Radium-226 (Ra-226) is a naturally occurring radioactive isotope of radium, part of the uranium-238 decay series. It has a half-life of approximately 1,600 years and decays by emitting alpha particles, eventually forming radon-222, a radioactive gas. Ra-226 also emits gamma radiation, making it detectable by gamma spectrometry. Due to its radioactivity, Ra-226 is both a radiological hazard and a valuable tool in specific applications.

Ra-226 was historically used in luminescent paints for watch dials, instrument panels, and other devices, though this practice was discontinued due to health risks. Today, Ra-226 is employed in medicine for cancer treatment, particularly in brachytherapy, where its strong radiation is used to target tumors. Additionally, it is used in calibration sources for radiation detection instruments and in some research applications.

Ra-226 is found in trace amounts in uranium ores, such as pitchblende, and contributes to natural background radiation. It may also be present in groundwater and soil in areas with high natural uranium content. Due to its long half-life and radioactive properties, Ra-226 is subject to strict regulation and careful handling to minimize environmental and health risks.