Wilhelm Roentgen (1845-1923)
The German physicist was first to observe x-rays in 1895. While working with cathode ray instruments, Roentgen was surprised to find a flickering image cast by his instruments. Further tests showed that paper, wood, aluminum and other materials were transparent to these strange rays. Although lead glass transmitted light and not these rays, wood stopped light but the rays passed through. Roentgen named the new rays "X" to indicate they were unknown. The basic principle of the x-ray tube has not changed significantly since Roentgen's 1895 discovery. Current applied to a metal cathode produces free electrons. The x-rays are produced when the rapidly moving electrons are suddenly stopped as they strike the metal target of the tube. Roentgen, awarded the first Nobel Prize in physics (1901) for discovering x-rays, did not realize the hazards--during the next decades more than 100 people died from exposure to x-rays. The image of Mrs. Roentgen's hand was the first x-ray to look inside the human body.

Henri Becquerel (1852-1908)
In 1896 Becquerel put his wrapped photographic plates away in a darkened drawer along with some crystals containing uranium. Much to his Becquerel's surprise, the plates were exposed during storage.  The phenomenon was found to be common to all the uranium salts studied and was concluded to be a property of the uranium atom.

Marie Curie (1867-1934)
Pierre Curie (1859-1906)

Working in the Becquerel lab, Marie and Pierre Curie began what became a life long study of radioactivity. Becquerel had already noted that uranium emissions could turn air into a conductor of electricity. Developing sensitive instruments to measure conductivity allowed the Curies to study the  radiation generated by various elements. In 1898 the Curies tested pitchblende, an ore of uranium, for its ability to turn air into a conductor of electricity. The Curies found that the pitchblende produced a current 300 times stronger than that produced by pure uranium. They tested and recalibrated their instruments, and yet they still found the same puzzling results. The Curies reasoned that a very active unknown substance in addition to the uranium must exist within the pitchblende. In the title of a paper describing this hypothesized element (which they named polonium after Marie's native Poland), they introduced the new term: "radio-active."

After much grueling work, the Curies were able to extract enough polonium and another radioactive element, radium, to establish the chemical properties of these elements. The Curies shared the Nobel Prize for Physics (1903) with Becquerel. It remained to be shown whether radium was indeed a chemical element. After Pierre's death, Marie isolated a small amount of radium from several tons of minerals and determining its atomic weight. In 1911 Marie received a Nobel Prize in Chemistry.

The Curies suffered from exposure to radiation. Pierre was in constant pain and had difficulty standing upright. The skin on Marie's fingers was cracked and scarred. Both of them constantly suffered from fatigue. They evidently had no idea that radiation had a detrimental effect on their health. Pierre, who liked to say that radium had a million times stronger radioactivity than uranium, often carried a sample in his pocket to show his friends. Marie liked to have a little radium salt by her bed to shine in the darkness. The papers they left behind gave off pronounced radioactivity. Even today, researchers wishing to view the Curie's three black notebooks from 1897-1900 must sign a certificate to work at their own risk. Pierre died in 1906 after being run over by a horse-drawn wagon; almost certainly due to radiation exposure, Marie died of leukemia at age 66 in 1934.

Ernest Rutherford (1871-1937)
In 1897 Rutherford directed the radiation emitted by radium between charged electric plates. He found 3 types of radiation emitted:

Alpha (a) rays identified as positively charged helium ions (He+2).
Beta (b) rays identified as high speed electrons (e^-1).
Gamma (g) rays identified as electromagnetic radiation of very short wavelength (high energy).

In 1911, Rutherford bombarded gold foil with alpha particles. Because most alpha particles passed through the foil undisturbed, Rutherford concluded that atomic particles consist primarily of empty space surrounding a well-defined central core (nucleus).
In experiments that newspapers referred to as "splitting the atom," Rutherford (1919) bombarded nitrogen with alpha particles to prepare oxygen-17.

Rutherford believed physics to be the only science requiring real discoveries and said, "In science there is only Physics, all else is stamp collecting." Ironically, Rutherford was awarded the 1908 Nobel Prize in Chemistry. After his death in 1937, Rutherford was buried in Westminster Abbey near Sir Isaac Newton.

Positrons
In 1930 Paul Dirac calculated the existence of electrons with positive charges. These "anti-electrons" would be expected to have the same mass as the electron, but opposite electric charge. In 1932 Carl Anderson was examining tracks produced by cosmic rays in a cloud chamber. One particle made a track like an electron, but the curvature of its path in the magnetic field showed that it was positively charged. He named this positive electron a positron. We know that the particle Anderson detected was the anti-electron predicted by Dirac. An electron and positron annihilate one another producing two gamma rays (b^- + b⁺® g + g).

Irene Curie-Joliot (1897-1956), the daughter of Marie & Pierre, and her husband Frédéric Joliot prepared phosphorus-30 by bombarding aluminum with alpha particles..

Phosphorus-30 does not occur in nature and is radioactive. This was the first artificial radioactive substance ever prepared. Aside from the three natural types of radioactivity (a,b,g), artificially made nuclei can undergo:

Both positron emission and electron capture tend to occur for radioactive isotopes that need to convert a proton into a neutron. The Curie-Joliots were awarded the Nobel Prize in Chemistry in 1935 for discovering artificial radioactivity.
Tunneling Effect
Consider 200 people in a room and 100 can exit in one minute. Now consider 100 people in the same room but only 50 can exit in a minute. And if 50 people are in the same room, now only 25 can exit in a minute. This is essentially how radioactive decay works. Using a statistical approach, Rutherford invented the term half-life (time needed for half the nuclei to decay). Although he could not predict the time needed for a single atom to decay, Rutherford was able to calculate the time required for a group of atoms. Life insurance companies use this approach to predict average life spans but not year of death for an individual.

A theory known as the "tunneling effect" has been used to explain radioactive decay. If an a particle in motion strikes a barrier many times, there is a probability that the particle will pass through the barrier--an a particle must strike the nuclear barrier about 10³⁰ times before leaving. Assume QQ undergoes a emission with a half-life of one hour. If 100 atoms of QQ present initially, 50 will be successful in tunneling their way out of the nucleus. During the next hour, 50 nuclei remain. Since half as many a particles strike the barrier during the second hour, only 25 can tunnel out.