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Discovery of radioactivity by Henri Becquerel 100 years ago - "Radioactivity" laboratory of PTR/PTB founded as early as 1912


On February 24, 1896, Henri Becquerel reported before the Académie des Sciences in Paris that a specific uranium-bearing crystal which had been brought to shine under the action of sunlight and had then been placed on a photographic plate wrapped in dark paper had blackened the photoplate. As early as one week later he recognized that the crystal produced the same blackening effect even when it was in the dark and did not shine at all. Even when thin glass plates or tin foils were placed between crystal and photoplate, this phenomenon did not disappear. The crystal obviously emitted a penetrating radiation similar to the X-radiation which had been discovered shortly before. With this, a new natural phenomenon had been discovered, which two years later was referred to by Marie Curie as "radioactivité". For the moment, radioactivity was observed only in uranium-containing substances.

The professional circles at home and abroad were informed about Becquerel's discovery, but they were not much interested in it, the less so as the radiation was weak and a great number of experts did not quite believe it. The scientists had thrown themselves into the investigation into X-radiation, for the world was fascinated by the perspectives which the application of this far more intense radiation opened up in the field of medicine. So there was not much time left for the study of radioactivity.

Only when the Curies appeared on the scene in 1898 and discovered the highly radioactive radium was life put into this area of research. The development that followed ultimately led to a new concept of physics. Radioactivity was the basis for the theories advanced in the 20th century about the structure of matter and the structure of the atoms. The credit for having laid the foundation of this goes to Becquerel.

In Germany, in the first years after radioactivity had been discovered, it was chiefly Julius Elster and Hans Geitel, teachers at the Wolfenbüttel Grammar School, and Fritz Giesel from Braunschweig, who devoted themselves to the study of the properties of the "Becquerel rays" and whose publications won international recognition.

Today, after a hundred years of development in this area, on account of our knowledge about the potential hazards of the radiation emitted by radioactive substances, after the discovery of nuclear fission and after catastrophes like those of Hiroshima and Chernobyl, almost anything having to do with radioactivity arouses fear. This is understandable. But on the other hand, we must realize that we are always surrounded by natural radioactivity in the air (radon), in rocks and in ourselves (potassium-40) and that any living thing has been exposed to ionizing radiation since it came into being. Furthermore, radioactive substances are today being used to good effect in research, medicine and industry.

As early as 1912, the Physikalisch-Technische Reichsanstalt (PTR) in Berlin founded its Laboratory for Radioactivity whose first head, until 1925, was Hans Geiger, the inventor of the Geiger counter. Since World War II, the Physikalisch-Technische Bundesanstalt (PTB), the direct successor to the PTR, has carried on this work. The intensity of the radiation of radioactive substances is precisely determined with the aid of sophisticated measuring equipment, and so-called activity standards for the calibration (verification) of measuring equipment are made available to bodies interested. The quantity measured is the "activity", the unit is the becquerel, symbol: Bq. In a radioactive substance with an activity of 1 Bq, one radioactive nuclear transformation per second, with radiation being emitted, takes place on an average.

The PTB's work is an important prerequisite for the reliable and uniform measurement of the activity in many authorities of the federal and state governments responsible for the monitoring of radioactivity in the environment, and at the premises of the manufacturers and users of radioactive sources and drugs.

On March 7, 1996, at 14.00 hours, on the occasion of the centenary of the discovery of radioactivity, the PTB will hold a colloquium on

"Radioactivity - discovery, measurement, application".

to which all those interested are invited.