In this installment on the history of atom theory, physics professor (and my dad) Dean Zollman demonstrates a truism of science—how one discovery leads to even more questions. In this case, researchers study the nature of the rays they’re observing and find that one element can indeed change into another.—Kim
By Dean Zollman
- What are the characteristics of the radiations emitted by the radioactive substances?
- What was the source of energy for the emissions?
- From where did the emissions come—inside the atom or outside?
People did not even know what to call the “stuff” that researchers such as Alexandre Becquerel and Pierre and Marie Curie were observing. The term “rays” was used by many people because of an analogy with the recently discovered X-rays. But most scientists did not know if the rays coming from uranium were similar to X-rays or something quite different. Further, they did not know if the rays from uranium differed from the rays from polonium or thorium. Sorting this situation out took several years and several physicists and chemists.
Ernest Rutherford (1871-1931) was a critical player in determining what the rays were and what was left behind when they were emitted. He was born in New Zealand and came to the Cavendish Laboratory in the University of Cambridge in the U.K. in 1895. There, he was a doctoral student mentored by J.J. Thompson, who was to discover the electron during this time period. During his stay in Cambridge, Rutherford became interested in radioactivity. After finishing his studies, he moved to McGill University in Canada. In both the U.K. and Canada, he made some remarkable measurements.
He began his investigations with uranium. Because he had no reason to believe that the emanations (as Rutherford called them) from uranium should be the same as those from other radioactive elements, he specified that he was investigating the uranium rays. Early in his research, he learned that uranium emitted two types of rays. In his experiments, he allowed the rays to pass through different types of materials. He found that some of the rays would be stopped by thin pieces of materials, even a sheet of paper. But others would penetrate through reasonably thick material. He named the ones that were easily stopped alpha rays and the ones that penetrated deeply beta rays.
The way in which they penetrated material was just the first step in finding differences between the two types of rays. At an 1899 meeting of the French Academy of Sciences, Becquerel showed that the more penetrating rays of uranium (the beta rays) could be deflected by a magnetic field. To be deflected in this way an object needs to have an electrical charge. So this result indicated that the beta rays carried an electric charge.
The Curies carried out experiments to determine the type of electric charge. Recall that Pierre Curie had devised a very sensitive instrument for measuring electric charge. In early 1900, the Curies were able to report that the beta rays from radium had a negative charge. Becquerel was able to show that these rays could also be deflected by a high voltage. Using the direction and amount of the deflection, the researchers concluded that the highly penetrating beta rays were identical to the cathode rays (electrons) J.J. Thompson had discovered. They also concluded that the rays from one radioactive element were the same as those from another.
Introducing…the Gamma Ray
Many other people were also working on similar experiments. So, while the Curies and Becquerel were making measurements in Paris, Stefan Meyer (1872-1949) and Egon Schweidler (1873-1948) in Vienna made similar measurements about the deflection in a magnetic field. They were working with a small amount of radium that they had obtained from the chemist Friedrich Oskar Giesel (1852-1927). Thus, the identification of beta rays as electrons was well established by independent measurements.
During the same time period, several researchers concluded that alpha rays were not deflected by magnetic fields. If that were correct, alpha rays would not have an electric charge. We will return to this issue in a few paragraphs.
About the same time, Paul Villard (1860-1934) wanted to study some of the properties of cathode rays and beta rays. Because he was in Paris, he was able to obtain a small quantity of radioactive material from the Curies. He found that in addition to the alpha and beta rays, another emission was highly penetrating and could not be deflected by a magnetic field. Villard stated that these rays must be similar to X-rays. (This statement must have been at least partially speculation by Villard because he had made only a few measurements. But he was right. The radiation that we call gamma rays today is electromagnetic radiation just like X-rays but they have very high energy.)
Becquerel was not ready to believe Villard’s work. He stated that “the existence of these rays could not possibly have escaped attention in the experiments of Mr. and Mrs. Curie, nor in my own experiments.” However, a few months later he had to admit that the highly penetrating rays which could not be deflected were real.
The name gamma rays for these emissions apparently came from Rutherford. Historians cannot find a specific publication where Rutherford coins the name. However, in her dissertation Marie Curie attributes the name for all three radiations to Rutherford.
Alpha rays presented a little bit of a challenge. Eventually, careful experimentation with large magnetic fields showed that the original conclusion that they were not deflected was incorrect. The deflection was small and in the opposite direction of the beta rays. The small deflection meant that alpha particles had much more mass than beta particles. The opposite direction indicated that their charge was positive instead of negative. Using magnetic fields, one can measure the ratio of the charge to the mass of a particle. Rutherford did this for the alpha particles and noted that the ratio was the same as for a helium atom. The conclusion was that the alpha particles were related to helium atoms but had a positive charge. (Later, we would learn that alpha particles are the nucleus of the helium atom. However, at this time no one knew about the nucleus.)
Transmutation Is Real, but Not What Alchemists Wanted
For further investigations, Rutherford teamed up with a chemist Frederick Soddy (1877-1956). They looked at some properties of the material that was left behind after substances such as radium and thorium emitted the alpha, beta, or gamma rays. They eventually concluded that the original materials were transforming into different elements. The solid radium, for example, would emit an alpha particle and become the gas radon. Rutherford was the first to recognize that their data were conclusively showing that one type of atom was turning into another.
Later Soddy wrote, “I remember quite well standing there transfixed as though stunned by the colossal impact of the thing and blurting out. . . . ‘Rutherford, this is transmutation.’ Rutherford’s reply was, ‘For Mike’s sake, Soddy, don’t call it transmutation. They’ll have our heads off as alchemists.’” (From “Ernest Rutherford: The ‘True Discoverer’ of Radon,” by James R. and Virginia R. Marshall, Bulletin of the History of Chemistry, Volume 28, Number 2, 2003)
Indeed, the alchemists of medieval times looked for ways to transmute elements, particularity change lead into gold. They failed, but hundreds of years later, scientists found that nature was transmuting elements. However, the end product of many transmutations was lead. Instead of changing a common element (lead) into a precious one (gold), nature was starting with a precious substance such as radium and converting it into lead.
The discovery of transmutation laid another issue to rest. If a radioactive element was changing into a different element, one could conclude that the emitted rays were coming from inside the atom. (Today we know that the particles are coming from the nucleus of the atom.)
By the early years of the 20th century, the source of energy for radioactivity was still a mystery. The solution to that mystery needed to wait for Albert Einstein and his Special Theory of Relativity.
Rutherford returned to England and became a professor at the University of Manchester. There, he used alpha particles to probe into matter at a smaller level than anyone had done. We will look at that experiment in a couple of posts. Next time, we will look at some of the models of atoms that were devised based on knowledge around the beginning of the 20th century and were the motivation for Rutherford’s continued investigation of matter at a very small scale.
Dean Zollman is university distinguished professor of physics at Kansas State University where he has been a faculty member for more than 40 years. During his career he has received four major awards — the American Association of Physics Teachers’ Oersted Medal (2014), the National Science Foundation Director’s Award for Distinguished Teacher Scholars (2004), the Carnegie Foundation for the Advancement of Teaching Doctoral University Professor of the Year (1996), and AAPT’s Robert A. Millikan Medal (1995). His present research concentrates on the teaching and learning of physics and on science teacher preparation.