The discovers of radioactivity

H. BECQUEREL (1852-1908) (source from Nobel Lectures, Physics 1901-1921) H. BECQUEREL (1852-1908) (source from Nobel Lectures, Physics 1901-1921)

Radioactivity is a natural phenomenon that has existed every since atoms were formed several billion years ago, at the beginning of the universe. All atoms are built on the same model, with electrons orbiting around a nucleus made up of protons and neutrons.

 

But different atoms have different properties. Some are stable and remain forever the same. Others are unstable and, at some point in time, release part of their mass and energy to gain greater stability. We refer to this phenomenon as radiation and more specifically as radioactivity.

 

Radioactivity was discovered in 1896 by Henri Becquerel (1852-1908) while working on X-rays, discovered accidentally by Wilhelm Rongten (1845-1923). Becquerel wanted to see if fluorescent substances emitted radiation capable of exposing a photographic plate through black paper.

 

One day, Becquerel left a uranium compound near one of his photographic plates and later discovered that the plate was fully exposed. After several other experiments, he decided that the phenomenon was related to a property specific to uranium and its compounds. Deducing that an atomic property was at work, he concluded that uranium emitted a specific form of radiation he referred to as uranic rays. This marked the discovery of radioactivity.

In 1897, Ernest Rutherford (1871-1937) observed that the radiation emitted by different radioactive bodies had different behaviors in matter. He distinguished between what he referred to as alpha and beta radiation.

P. & M. CURIE (1859-1906 and 1867-1934, respectively) (source unknown) P. & M. CURIE (1859-1906 and 1867-1934, respectively) (source unknown)

Marie Curie (1867-1934) carried out quantitative measurements of radioactivity based on the ionization that this radiation produced in air. She demonstrated, at the same time as Gerhard Carl Schmidt (1865-1949) in Germany, that thorium also emitted radiation. In 1898, together with her husband Pierre Currie (1859-1906), she announced the presence of a new element, polonium (z=84), and later the existence of radium. She coined the term radioactivity to describe the phenomenon discovered by Becquerel.

N. BOHR (1885-1962) and A. EINSTEIN (1879-1955) (source from http://www.dfi.dk/) N. BOHR (1885-1962) and A. EINSTEIN (1879-1955) (source from http://www.dfi.dk/)

The succession of discoveries that marked the end of the 19th century (e.g. X-rays, radioactivity, electrons) led Ernest Rutherford (1871-1937) to demonstrate, in 1911, the existence of a positively charged nucleus at the center of the atom. On the basis of this discovery, Niels Bohr (1885-1962) defined the first model of the atomic structure, a nucleus surrounded by electrons, in a configuration similar to that of a miniature solar system. This laid the groundwork for the development of atomic physics (physics of the atom), nuclear physics (physics of the nucleus) and, in the 1930s, particle physics.

 

These disciplines progressed by integrating the newly discovered laws of relative mechanics (Albert Einstein) governing the motion of very rapid bodies and those of quantum or wave mechanics (Planck, de Broglie, Heisenberg, Bohr, etc.) governing the motion of very small objects.

 

In 1932, the work of James Chadwick (1891-1974) on the neutron led to today's representation of the nucleus as a collection of protons and neutrons. A great many experiments were carried out to determine the laws of radioactivity.

 

Alpha, beta and gamma radiation was rapidly characterized, but it was not until 1934 that Irène (1897-1956) and Frédéric Joliot-Curie (1900-1958) discovered artificial radioactivity and a new mode of radioactive decay referred to as beta plus decay. In 1939, work on nuclear fission by Otto Hahn (1879-1968) and Fritz Strassman (1902-1980) led to the energy and military applications we know today.