Deuterium

• 1. Introduction

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  Deuterium, a heavier sibling of ordinary hydrogen, once traveled in twenty-six steel drums. Those drums held the entire world supply of heavy water, and the water had been made in Norway. Shortly before the war, Hans von Halban and Lew Kowarski moved their neutron research from France to Britain. They smuggled the drums out with them.

  Why would anyone smuggle water? The answer begins with a single extra particle in the heart of an atom. It leads to questions that stretch across every scale of science. Where did this rare form of hydrogen come from, and why is so little of it left? How did chemists first find it, and who got the credit? And what is it worth to a physicist, a doctor and a bomb designer?

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• 2. An Odd Couple Nucleus

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  Two particles, one proton and one neutron, make up the deuteron, which is the nucleus of deuterium. Ordinary hydrogen, called protium, has no neutron at all. Together the two are the only stable isotopes of hydrogen.

  Scientists write deuterium as D or as hydrogen-2, and the international chemistry body allows both while preferring the second. A separate letter exists for convenience, and also because the doubled mass brings real chemical differences. Deuterium has a mass of 2.014102, roughly twice protium's 1.007825.

  The nucleus itself breaks a rule of thumb. Deuterium is one of only five stable nuclides with an odd number of protons and an odd number of neutrons. The others are lithium-6, boron-10, nitrogen-14 and a long-lived state of tantalum-180. Most nuclei built that way fall apart by beta decay.

  Deuterium survives because its proton and neutron lock into a spin-1 arrangement, which pulls them together more strongly. Nothing like that exists for two protons or two neutrons, so those pairs never hold together. The spin of 1 also makes the deuteron a boson.

  Measurement backs up this picture. The deuteron's magnetic dipole moment comes out at 0.857, which is 97.5% of the 0.879 you get by simply adding the proton and neutron contributions. The small shortfall shows a slight mixing of a second state into the nucleus.

  The deuteron has also been put to work as a target. The Sudbury Neutrino Observatory used deuterium because neutrinos can break its proton and neutron apart. The atom has a quieter signature too: every deuterium spectral line sits 0.0272% shorter in wavelength than the matching hydrogen line.

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• 3. Finding and Naming

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  In 1919, Ernest Rutherford made a passing mention of matter with mass two and charge one in his Bakerian lecture. In his picture, such a nucleus would hold two protons and one nuclear electron. The same year, Otto Stern and Max Volmer reported water diffusion experiments that they said ruled out isotopes of hydrogen. Scientists took their conclusion as settled throughout the 1920s.

  Late in 1931, Harold Urey, a chemist at Columbia University, detected deuterium by spectroscopy. His collaborator Ferdinand Brickwedde boiled five liters of cryogenically produced liquid hydrogen down to one liter. He used the low-temperature laboratory recently set up at the National Bureau of Standards in Washington, DC. The same boil-off trick had already separated heavy isotopes of neon, and it concentrated the mass-2 hydrogen enough to make the identification unambiguous.

  There was a good reason nobody had noticed it before. Seawater holds only about 1 deuterium atom for every 6400 hydrogen atoms, around 156 parts per million. That trace was too small to nudge earlier measurements of hydrogen's average atomic mass.

  In 1934, Urey published the names protium, deuterium and tritium. Gilbert N. Lewis advised him, and Lewis had floated the form "deutium". The root is the Greek deuteros, meaning second. Some British scientists, Rutherford among them, preferred "diplogen", from the Greek diploos, meaning double, with a nucleus called a "diplon". Tradition let the discoverer pick the name, and Urey's choice won.

  The timing mattered for theory. The isotope was found before the neutron was reported in 1932, which made it an experimental shock. Once the neutron appeared, deuterium made far more sense. Urey received the Nobel Prize in Chemistry in 1934, only three years after isolating it.

  Lewis had been Urey's graduate advisor at Berkeley, and in 1933 he had prepared and characterized the first samples of pure heavy water. He was deeply disappointed by the Nobel Committee's decision. Several senior Berkeley administrators believed that disappointment played a central role in his suicide a decade later.

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• 4. Twenty Minutes of Fusion

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  Twenty minutes after the Big Bang, the universe cooled past the point where nuclei could keep fusing. That short window decided how much deuterium, helium and hydrogen the cosmos would ever have. Calculations from the temperature and expansion rate point to seven protons for every neutron when nucleus-building began. The lighter proton was favored, and that ratio stayed fixed afterward.

  Helium-4 was the energetically favored destination for those particles. Reaching it required an intermediate step, which was making deuterium. For much of the available time, the mean energy of each particle exceeded the weak binding energy of deuterium. Any deuterium that formed was destroyed at once. Physicists call this the deuterium bottleneck.

