Rutherford is a hero for physicists because he did so much to shine a light on the structure of the atom. But he’s held in special regard by star builders because of experiments that went even deeper into the nascent field of nuclear physics.
One of the ways that physicists in Rutherford’s time were studying atoms and nuclei was by accelerating them to high speeds, and thus high energies, before smashing them into other atoms to see what happened. These very high energies allowed the nuclei of atoms to get close enough to interact, rather than more passively bumping off one another. A lot of knowledge about subatomic physics has been gleaned in this way, and the tactic of smashing particles for knowledge continues today in experiments conducted at the Large Hadron Collider at CERN, the European Organization for Nuclear Research.
In those early days of atomic research, the machines were a lot smaller, but the discoveries were just as big, if not bigger. Rutherford himself oversaw a series of experiments in which nuclei were accelerated to very high energies and smashed into other nuclei.
In one, in 1917, Rutherford performed the first artificial nuclear reaction; he transmuted nitrogen by firing a helium nucleus into it. Rutherford didn’t quite realize what he had done at the time: he thought he’d split the nitrogen apart, but in actual fact he had turned it into oxygen, releasing a hydrogen nucleus in the process.7
But it is two subsequent experiments that Rutherford was involved in that really started the quest for nuclear energy.
Atomic Energy
First, in 1932, two physicists working under Rutherford’s guidance at the Cavendish made a huge nuclear discovery that made headlines around the world and provided the first proof that energy could be unlocked from within the atomic nucleus.
The experiment was performed by John Cockcroft and Ernest Walton. Cockcroft and Walton were accelerating protons to high energies and slamming them into lumps of lithium. Most protons didn’t do anything interesting. But just occasionally—ten for every 1 billion fired—a proton would strike a lithium nucleus and split it clean in two. A nucleus of lithium-7 (the most common isotope of lithium, with three protons and four neutrons) became two helium-4 nuclei. The proton caused the lithium nucleus to have a surfeit of positive charge and so repulse itself, elongate, and break apart. It was pure alchemy; the researchers put in protons and lithium, and got out helium. They’d discovered that some atoms are fissile—that is, they can be split apart—an extraordinary insight into what is possible with the matter that makes up, well, pretty much everything.8
Cockroft, Walton, and Rutherford had split the lithium atoms. But the experiment was most interesting for what they saw when they added up the energies involved. They’d put 0.6 mega-electron volts (MeV) of energy into accelerating each proton—a tiny amount on a human scale, one-tenth of a million millionth of a joule. While you’ve been reading this sentence, you will have used up as many as one hundred joules. But at the atomic scale, giving a proton 0.6 MeV of energy is a bit like strapping a fly to a rocket. As big a number as this is, it’s dwarfed by the energy that came out of the reaction. In producing the two helium nuclei, 17 MeV of energy was liberated. The colliding protons had released almost thirty times the energy that had been put into them!
Cockcroft and Walton won a Nobel Prize. Later, it was discovered that more massive isotopes could be split to yield energy in a repeating chain of nuclear fission reactions, knowledge that would ultimately lead to the atomic bomb and today’s nuclear fission reactors. Cockcroft became director of the UK’s Atomic Energy Research Establishment, where he arranged for the very first star machines to be housed and supported the construction of what the star builders of the day hoped would be the world’s first net-energy-gain fusion device. Those hopes were dashed, but Cockcroft was nonetheless one of the most important early star builders.
But it’s his second experiment that’s the most important for the star builders, and the one that they can trace their entire creed back to. In 1934, Rutherford and two other physicists, Mark Oliphant and Paul Harteck, were using an improved version of the Cavendish’s particle-smashing machine. They were firing so-called heavy hydrogen—what we now call deuterium, the isotope of hydrogen with one neutron in the nucleus—at other atoms of deuterium in a target. What they found was that—every so often—a reaction happened in which a deuterium nucleus with 1 MeV of energy combined with another deuterium nucleus to transform into a tritium nucleus, a proton, and 4 MeV of energy.II They’d put one unit of energy and two deuterium atoms into their reaction and somehow ended up with a bigger nucleus (tritium) and four times as much energy out. They’d fused the deuterium together to make a new isotope, tritium: they had discovered that fusion of atomic nuclei was possible.9 They’d shown beyond doubt that simple, common elements, like hydrogen, could be combined to make bigger, more scarce elements. This single idea could explain how so much of the rich zoo of atoms in the universe came to be, and it would soon be used to explain how the Sun shone. It was a revelation. What’s more, it put the power to re-mold creation into the hands of humanity.
