve. A collection apparatus provides for harvesting and storing nail clippings in a dry, oxygen-reduced, environment to prevent sample degradation. The collection apparatus includes a container with an atmosphere isolated from external atmosphere and a sample bag impermeable to oxygen and water vapor. The sample bag includes an oxygen absorber and a desiccant for storing nail clippings with minimal exposure to oxygen and water vapor, thereby retaining a stable EPR signal.