  The bottleneck held up helium until the universe cooled to the equivalent of about 100 keV. Then element formation surged. Deuterium appeared and immediately fused into helium. Very soon the temperature fell too low for more fusion, and the abundances were nearly frozen, changing only as radioactive leftovers such as tritium decayed.

  Nature offers no stable nucleus with a mass number of 5 or 8, so helium had no easy way to combine with hydrogen or with itself. As a result, an insignificant amount of carbon or anything heavier came out of the Big Bang. Those elements had to be forged later in stars.

  The failure of the Big Bang to burn through its hydrogen had a happy side effect. It left plenty of hydrogen behind to fuel long-lived stars such as the Sun.

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• 5. Cosmic Fingerprints

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  The Galileo space probe measured deuterium in Jupiter's atmosphere at 26 atoms per million hydrogen atoms. The gas giants carry roughly the primordial ratio, which is about 26 deuterium nuclei per million hydrogen nuclei. Observations from the ISO-SWS instrument found 22 per million in Jupiter, a figure thought to sit close to the ratio of the early Solar System. That is about 17% of the terrestrial value of 156 per million.

  The gap exists because stars are deuterium destroyers. Inside the Sun and other stars, the fusion reactions that consume deuterium run much faster than the slow proton-proton chain that creates it. Little deuterium is thought to remain in stellar interiors. Yet it lingers in the outer solar atmosphere at about Jupiter's concentration, probably unchanged since the Solar System formed.

  Comets tell a different story. Hale-Bopp and Halley's Comet contain about 200 deuterium atoms per million hydrogens, enriched compared with the early solar nebula, probably because of heating. A measurement of 161 per million in Comet 103P/Hartley, a former Kuiper belt object, lands almost exactly on the ratio in Earth's oceans. The oceans measure 155.76 per million, within a range of 153 to 156. That match supports the idea that much of Earth's surface water arrived on comets.

  Then came Rosetta. The probe found that Comet 67P/Churyumov-Gerasimenko carries about three times the deuterium of Earth water, the highest figure yet seen in a comet. The result revived the suggestion that some of Earth's water came from asteroids. Deuterium ratios remain an active research topic in both astronomy and climatology. Mars and Venus also show deuterium concentrated above the mean solar abundance.

  Closer to home, the Milky Way offers a puzzle. Ultraviolet spectra of undisturbed gas clouds show as much as 23 atoms of deuterium per million hydrogen. That is only 15% below the primordial figure of about 27 per million estimated from the WMAP survey. Astronomers read this as less deuterium destroyed than expected, or fresh primordial hydrogen falling in from outside the galaxy. A few hundred light years from the Sun, the value drops to 15 per million, probably because deuterium sticks to carbon dust grains.

  One feature of all this data carried an argument. Deuterium shows up at a low but nearly constant primordial fraction in hydrogen wherever it is found. That pattern favors the Big Bang over the Steady State theory of the universe.

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• 6. Heavy Water

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  Ice made from heavy water sinks in ordinary water, because heavy water is about 10.6% denser. Heavy water forms when deuterium takes the place of hydrogen in a water molecule. It is also more viscous than normal water. Bonds involving deuterium are somewhat stronger than the matching bonds in protium, and the difference in bond energy and length is larger than for any other element.

  Those stronger bonds are enough to change biological reactions. In animals with complex cells, replacing 25% of body water with heavy water causes cell division problems and sterility. At 50% it causes death through failure of the bone marrow and the lining of the gut. Bacteria cope far better. They can survive and grow in pure heavy water, although slowly.

  Despite that toxicity, ordinary exposure is no health threat. A 70 kg person could probably drink 4.8 liters without serious consequences. A few grams, an amount of deuterium comparable to what the body already holds, serve as a harmless metabolic tracer in people and animals.

  Producing it starts with ordinary water, a small fraction of which is already heavy. Makers separate the heavy part by the Girdler sulfide process, by distillation or by other methods. A reactor could in theory make deuterium, but separating it from plain water is the cheapest way to produce it in bulk.

  Atomic Energy of Canada Limited was the world's leading supplier until 1997, when the last heavy water plant there shut down. Canada uses heavy water to slow neutrons in its CANDU reactor design. Reactors of this kind use deuterium because it slows neutrons without soaking them up the way ordinary hydrogen does.

  India is another major producer. All but one of its atomic energy plants are pressurized heavy water plants, and they run on natural uranium. It has eight heavy water plants, of which seven operate. Six of the eight use deuterium-hydrogen exchange in ammonia gas, and the other two draw deuterium from natural water with high-pressure hydrogen sulfide. India covers its own needs and also exports reactor-grade heavy water.

