A new physical soft error mechanism in dynamic RAMs and CCDs is the upset of stored data by the passage of heavily-ionizing radiation through the memory array area. Alpha particles are emitted in the radioactive decay of uranium and thorium present in parts-per-million levels in packaging materials. When an alpha particle penetrates the die surface, it can create enough electron-hole pairs near a storage node to cause a random, single-bit error. Results of experiments and measurements of alpha activity of materials are reported and a physical model for the soft error is developed. Implications for the future of dynamic memories are also discussed.