A radioactive source with a high energy density (10⁵kJ/cm³) can generate thermal energy due to the kinetic energy of emitted particles. Sources such as cesium-137 have a half-life of 30 years and a power capacity of 0.015 W/gm. This method can generate power in the Watt-to-kW range, but isn't practical in low-power sensor levels for IoT deployments. Space vehicles have used this technology for decades. Promising developments using MEMS piezoelectronics that capture electrons and force a micro-armature to move can create mechanical energy that may be harvested. A secondary effect of radioactive decay is the relatively weak power density profile. A radiation source with a long half-life will have weaker power density. Thus, they are suitable for bulk-charging supercaps to provide momentary energy when needed. The final issue with radioactive sources is the significant weight of lead shielding required. Cesium-137 requires 80 mm/W of shielding, which can add significant cost and weight to an IoT sensor.