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• 7. The Vemork Raid

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  Vemork, in Norway, housed a plant that produced and enriched deuterium, and the Allies decided it had to go. Nazi Germany was known to be testing heavy water as a moderator in a reactor design. Such a reactor could breed plutonium for an atomic bomb. The Allied response was the operation known as the Norwegian heavy water sabotage.

  After the war, the Allies learned that Germany had put in less serious effort than they feared. The Germans had finished only a small, partly built experimental reactor, which they had hidden away. They never managed a sustained chain reaction. By the war's end, they did not have even a fifth of the heavy water needed to run it, partly thanks to the Norwegian raid.

  Even a working German reactor would have left them years short of a bomb. The United States started up Chicago Pile-1 in late 1942. Both the United States and the Soviet Union then needed about two and a half years to go from a first working reactor to a bomb, even with maximal effort and funding.

  The real payoff for deuterium on the battlefield of physics came later. Light hydrogen has never fused under conditions achievable on Earth, so every artificial fusion device, hydrogen bombs included, needs deuterium, tritium or both.

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• 8. Ivy Mike and Fusion

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  On the 1st of November 1952, the United States exploded a 62-ton device called Ivy Mike. It was the first fully successful hydrogen bomb, and most of its energy came from fusion stages that followed a fission primary. It was less a weapon than a factory-like building. At its center, a huge insulated cryostat held about 1000 liters of liquid deuterium, which would have weighed 160 kilograms if completely full. A conventional atomic bomb at one end created the extreme heat and pressure that triggered fusion.

  Within a few years, designers built "dry" hydrogen bombs that needed no cryogenic fuel. Released information suggests every thermonuclear weapon since then uses lithium deuteride in its secondary stage. The lithium is the isotope lithium-6. Fast neutrons from the atomic primary turn it into tritium, which fuses with the deuterium and releases helium-4, plentiful energy and still more neutrons.

  "Pure" fusion weapons such as the Tsar Bomba are believed to be obsolete. In most modern boosted weapons, fusion supplies only a small share of the energy directly. Fast neutrons from the deuterium-tritium reaction split a tamper of natural uranium-238, and that fission releases far more than the fusion itself.

  Peaceful fusion has taken longer. Deuterium is the most common fuel in experimental reactor designs, usually paired with tritium for the large reaction rate and high energy yield. A deuterium and helium-3 reaction yields even more, but its breakeven point is higher and helium-3 is scarce, which makes it implausible as a practical power source. Commercial nuclear fusion is not yet an accomplished technology.

  A deuterium frequency of 61 MHz, when protium sits at 400 MHz, is what makes a chemist's solvent invisible. In proton NMR spectroscopy, samples are dissolved in deuterated solvents such as heavy water or deuterated chloroform. An instrument tuned for light hydrogen does not see the deuterium, so only the compound of interest shows up. Solid-state deuterium NMR can also measure the arrangement of hydrocarbon chains in the lipid membranes of cells.

  Deuterated compounds serve as cheap internal standards in mass spectrometry, improving accuracy. In neutron scattering, the cross sections of hydrogen and deuterium differ in size and sign. This lets researchers vary contrast. Normal hydrogen also adds scattering noise through its large incoherent cross section, which is zero for deuterium.

  Geographers use deuterium to trace water. Rainwater grows richer in heavy isotopes as local temperature rises, so the ratio falls along a predictable line called the global meteoric water line. Maps of these ratios are called isoscapes. The same analysis in a migrant bird or insect can hint at where it came from.

  Medicine has taken notice because deuterium is harder to pull off carbon than hydrogen. Swapping it into a drug can slow metabolism and extend its half-life. In 2017, deutetrabenazine became the first deuterated drug to win approval from the FDA.

  The idea reaches into nutrients. Deuterium can armor fragile carbon-hydrogen bonds in polyunsaturated fatty acids such as linoleic acid, slowing the chain reaction of lipid peroxidation that damages cells. A deuterated ethyl ester of linoleic acid called RT001, developed by Retrotope, is in a compassionate use trial for infantile neuroaxonal dystrophy. It has also completed a Phase I/II trial in Friedreich's ataxia.

  Other effects are stranger. Deuterium can stabilize live vaccines such as oral polio vaccine, alone or alongside magnesium chloride. In rats, hamsters and Gonyaulax dinoflagellates it lengthens the period of the circadian clock. In rats, a diet of 25% heavy water disrupts the rhythm by acting on the suprachiasmatic nucleus in the hypothalamus.

  In August 2018, scientists announced they had turned gaseous deuterium into a liquid metal, which could help explain the magnetic fields of Jupiter, Saturn and some exoplanets. The antimatter version is further out. The antideuteron, built from an antiproton and an antineutron, was first produced in 1965 at CERN and at Brookhaven. A complete antideuterium atom, with a positron orbiting the nucleus, had not been created as of 2019.

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