Fusion is the reaction that alchemists dreamed of—it really can turn base elements into gold.III10 It’s the reaction that makes the entire universe a Lego set—with the right base blocks, you can build anything. Is it any wonder that star builders are so captivated by it?
However, fission and fusion are more than just ways of rearranging the atomic building blocks that Rutherford and his colleagues had helped discover. Some rearrangements, like the fusion reactions that make the Sun shine, the splitting of lithium, and the fusion of deuterium with itself performed by Rutherford, release energy. Not chemical energy, because they don’t involve electrons, but nuclear energy, and a lot of it. What scientists had discovered was a new energy source hidden deep within the atom.
Star builders like Mark Herrmann, at NIF, and Ian Chapman, at the Culham Centre for Fusion Energy, are using deuterium-tritium fusion reactions, which produce even more energy than Rutherford’s. The two isotopes of hydrogen fuse together to create a helium nucleus, a neutron, and energy. In every successful reaction, scientists put just 0.1 MeV in and get a whopping 17 MeV out; that’s 170 times as much energy out as went in. On an atomic scale, this is huge: 17 MeV is two million times more per reaction than you’d get from a single fossil fuel reaction.
But physics is about understanding, not just documenting. Even after the discovery of fusion, physicists didn’t know why or how it was happening—they didn’t know what combinations of atomic Lego blocks would fit together, or where the energy was coming from. If you’re a star builder and you’re interested in both releasing and using nuclear energy, you need to know more about how fusion happens.
A first clue as to what was happening came from one of J. J. Thomson’s other students, a physicist called Francis Aston. In 1920, Aston came up with a more accurate and precise way to measure the masses of atoms. Surprisingly, he found that the masses of atoms weren’t in exact whole number ratios, with, say, helium being four times the mass of hydrogen because it has four times as many particles in its nucleus. That’s what you’d expect if you thought that protons and neutrons were like Lego blocks. Instead, the ratios were very slightly different: helium had a mass that was 99 percent of the mass of four hydrogens. This difference seems small and inconsequential, but it is the very reason why nuclear fusion works.
The physicist and great popularizer of science Arthur Eddington was struck by this apparent mistake in the arithmetic of the universe. Eddington was a nuclear visionary, suggesting long before Rutherford’s fusion experiment that the power source of stars was subatomic in nature. Eddington reasoned that four hydrogen atoms should really have exactly the same mass as one helium atom, not 99 percent of the mass. The only explanation was that in going from a single building block (in the form of hydrogen) to four (in the form of helium) some of the mass must disappear. This was pure heresy: mass didn’t just disappear. Matter might change form, say from a solid to a liquid, but the
mass of the end products was always the same as the mass of the reactants. Conservation of mass was a central tenet of physics.
Eddington wondered whether a theory published by a brilliant physicist he greatly admired could shed light on the puzzle. It said that mass and energy were different sides of the same coin. Mass could become energy, and energy could become mass. It was a wild idea. Impressively, the physicist in question had published this theory along with three others that challenged fundamental concepts in physics in 1905. His name was Albert Einstein.11
The Secrets of Atomic Energy
The theory of Einstein’s that Eddington had in mind to explain the minute differences in mass of atoms said that the relationship between an object’s mass, when it is not moving, and its energy is
E=mc2
where E is energy, c is the speed of light (a gut-wrenching 300 million meters, almost a billion feet, per second), and m is the mass difference that Aston found. With effortless parsimony, this simple equation explains what’s going on in both nuclear fusion and nuclear fission.
