The Discovery of Spontaneous Radioactivity

In 1896, Becquerel had registered the very first spontaneous radioactivity while working on x-rays. A mere accident allowed him to detect remarkable, not-seen-before phenomenon.

To some degree, all discoveries are like rolling a dice. What, once thought out, is kind of funny; because all initial conditions must be met, without any knowledge about them. Like walking around a place would surprisingly lead to discovering Atlantis. From radioactivity through penicillium and far beyond, this scenario repeats all the time.

Becquerel’s experimenting set was:

• photographic plates wrapped in thin black paper to prevent appearing of casual images;
• fluorescent mineral, for they can absorb radiation: potassium uranyl sulfate.

He used this mineral to expose it to the sunlight for absorbing radiation, which became much brighter with time; I am sure you have played with fluorescent materials in your lifetime. Once it was bright enough, he put it on the plate wrapped in the protective black paper and waited. He let it be that way for a moment, after which he took the orb aside and unwrapped the plate from its protective layer. Unsurprisingly, the orb’s shadow image appeared on there, meaning emission of the earlier absorbed radiation had to get through the protective layer; at least, he thought that was the case.

However, Becquerel was about to change his mind. On 26th February 1896, he had prepared probes, as usual, but decided to abandon testing as it was an overcast day, so he put them in a drawer. He had not known yet, but this decision was his life’s work.

As he was unwrapping the plate from protective paper, he was expecting a very dim image at best, and I can only imagine how shocking seeing the bright one had to be for him.

There is only one explanation for this fact; namely, the orb had to radiate itself. Becquerel had added two to two and put it forward as a hypothesis.

To work it out, Becquerel changed the experiment slightly. He enclosed the orb inside a lead block that had a narrow but deep opening made to let it in. If the hypothesis was right, an image should appear on the screen only above it. And indeed, it was the case.

But, it is not over yet. At that time, Maxwell’s equations were few decades old, and so, he knew that magnetic fields influence paths of electrical charges.

He had repeated the experiment in the presence of a magnetic field and discovered that a single point on the screen now had appeared at three places. Meaning that a single beam had to triple: today are known as α, β, γ rays.

This way, he discovered spontaneous radioactivity for which got a Nobel Prize in 1903, along with Marie and Pierre Curie, who advanced investigation upon it.