The maltese cross

The first clue of an unknown phenomenon

We know today that "uranium rays" which acted on the Becquerel plate were not the alpha particles of uranium - stopped by the paper envelope - but the beta and gamma rays from close offspring.

Image of the Maltese Cross.
One of Henri Becquerel photographic plates - developed on 1 March 1896 - shows the image of a Maltese cross. The copper cross was interposed on February 26, 1896 between the photographic plate wrapped in paper and a thin glass slide supporting salts of uranium. All ended up in a dark drawer. The appearance of the cross is due to the fact that the copper has absorbed the radiation emitted from uranium salts.

Henri Becquerel had used a flask containing salts of uranium, a rare product, however available to the National Museum of Natural History. Given the radioactive period of uranium-238 and its descendants, the flask should contain the first filiation elements, that is to say, thorium-234, protactinium-234 and uranium-234.

Uranium-238 (main isotope of uranium) decays by emitting alpha particles which are quickly absorbed by the material. As Henri Becquerel's photographic plates were covered of thick paper, the alpha particles could not be the cause of the plate marks.

Among the descendant of uranium-238, protactinium-234 decays by emitting a highly energetic beta. Five to six millimeters of glass are required to completely absorb these electrons. Thus they have contributed to draw the Maltese Cross interposed by Henri Becquerel between the plate wrapped in paper and a thin glass plate of 0.1 mm withstanding uranium salts.

Remain different gamma radiations emitted by the descendants of uranium. These gamma rays have contributed to act on photographic plates. They were attenuated by the interposition of glass slides more or less thick, and sheets of aluminum or copper. Despite their low intensity and the values of corresponding attenuation coefficients, they provided the "continued success" of the experiences of pioneers.

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