13. Gamma rays and X-ray

Wilson also tried using radium bromide to produce gamma-radiation. He saw the same cloud tracks as he had seen for beta-particles, but the tracks were in all directions, rather than directed from the source. The walls of the chamber were absorbing the gamma-radiation and emitting beta-radiation from all directions.

Photograph of a typical X-ray cloud

Finally, Wilson tried using X-rays. He saw 'minute streaks and patches of cloud' throughout the region in the path of the beam. The clouds were mainly short thread-like objects, few of them moving in straight lines and some of them even looping round. Wilson saw this as 'a very direct proof' that X-rays liberated energetic electrons in the gas, and that these electrons caused ionisation along the path of the X-rays.

Wilson perfected his cloud chamber by 1912, and used it to produce many wonderful photographs of the alpha, beta and X-ray tracks. With his cooperation cloud chambers were manufactured commercially by the Cambridge Scientific Instruments Company, and remained the principle detector for studying particle tracks until the invention of the bubble chamber in the 1950s.

The original cloud chamber is on display in the museum at the Cavendish Laboratory.

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