Films

Photographic films are not only sensitive to light but also to all kinds of ionizing radiation, i.e. X-rays, gamma rays and energetic particles.

When a radioactive object is placed on a photograhic film for some time and the film is then dveloped, it is more or less blackened depending on the spatial distribution of radioactivity and absorbing material in the object. The object x-rays itself, in a way. This method is called autoradiography.

This image was produced with an x-ray film that was wrapped light-proof with aliminium foil. I put three radioactive sources directly on the wrapped film and left them there for three days. The film was then developed in the usual way. The three sources were (from left to right):

An old watch containing radium paint with the glass removed, dial towards film. Detail
Another watch, slightly less active, with front glass. Detail
A plastic box containing thorium nitrate, and a metal toy figure between film and box. Detail

Autoradiography is used extensively in biotechnology. E.g., a mixture of DNA fragments, of which some or all are radioactively marked, can be spatially separated using gel electropheresis. Autoradiography of the gel makes the marked fragments visible as dark spots. As the exposure time can be varied over many orders of magnitude, the sensitivity of the method can be adapted to the number of marked fragments to be detected, from very high to very low concetrations.

Another application is analysis of air for "hot particles", small dust particles that are highly radioactive. The number of such particles per volume of air may be low, and the averaged radioactivity of the air therefore low, but when a particles gets trapped in the lung, its concentrated radioactivity can cause cancer. For analysis, air is sucked through a filter and the filter is then autoradiographed. Any "hot particles" will be visible as prominent dark spots.

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