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| Fig. 1: By accidentally exposing his photographic plate to uranium, Becquerel discovered the phenomenon of radioactivity. (This photograph is in the public domain, due to copyright expiration. The original is published in Becquerel's 1903 Nobel lecture.) |
Around a hundred years ago, the dedicated work of a group of scientists changed the world by ushering in the nuclear age. What started out as merely an accident, the discovery of radioactivity would lead to great and ominous innovations from energy sources to weaponry. In this article, I plan to give a brief description of the beginning of this period as well as explain the basics of the three modes of radiation.
In March of 1896, French physicist Henri Becquerel made an incredible discovery purely by accident. At the time, Becquerel was studying the phenomenon of phosphorescence (the property of some materials to later reemit some of the light they absorb) using crystals and photographic plates. These plates, like photographic film, leave the image of a particle, such as light, that interacts with it.
In preparation for an experiment that required bright sunlight, Becquerel wrapped these crystals, which contained uranium, in the plates and placed them in a dark drawer. However, he later discovered that the plates had been exposed while in the drawer, meaning that some element in these crystals themselves emitted rays—would later be termed nuclear radiation [1]. His "photograph" can be seen in Fig. 1.
Nuclear radioactivity is the outward emission of radiation in an unstable nucleus. This radiation can take the form of sub-atomic particles (α or β particles) or non-visible light (gamma rays). The nucleus, which forms the heart of an atom and is surrounded by electrons, is made up of a certain number of protons and neutrons. An unstable nucleus is one where there is an excess in the number of protons, neutrons, or energy. In an effort to become more stable, the atom undergoes a form of sudden change through nuclear radiation.
Through the efforts of many scientists, the properties of this phenomenon were slowly discovered. Among those, Marie Curie discovered that only certain elements emitted radiation. It was also discovered that nuclear radiation occurs in three modes (α, β, and γ), each with their own properties.
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| Fig. 2: The number of counts versus the alpha particle’s energy in Be-8 decay. [2] The sharp peak shows that most of the particles are emitted with the same energy. |
An alpha particle consists of a Helium nucleus — 2 protons and 2 neutrons bound together. Some unstable nuclei will emit an alpha particle to become more stable. For example, Be-8 has a total of 4 protons and 4 neutrons and is unstable. Via α decay, it expels 2 protons and 2 neutrons, becoming He-4 (which also consists of 2 protons and 2 neutrons). α particles have little range and can typically be stopped with a sheet of paper or human skin.
The energy of the emitted alpha particles depends on the specific atom and its number of neutrons, or in other words, its isotope. It has been found that, for a certain isotope, the energy of the alpha particles emitted is stable. We can see this through Fig. 2. In this graph, the number of particle counts is plotted versus the alpha particle energy for Be-8 decay.
This data was collected by measuring how far the alpha particles traveled in a certain material, which, in this case was mostly Li-7. Through calculations, this then shows the starting momentum of the alpha particle, and hence the initial energy of the particle. From this graph, it can be seen that most of the particles emitted had a specific, constant energy [2].
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| Fig. 3: The number of counts versus the beta particle’s energy for C-14 decay. [3] The continuous spectrum shows that the beta particles are emitted in a wide range of energies. This eventually led to the discovery of the neutrino. |
β decay occurs when a neutron decays into a proton, beta particle, and a neutrally charged particle called a neutrino (or antineutrino). β particles can either take the form of an electron or its antimatter counterpart, the positron. β particles are emitted at much greater speeds than their α counterparts, and can be stopped by a few millimeters of aluminum.
One example of β decay occurs in C-14, which consists of 6 protons and 8 neutrons. This isotope is important in determining the age of archaeological and geological samples and is commonly referred to as carbon dating. Via β decay, a neutron in C-14 becomes a proton and an electron, with an antineutrino being emitted as well. As a result, the isotope gains a proton, loses a neutron, and becomes N-14 with 7 protons and 7 neutrons.
Unlike α particles, β particles can be emitted at different energies. Fig. 3 shows a graph of the number of β particles emitted versus the specific energies of these particles. Using similar methods as in Fig. 2, the energies of each particle can be deduced and then plotted. Looking at Fig. 3, one can see that there is no sharp peak [3].
Cases like these, where the energy of the emitted particle can be widely spread, is called a continuous spectrum, and the fact that this occurred with β radiation confused scientists for years. In order to explain this phenomenon, the famous physicist Wolfgang Pauli theorized that another particle must be released in β decay [4]. His theory turned out to be correct, and this particle was called the neutrino. Hence, it was discovered that with β decay, two particles were emitted, the β particle (electron or positron) and the neutrino or antineutrino.
γ rays are a form of electromagnetic radiation, or light rays, emitted at a frequency greater than that of the visible spectrum. γ radiation is an offspring of α or β decay when the nucleus is left in an excited state. The nucleus then releases the excess energy through a γ ray. This mode of radiation releases the unit of light, or photon, at very high energies, requiring a dense material like lead to shield from it.
The discovery of radiation was accidental, but subsequent scientific experiments paved the way to greater understanding of the phenomenon. Identifying the three modes of radiation helped in the discovery of elementary particles such as the neutron and neutrino. Soon after, scientists began to harness the power of radiation, from smoke detectors and nuclear power plants to the atomic bomb. Ultimately, this inadvertent discovery proved to be a powerful contributor to many innovations of the 20th century and beyond.
© 2009 Matthew Sahagun. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] A. H. Becquerel, "On the Invisible Rays Emitted by Phosphorescent Bodies," Comptes Rendus 122, 501 (1896).
[2] R. T. Frost, and S. S. Hanna, "Alpha Spectrum of Decay of Li8," Phys. Rev. 99, 8 (1955).
[3] A. Moljk, and S. C. Curran, "Beta Spectrum of C14 and S35," Phys. Rev. 96, 395 (1954).
[4] W. Pauli, "Letter to Lisa Meitner and Others," 4 Dec 30, in K. v. Meyenn, ed., Wolfgang Pauli - Scientific Correspondence with Bohr, Einstein, Heisenberg and Others, Vol. II (Springer, 1985).