The Beginnings of Nuclear Physics. In the final decades of the nineteenth and first few decades of the twentieth centuries, physicists would begin to unlock the mysteries of radioactivity.
In the beginning of radioactivity was French physicist Henri Becquerel. Like his father before him (Physicist Alexandre-Edmond Becquerel), Becquerel was fascinated by the subject of atomic phosphorescence (substances that glow in the dark, especially after having been energized by exposure to light), and performed tests on various phosphorescent compounds.
In 1896, Becquerel noticed an odd thing when he was experimenting on Potassium Uranyl Sulfate (the “uranyl” implies that the compound contains uranium, which is important). He found that, even after having not been exposed to the sun, something from inside the compound would “radiate” outward and leave traces on a photographic plate nearby, even if the compound was wrapped in an opaque material.
Whatever was coming out of the compound penetrated right through the material like X-rays (which had only just been discovered a year previously, and which were found to be merely high-energy forms of electromagnetic waves).
Because this had occurred even without being “energized” by the sun’s light, it was clear that whatever was causing this radiation had to be some heretofore undiscovered property of the chemical compound itself.
Though he did not yet realize the full potential of what he had found, Henri Becquerel had unknowingly discovered atomic radiation.
The Work of the Curies
It would take the efforts of several other people before radiation had begun to be understood, including Marie Curie (remarkably, one of very few women to have made her mark on the world of physics up until very recently), her husband Pierre, and Ernest Rutherford.
The Curies in particular began to perform further studies in radioactivity (a phrase which Marie Curie herself coined in 1898). Madame Curie showed that several different compounds could be shown to be radioactive, but that they all seemed to have the element Uranium (atomic number 92) in common. She later found that Thorium (atomic number 90) was also radioactive. These two elements are the heaviest which occur naturally on Earth, a fact which I’m sure gave Curie some pause. Why is it only the heavy elements which emit these strange “radioactive” particles?
Rutherford’s Three Types of Radiation
The next great step in understanding radioactivity came from New Zealander Ernest Rutherford (“The Father of Nuclear Physics,” he is sometimes called).
Rutherford (who would later be commemorated with his very own radioactive element – Rutherfordium) noticed in 1898 that when he allowed a sample of Uranium to radiate into a magnetic field, whatever particles were emanating from the sample were splitting and going in three different directions. So there were apparently three different types of radiation, which he called alpha, beta and gamma, after the first three letters of the Greek alphabet. What’s more, Rutherford noted the different properties of the three types of radiation and made some dramatic conclusions:
First, beta particles were deflected in such a way as to show that they must have been negatively charged and very light (for their angle of deflection was rather extreme). Perhaps, Rutherford correctly thought, they were electrons escaping from the atom at a high velocity, which would explain both properties (for these matched all the properties known for electrons at the time, so it made perfect sense).
Alpha particles, on the other hand, were deflected in such a way so as to be positive in charge, but the angle was rather slight, showing that they were much more massive than electrons. In 1920, Rutherford hypothesized that what was actually being ejected in alpha particles was the nucleus of a hydrogen atom, which consisted of a single particle (which he himself had discovered) called a proton. But Rutherford was actually wrong in this. It was later discovered that it was actually helium nuclei being ejected – consisting of two protons as well as two neutrons, but the physics community had no need for neutrons yet in Rutherford’s day, so this fact would remain unknown for some time to come.
The Gamma radiation, however, was an especially tricky one. It became clear to Rutherford as well as to other experimenters that gamma radiation does not seem to consist of particles at all. Apparently, gamma radiation existed in the form of an electromagnetic wave (light) of a very high frequency and energy, along with a very short wavelength which made it very penetrable. These gamma rays are much more intense than the X-rays or even the ultraviolet light which causes the sun’s rays to burn human skin.
Thus, with the work of these fine physicists and others, the strange phenomenon of radioactivity had begun to unravel, and now that it was understood what radiation consisted of, the next step would be to explain why. After all, there seemed no reason that certain atoms should just spontaneously begin emitting random particles, does it?