An apparatus includes a radiation source to output radiation after power is supplied. The apparatus also includes a power source to supply power. The apparatus includes an ambient-activated switch electrically coupled between the radiation source and the power source. The ambient-activated switch is to switch to an open position while a value for an ambient characteristic for the ambient-activated switch is at an ambient level for a surface of the Earth. The ambient-activated switch is to switch to a closed position while the value for the ambient characteristic for the ambient-activated switch is at a downhole ambient level, wherein the ambient-activated switch is to electrically couple the power source to the radiation source while in the closed position.