IN A PHYSICS laboratory in Dallas, Texas, a pinprick of hafnium is blasted by radiation from a second-hand X-ray machine. A few atoms of the silvery metal emit some gamma rays. Not much to get excited about, you might think. Yet what is being witnessed in this lab today may be nothing less than the birth of a new science. Dubbed “quantum nucleonics”, it could lead to atomic-scale electronic components, nuclear-powered rockets and gamma-ray lasers – sources of photons a million times more energetic than those from any ordinary laser.
To control gamma rays we must learn to control events within the nucleus, a domain with dimensions 100 000 times smaller than an atom’s, and we must do so in a far more sophisticated way than in a nuclear bomb or reactor. The nucleons (protons and neutrons) that make up any nucleus behave a lot like the electrons orbiting in an atom, in that their quantum-mechanical nature restricts them to a set of precise energy levels, like the rungs of a ladder. But the nuclear rungs are about a million times farther apart than the electronic rungs, so when a nucleon sheds its energy by dropping to a lower rung, it emits a photon a million times more energetic than an electron doing the same thing. Instead of a photon of visible light, it emits a gamma-ray photon.
The decay of a nucleus is a random event. Normally, all you can say about when it will happen is that each type of nucleus has a characteristic half-life, the time for half the nuclei in a sample to decay. The challenge in creating a controllable source of gamma rays is therefore to find a way to shorten the half-life of a particular nuclear species so that it decays before its time. “The textbooks say the half-life of a nucleus is set in stone and that nothing can change it,” says Carl Collins of the University of Texas at Dallas, who leads a team from the US, Russia, Ukraine, Romania and France. “We’ve shown this is wrong. In our latest experiment, we’ve persuaded nuclei of hafnium-178 to unleash a burst of gamma rays,” says Collins. “We’ve discovered the way to make a gamma-ray flashbulb.”
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The team used a special kind of nuclear state called an isomer. Excited nuclear states tend to last only a split second before decaying, and so are useless for energy storage. But some nuclei can become trapped in very long-lived excited states by a quantum mechanical property known as spin. When a nucleus is given extra energy in a collision, for example, the neutrons and protons within it gyrate more rapidly. Sometimes these gyrations add up to give the whole nucleus several quantum units of spin more than the ground state. If the nucleus is then to fall back to the ground state by emitting a gamma ray, the gamma-ray photon must not only carry away the extra energy, it must also carry away the extra spin. But photons can only rarely carry away more than one quantum unit of spin, so the nucleus stays stuck in its excited state.
Escaping the trap
Many isomers occur in deformed, cigar-shaped nuclei, where a second trapping mechanism operates. Quantum mechanics dictates that the spin must take up certain precise alignments relative to the axis of the nucleus. If these alignments are different for excited and ground states, an escaping gamma-ray photon must again change the spin in just the right way.
One such trapped nucleus is tantalum-180, a very rare but naturally occurring variety of element 73. Almost every tantalum-180 nucleus is stuck in an isomer state 0.075 million electronvolts (MeV) above the ground state, with a spin that is eight quantum units higher. It is so rare for a photon to take away this much angular momentum that the half-life of tantalum-180, rather than being a mere split second, is at least 1015 years – 100 000 times the age of the Universe. Each nucleus is like a battery charged up with 0.075 MeV.
In 1988, Collins and his multinational team set out to discharge the battery. The key to achieving this is the existence of a so-called “K-mixing state” at a higher energy than the isomer state. Quantum systems have the peculiar ability to be in two states with the same energy at once – called a mixed state. No one completely understands K-mixing states, but Collins suspects they occur when, for a given energy, a nucleus can have two different shapes – prolate like a cigar or oblate like a pancake – and the nucleus spends time changing between prolate and oblate. For an instant in between, the nucleus is spherical. Because a sphere has no preferred axis, the spin direction of the nucleus is undefined and there is no longer a constraint on the spin carried away by a gamma ray.
So, to unlock a long-lived isomer, you need only boost it up to a K-mixing state. From there it can freely decay to the ground state (see Diagram). Tantalum-180 has a K-mixing state about 2.5 MeV above the isomer, so Collins and his colleagues achieved the feat by zapping tantalum-180 with 2.5 MeV gamma rays. Think of the nucleus as a ball trapped in a valley (the isomer state) which is separated from another, deeper valley (the ground state) by a hill. Once the ball has been boosted to the K-mixing state at the top of the hill, it can roll down to the bottom of the neighbouring valley.
The great thing about tantalum-180 is that nature provides it ready charged-up. The nuclear battery discharged by Collins and his team in a modest laboratory in Texas was primed billions of years before the Earth was born, by one of the most violent events in creation, a supernova. Massive stars end their lives in these huge, hot explosions. At a temperature of more than a billion degrees, protons can force their way into heavy elements, creating new isotopes and nuclear states that are then scattered across the Galaxy.