Imagine a scale that measures the number of particles in an atomic nucleus. On one side of the scale, carbon (which has six protons and six neutrons) would measure exactly twelve units. Now put the inputs into fusion (deuterium and tritium) on one side of the scale and the outputs (a neutron and helium) onto the other.
On the side of the scale with deuterium and tritium, you’d have five particles between the two nuclei: three neutrons and two protons. But the scale would read 5.0304 units; almost 5, but, importantly, a little bit more. On the other side of the scale, the neutron and helium nucleus that are created have a mass of 5.0113 units. The scales would tip ever so slightly toward the deuterium and tritium. The difference between them and what comes out of the reaction is tiny, just 0.02 units of mass. Einstein’s equation tells us that it’s anything but tiny in terms of energy.
The m in Einstein’s equation takes the apparently small value of 0.02 units, but it gets multiplied by a very big number, the square of c, and this means that even a tiny amount of mass converts into an outrageous amount of energy. Plugging 0.02 units of mass into E=mc2 tells you that a whopping 17.6 MeV of energy was released from mass—just as found by experiment!
In fission, the same phenomenon occurs; the masses of the leftover pieces are a bit less than what went into the reaction. In both fission and fusion, Einstein’s theory says how much energy will come out for a given change in mass.
With Einstein’s equation, star builders are equipped to understand what combinations of isotopes will deliver energy if fused. One of the most amazing insights of these nuclear theories is that a little of the star builders’ preferred fusion fuel goes a very long way. Rich fuels produce more energy per amount used than poor fuels, or, put another way, they have higher energy density. Kilogram for kilogram, burning coal releases about twice as much energy as burning wood. That’s why coal was so important to the industrial revolution and, in part, why it’s the most popular source of energy on the planet today. Crude oil releases slightly more energy than coal. The most high-energy-density chemical fuel is hydrogen gas, which releases eight times as much energy as wood. Burning coal is a chemical reaction; it swaps electrons around. It doesn’t change nuclei. If you want to release a lot of energy, you need to go nuclear.
Deuterium-tritium fusion, the kind of fusion that most star builders are doing, releases 10 million times the amount of energy per kilogram as coal. Ten million. If you had a fusion reactor in your house, you’d have to go to the deuterium-tritium shed once for every 10 million times you went to the coal shed. What this means is that the mass of a single cup of water contains the equivalent energy of 290 times what the average person in the US uses each year. The mass of an Olympic swimming pool contains an amount of energy in excess of total world annual energy use.
Even the fission reaction that powers today’s nuclear plants isn’t quite as energy rich kilogram for kilogram as fusion is; fission releases only 25 percent as much energy. In fact, there is only one reaction in the universe that releases more energy per kilogram than fusion, and it’s the annihilation of matter and antimatter into pure energy so that no mass at all is left over. This reaction needs antimatter, which doesn’t exist anywhere in the universe in large quantities (let alone on Earth). Which means that nuclear fusion is the most energy-rich fuel available to humanity.
Now it’s clear why fusion fuel could provide energy to the planet for millions, perhaps billions, of years: not only is it plentiful, we don’t need very much of it.
But there’s a problem. Einstein’s equation can tell star builders how much energy they can expect to release, if the reaction happens. But how do they know what reactions can happen? Are some allowed and some not?
Fortunately, the star builders can rely on decades of work that have gone into understanding nuclear reactions. At the root of what can and cannot happen are the fundamental forces of nature. We’ll be seeing them again and again in this book, not least because they dictate how everything in the universe happens. And I do mean everything. They lie behind phenomena as diverse as the chemical reactions in a car engine, the creation of the elements, the way a stone falls, and the ability to see. It is these forces that allow objects to interact with each other; if what you’re sitting on now did not hold you up with a force, then you’d simply pass through it like a ghost.
The four forces are gravity, the electromagnetic force, the weak force, and the strong force. They are what make sense of the abstract properties that particles have, like charge and mass. Electrical charge determines how strongly a particle interacts through light and electromagnetism; mass determines how strongly a particle feels gravity.
