Radiation
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In physics, radiation is the emission or transmission of energy in the form of waves or particles through space or a material medium. This includes:
Some kinds of ionizing radiation can be detected in a cloud chamber.
Radioactive decay
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Radioactive decay is the process by which an unstable atomic nucleus loses energy by radiation. A material containing unstable nuclei is considered radioactive. Three of the most common types of decay
Pierre and Marie Curie in their Paris laboratory, before 1907
Taking an X-ray image with early Crookes tube apparatus in 1896. The Crookes tube is visible in the centre. The standing man is viewing his hand with a fluoroscope screen; this was a common way of setting up the tube. No precautions against radiation exposure are being taken; its hazards were not known at the time.