The invention provides a method of detecting and monitoring ionizing radiation such as nuclear radiation. The method comprises providing a diamond radiation sensor element having a nitrogen concentration not exceeding 20 ppm and which is optimized for phosphorescent response, and exposing it to ionizing radiation. The resulting phosphorescent response of the sensor element is then monitored, typically over a period of at least 20 seconds. The invention extends to the sensor element itself, and to apparatus employing the sensor.