WHAT IS RADIATION?

The story of radiation is the story of the atom and of subatomic particles. I should warn you that I love this story because it is one of the most fascinating and com­pelling stories in the history of science, it involves a cast of brilliant scientists, and it changed the world. So I get enthused and want to go into too much detail—at least that is what my students think. But to really understand the story, it will be necessary to learn some complicated and apparently nonsensical ideas in phys­ics. I will try to keep the technical details to a minimum, but if you really want to understand what radiation is and where it comes from, stick with me as we explore the story. I hope you will be fascinated, too.

The beginnings of the story go back to Indian and Greek philosophers who postulated that the universe consisted of space and indivisible particles that could combine to form more complex matter. The term atomos, meaning uncuttable or indivisible, was coined by the Greek philosopher Democritus in the fourth cen­tury B. C.E. While this was purely a philosophical speculation, it was a remarkable insight. More than a thousand years later, science and experimentation began to uncover just what this meant. John Dalton is credited with being the father of modern atomic theory in chemistry. In 1803 he developed the idea that elements consist of atoms, that different atoms have different weights, and that the atoms of a specific element are all alike but are different from those of other elements. He also proposed that atoms can combine in specific proportions to make up com­pounds or molecules (1).

But are atoms really indivisible, and if not, what are they made of? Nearly a hundred years after Dalton, this question began to be answered in a burst of experiments and insights at the end of the nineteenth century and the begin­ning of the twentieth century, starting with the discovery of radiation. In 1895 the German physicist Wilhelm Conrad Rontgen was studying emissions of light coming from evacuated glass tubes that contained a small quantity of gas and had a voltage applied to them between a cathode (negative) and anode (posi­tive). When he covered the tube with a black cardboard hood and applied a high

voltage across the tube he found that there were unknown rays coming from the tube that caused fluorescence in a nearby cardboard screen coated with a special material. This was astonishing because the black cardboard should have stopped any light. He concluded that there were some mysterious rays, which he named X-rays, coming from the tube, causing the fluorescence. He proceeded to insert various materials such as paper and aluminum between the tube and the screen and discovered that different materials absorbed these X-rays to differing degrees (2). He took a picture of his wife’s hand that clearly showed the bones in her fingers and a ring she was wearing. In a public lecture in January 1896, he took a picture of the hand of a colleague and thus began the field of radiology (Figure 6.1). Rontgen received the first Nobel Prize in Physics for his discovery of X-rays.1

This was just the beginning of an incredibly fertile revolution in physics, and it is all the more remarkable because, just prior to this time, physicists thought they knew nearly everything there was to know about physics. They understood—or thought they understood—mechanics, gravity, electricity and magnetism, optics, thermodynamics, and the statistical nature of gases. Lord Kelvin, the famous Irish-born Scottish mathematician and physicist, is reputed to have said in 1900, “There is nothing new to be discovered in physics now. All that remains is more and more precise measurement” (3). How wrong he and other physicists were!

The French physicist Henri Becquerel discovered natural radioactiv­ity within months after Rontgen discovered X-rays. He was actually studying

phosphorescence of certain minerals because he knew that some minerals glow when they are exposed to sunlight, and he was interested in the radiation reported by Rontgen. He had already determined that if you exposed crystals of uranium salt (potassium uranyl sulfate) to sunlight for several hours while they were lying on a photographic plate that was wrapped in sheets of heavy black paper, the pho­tographic plate became exposed. He assumed that this was because of the phos­phorescence of the uranium salt caused by exposure to the sun. He was going to do further experimentation, but because of cloudy weather he wrapped the uranium salts in paper and put them in a drawer on top of wrapped photographic plates. A few days later he took the plates out and, in one of those serendipitous moments that often result in great discoveries, decided to develop the plates. Lo and behold, he found that they had been exposed, even though the uranium crys­tals had never been exposed to the sun. He concluded that the uranium was emit­ting a type of natural radiation that could pass through the paper and expose the plates. He also showed that the radiation was different from the X-rays discovered by Rontgen, but he didn’t know what it was (4, 5).

These dramatic new discoveries by Rontgen and Becquerel were followed by the discovery of two new radioactive elements—polonium and radium—by Marie and Pierre Curie in 1898. They obtained tons of pitchblende from the mines at St. Joachimstal in the Cruel Mountains bordering Germany and the Czech Republic (see Chapter 2) and very laboriously extracted a minute amount of radioactive material that was even more active than uranium. They named the first ele­ment polonium after Marie’s native country, Poland, and the second they named radium. The story of their work is one of the most compelling in the history of science. They worked in a drafty shed—with water dripping through the roof and virtually no heat in the winter—to chemically separate a fraction of a gram of radioactive material from a ton of pitchblende (6-8). The Curies and Becquerel were jointly awarded the Nobel Prize in Physics in 1903 for their discoveries of natural radioactivity.

Sandwiched between the discoveries of Becquerel and the Curies was another critical discovery by the English physicist J. J. Thomson in 1897. He was studying what were known as cathode rays in evacuated tubes, the same kind of tubes used by Rontgen to produce X-rays. He was able to show that the rays were deflected by electric and magnetic fields and ultimately determined that they were composed of particles that had a negative charge and were much lighter than atoms.2 He also showed that the X-rays discovered by Rontgen could create these particles in air (4). He initially named the small particles “negative corpuscles,” but fortunately ended up calling them “electrons.”

Now the stage was set to upend the whole understanding of classical physics because there was no theory that could explain these discoveries of X-rays and natural radioactivity. Where, exactly, did the radiation come from? Pursuing the answer to this question led to the development of a revolutionary new branch of physics known as quantum mechanics, which explained the structure of the atom and the nucleus and led to the development of the atomic bomb and nuclear power reactors.