Electron spin resonance (ESR) measures the number of charges occupying deep traps in the crystal bandgap. By measuring the change in absorption of microwave energy within a continuously varying strong magnetic field, this method detects the number of "unpaired spins" of electronic charges trapped at various defects in the mineral lattice. The principle of ESR dating is that radiation damage occurs in minerals as a result of uranium uptake, and external effects. This damage is usually repaired in living tissue, but in dead tissue it accumulates. If the method of uptake can be judged, then the approximate age of the tissue can be deduced from the extent of the radiation damage.