The most familiar force is gravity because it keeps you from floating off into space. Electromagnetism describes electricity and magnetism—you can see how the two are related by placing a compass next to the cord of a hair dryer or electric kettle with the power on; the needle stirs due to the magnetic field created by electrons moving in the power cord. The weak force is responsible for radioactive decay. The strong force is the glue that holds together most of an atom’s mass in the nucleus.
The forces have different strengths. The strong force is the strongest, then the electromagnetic force, then the weak force, and finally gravity. It might be surprising that gravity is the weakest of the four forces, especially as astronauts have to use rockets to escape the Earth’s gravity. But even a puny bar magnet can overcome the gravitational attraction of the entire planet when you use it to suspend a paper clip above the ground.
The forces act over different ranges too. Gravity has infinite reach and is solely attractive. It doesn’t get canceled out by an “antigravity.” These unusual properties are why it’s gravity that determines how the structure of the universe evolves, despite its relative weakness. Electromagnetism also has infinite range but comes in positive and negative versions that tend to cancel out on large scales.
In contrast, the strong force has a very short range, and it’s this short range that determines how big atomic nuclei can get. It is so strong that it can bind protons together in nuclei even though they repulse each other due to their same electromagnetic charges. When the strong force is acting between particles within an atomic nucleus, it has a different name: the nuclear force.
The nuclear force says that for energy to be released by fusion, two slightly more unstable atoms—that have proportionately less glue-like nuclear force—must be combined to create a more stable nucleus. For energy to be released from fission, a big, ponderous, and unstable atom must split into more stable smaller atoms. The common atom that is the most stable of all, the Goldilocks of nuclear physics, is the isotope of iron with fifty-six particles in the nucleus, or Fe-56. Atoms smaller than this tend to release energy if fused together (like hydrogen and its isotopes) because the reaction makes bigger atoms that are closer to Fe-56. Similarly, atoms bigger than Fe-56 tend to release energy when split.
&n
bsp; The forces determine which atoms we need to unlock the most energetic reaction we can ever hope to use as a species. Great, let’s do some nuclear physics!
The Problem
Not so fast. There’s a catch, a big one, and it’s why getting net energy from nuclear reactions is hard.
Remember Cockcroft and Walton’s nuclear reaction that produced twenty times the energy they put in? It only succeeded ten times for every 1 billion protons that they accelerated. To produce net energy gain from the experiment as a whole, they’d have needed it to succeed more than one time in every thirty. Their experiment actually used substantially more energy than it produced. The fusion reaction that Rutherford first demonstrated was similar, with a chance of one in a million—so low that the machine used much more energy than it produced.12
Doing nuclear physics the particle smashing way is like throwing darts blindfolded: you might know the rough direction, but you’re not going to get many bull’s-eyes. Einstein himself said that the “likelihood of transforming matter into energy is something akin to shooting birds in the dark in a country where there are only a few birds.” Rutherford agreed, saying of one nuclear experiment that “We might in these processes obtain very much more energy than the proton supplied, but on the average we could not expect to obtain energy in this way. It was a very poor and inefficient way of producing energy, and anyone who looked for a source of power in the transformation of the atoms was talking moonshine.”13
Just like throwing darts at random, there are probabilities for “hitting” the target when you’re firing one particle at another and hoping that you’ll get a nuclear fusion reaction to happen. And just like when I throw darts, the probability of a hit is low.
The reason is that the electromagnetic force repulsion between the protons of the two nuclei kicks in much sooner than the nuclear force that snaps them together. From the point of view of the accelerated particle, it’s like climbing a steep hill (electromagnetic repulsion) with a deep valley (nuclear force attraction) on the other side. Get the approach just right, and the incoming particle will have enough momentum to make it over the top of the hill and into the deep valley on the other side: fusion can happen. But most of the time, the incoming particle doesn’t make it—even when it has the perfect amount of energy, it’s still much more likely that the two nuclei will glance off each other rather than fuse together.
The Star Builders Page 6
