We will be fusing deuterium gas in our nuclear reactor. But what is deuterium and what makes it special? Deuterium is one of two stable isotopes of hydrogen. Deuterium has one proton and one neutron in the nucleus. Protium, the other stable isotope, has a proton and no neutrons. Thus, deuterium is twice as massive as protium. Tritium, while an unstable isotope, is the only other naturally occurring isotope and has two neutrons in addition to a proton in the nucleus.
Unlike protium and deuterium, tritium is radioactive and decays into helium-3 through beta decay. Protium is the most abundant form of hydrogen. Protium makes up about 99.98% of naturally occurring hydrogen in the ocean, leaving less than 0.02% for deuterium, and even less for tritium. Naturally occurring tritium is extremely rare.
Figure 1 – Atomic structure of the three naturally occurring isotopes of hydrogen
Deuterium was first discovered in the 1930s. After the isotopes of oxygen were discovered in the 1920s, it was theorized that hydrogen also had isotopes. Harold Ulrey and George Murphy first found evidence in 1931. They used the Balmer series to find how much the heavy isotope should be redshifted. They used a spectrograph to measure the lines. Because the isotope is very rare compared to protium, the lines indicating deuterium were faint and hard to find. As a result, Ulrey did not feel confident publishing his results. In 1932, neutrons were discovered by James Chadwick, answering many questions about isotopes and making their existence more significant. Eventually, with the help of Ferdinand Brickwedde making the lines easier to read, Ulrey, Murphy, and Brickwedde jointly published their findings in 1932. In 1934, Ulrey was awarded with the Nobel Prize in Chemistry for the discovery of deuterium.
Figure 2 – Harold Ulrey
There are a couple different deuterium reactions that we could choose from: deuterium-deuterium, deuterium-tritium, and deuterium-helium-3. Deuterium-tritium fusion actually creates the most energy of any of the strictly hydrogen isotope fusions. Deuterium-helium-3 creates the most energy, but helium-3 is very hard to get. Using radioactive material (tritium) is an added layer of danger that is unnecessary for our educational fusor. Therefore, we will be doing a more safe and simple fusion of deuterium-deuterium.
Since deuterium is stable (it won’t decay into another element after a finite amount of time), it has been around since minutes after the Big Bang. However, deuterium can fuse with itself and create helium. This commonly happens in star and galaxy creation/formation. Therefore, not all the deuterium in the early universe is still around today. Additionally, any deuterium around today has never been burned by stars and is “pure”. The fact that deuterium fuses with itself makes it a perfect source of fusion for our nuclear reactor. There are two reactions from deuterium-deuterium that are equally likely.
Figure 3 – The two different deuterium-deuterium fusion reactions and corresponding diagrams
Since the purpose of our fusor is educational rather than trying to optimize the energy output, we care more about the first reaction. This reaction produces helium-3 and neutrons. If we can create neutrons, we will have achieved fusion and we will be able to do further experiments to learn more about nuclear physics!