Answer to Question #654 Submitted to "Ask the Experts"

Q

I understand Wilhelm Roentgen's contribution to physics. My question is more directed toward the field of nuclear medicine. How were gamma rays then discovered because of x rays?

A

Your question is a very good question. Most people do not know by whom or when gamma rays were discovered. To be sure, much of modern nuclear medicine relies on the use of gamma rays for scans, studies, and PET. So, let me try and give you an answer.

Paul Villard, a French physicist working in Paris at the same time as Marie and Pierre Curie, is credited with discovering gamma rays. In 1895, Roentgen discovered x rays and shortly thereafter Becquerel discovered radioactivity of uranium salts. Many researchers in France, England, Germany, and the United States jumped on the new discoveries trying to find new things, elements, and "rays." Moreover, they were delving into the atom, discerning structure and processes. It was a very exciting time in physics. Names of the top scientists of that time reads like a Who's Who—Bohr, Rutherford, Becquerel, Curies, Planke, Thompson . . . and shortly thereafter, Einstein.

Paul Villard's main interest was in chemistry, which guided him into his studies of cathode rays, x rays, and "radium rays." His experiments in radioactivity led to the unexpected discovery of gamma rays in 1900. Villard recognized them as being different from x rays because the gamma rays had a much greater penetrating depth. He had discovered they were emitted from radioactive substances and were not affected by electric or magnetic fields. These came to be called gamma rays by another scientist, Ernest Rutherford. It wasn't until 1914 that Rutherford showed that they were a form of electromagnetic (EM) like light only with a much shorter wavelength than x rays.

Now we know that gamma rays are a form of EM radiation similar to x rays. Gamma rays tend to have a higher energy and a shorter wavelength than x rays do. However, the dividing line between these two forms of radiation is not clearly defined. Scientists typically apply the term gamma ray to EM radiation with energies above several hundred thousand electron volts. One electron volt is the amount of energy gained by an electron as it moves freely between two points with a potential difference of 1 volt. What I like to try to think of is that an unstable nucleus or nuclear process (like annihilation, isometric transitions, etc.) gives off gamma rays, and x rays are involved in energy transformation of electrons.

Uranium and other radioactive elements emit alpha particles or beta particles from their nuclei when they transform into new elements. An instant later, these nuclei may give off gamma rays.

A nucleus may also emit a gamma ray alone in an isomeric transition. In this transition, the emission of the ray does not follow a change in the composition of the nucleus. Rather, the nucleus merely loses a certain amount of energy.

It is of interest to me that Villard did not receive a Nobel Prize for his work, while Rutherford, the Curies, Bequerel, and Roentgen all did.

A good Web site on the history of the discovery of radiation, written by Michael Fowler at University of Virginia, is Rays and Particles.

There is a good article in Comptes rendus de l'Académie des Sciences - Série IV - Physique Astrophysique ISSN: 1296-2147 2000 - IV - Volume 1 - Number 7 - pp 965-973, published by Elsevier Science. This article is by Leif Gerward and André Rassatas and is a centennial tribute to Villard's discovery of gamma rays and can be seen (in French) at La Découverte des Rayons Gamma.

Bruce Busby
RSO, Genentech