HEAVY HYDROGEN: DEUTERIUM, TRITIUM, AND BEYOND
Although hydrogen is known as the lightest element, it has iso-topes that defy this description. From deuterium to 5H, hydro-gen with added neutrons has fascinating properties that assist research and technology in many regimes.
Researchers have been interested in heavy hydrogen since the early 1900s when Harold Urey, a chemistry professor at Colum-bia University, developed a method of distilling liquid hydrogen to make deuterium—hydrogen with a neutron connected to the proton—an achievement for which he won the 1934 Nobel Prize in chemistry.
When used in place of hydrogen, deuterium or 2H (some-times designated as D) results in water approximately 10 percent denser than normal. Termed “heavy water,” D2O is harmless in small doses and can therefore be used safely as a tracer in the body, most commonly in measuring a subject’s metabolic rate. Heavy water is also used as a neutron moderator, meaning it is able to slow neutrons by collisions without absorbing them. This process is crucial for the chain reaction in nuclear reactors, where fast neutrons are produced by the fission process, but slow or thermal neutrons are more likely to induce fission.
The universal abundance of deuterium is the subject of ongoing investigation. All deuterium nuclei formed a few minutes after the big bang, providing the basis for the heavier elements. It is understood to be consumed solely by stellar burning—the nuclear fusion process in stars. New observational results, how-ever, show that universal abundance is about 20 percent more than projected under this scenario. This offers spectroscopists and theorists new channels of exploration.
Hydrogen with two neutrons, named tritium (3H or T), is less stable than the lighter isotopes and is radioactive, with a half-life of 12.26 years. Produced by cosmic ray protons colliding with nitrogen in the upper atmosphere, trace amounts are found in air and less abundantly in water. Tritium can also be made in the laboratory by bombarding lithium atoms with neutrons, which is the source of tritium for use in fusion energy research.
Deuterium-tritium fusion is considered the most likely process to result in a fusion reactor suitable for electricity production. In this process, a confined gas of deuterium and tritium atoms must be heated to nearly 100,000,000 K. Each fusion of D with T produces a helium nucleus, or alpha particle, and a neutron. The 17.6 million electron volts (MeV) of energy released per reaction are substantial, but only 3.5 MeV are carried away by the charged particle—the more useable form of energy for electricity. Although the method is well understood, it is still highly inefficient; much more energy must be put into the process than is produced. Fusion is not expected to be a viable source of power for humankind for at least the next 50 years.
The heavier isotopes of hydrogen, 4H and 5H, have been produced in the laboratory via tritium collisions with deuterium and tritium, respectively. Extremely short-lived (on the order of 10–23 seconds), these atoms most likely also exist in extreme temperature and density regimes in stars, though they cannot be detected spectroscopically.
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| Schematic of deuterium-tritium fusion |
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