Unfortunately, the isomer state of tantalum-180 is only 0.075 MeV above the ground state, whereas the K-mixing state is a whopping 2.5 MeV above the isomer. This means that almost as much energy has to be put into each nucleus as comes out. “This is a bit of a dead loss,” says Phil Walker of the University of Surrey.
Collins and his colleagues have now remedied this with another nucleus, an isomer of hafnium-178 that is 2.44 MeV above the ground state and has a half-life of 31 years. In this state, two protons and two neutrons orbit around the middle of the cigar-shaped nucleus. Physicists at the Los Alamos National Laboratory in New Mexico have made 4 micrograms of the isomer by irradiating a target with a proton beam. “It’s very precious stuff,” observes Collins.
It turns out that a K-mixing state sits only about 20 thousand electronvolts (keV) above the isomer, so a nucleus elevated from the isomer to the K-mixing state will release about 100 times as much energy as was put in. To give it the requisite amount of energy, Collins’s team simply hit their hafnium with 1.5-second pulses from an old X-ray machine, which had once occupied a dentist’s surgery. “This is tabletop physics,” he says. “It’s almost too good to be true.”
In January, Collins reported that when hafnium-178 decays from the K-mixing level it bumps from rung to rung down the energy ladder, emitting several photons, including one with an energy of 495 keV (Physical Review Letters, vol 82, p 695). The team detected an increase in the emission of 495 keV photons, and by inference the decay rate of hafnium-178, of about 4 per cent over the normal spontaneous rate. Not much, it’s true, but still a step towards a controllable, bright source of gamma rays. “It was an enormous break getting such a big energy release with such a low-energy trigger,” says Collins.
Other physicists sound a note of caution. “There is certainly some evidence that more energy was got out than was put in, but it’s still too early to be sure,” says Walker. “If it’s true, however, it is very exciting.” He points out that, in principle, isomers may exist that can be triggered by even lower energy photons, in the ultraviolet or optical wavebands.
So why the fuss? If you do manage to get a controlled source of gamma rays, what can you do with it? Well, for one thing it will allow you to manipulate matter on an atomic scale. This will make it possible to fashion nanomachines and incredibly small electronic devices, perhaps to be used in quantum computers. “It’s hard to imagine doing this without a bright source of gamma rays you can turn on and off,” says Collins. At the moment, electronic components and even three-dimensional structures such as tiny motors are made by shining light through a mask, which casts shadows onto a substrate. Where the light falls, it causes chemical changes, making the substrate resistant to acid. Dunk it in an acid bath and the unexposed part is eaten away, leaving the desired component.
The snag is that light bends around structures that are a similar size to its wavelength, blurring the shadows and so limiting the precision with which small components can be made. The obvious way to make smaller structures is to use shorter-wavelength light. For atomic-scale components, a thousand times smaller than the best that can be achieved today, this means using gamma rays.
Nuclear rockets
According to Collins, this is only the beginning. Everything you can do with atoms and chemistry ought to have a parallel at the nuclear level. But control of the nucleus means control of the densest form of energy known, a million times more concentrated than any chemical fuel.
“We are talking about safe nuclear energy release, with no nuclear waste,” says Walker. Big isomer-fired power stations are probably not on the agenda, however. Tantalum-180 could in principle be mined from the Earth’s crust, but it is rare and the energy release is highly inefficient. More plausible is the idea that quantum nucleonics could be the basis of a new form of nuclear rocket propulsion. Fission and fusion-powered rockets were considered in the 1950s, but dismissed because of the risks of lifting nuclear reactors or bombs into orbit. A nuclear isomer rocket should be a lot safer. Induced gamma radiation would heat an absorber to about 3000 °C, which in turn would heat a working fluid such as water or ammonia, vaporising and dissociating it so that it expands at ultrahigh speed through a rocket nozzle to generate thrust. Who knows, isomer rockets might one day carry probes to other stars.
But the Holy Grail of quantum nucleonics is the gamma-ray laser, an idea proposed in 1961 by Collins’s colleague, Lev Rivlin of the Moscow Institute of Radioengineering Automation. The first step towards such a device would be to achieve stimulated emission of gamma rays. That is, the photons emitted by a decaying nucleus must trigger other nuclei to decay, emitting more of the same photons in a chain reaction. If this process is efficient enough, it should produce a flood of gamma-ray photons, all in step – a gamma-ray laser.
A black art
This process requires a “population inversion”. That is, there must be a lot of nuclei in a raised energy state, and relatively few in the state it decays to. Then a photon of the transition energy – the difference between the states – will be able to trigger nuclei to emit other photons, which are not likely to be absorbed by nuclei in the lower energy state.
Collins hopes that a population inversion might appear in the eight-photon decay ladder of hafnium-178. If the rate of decay from a high energy level to a middle level is slow and the decay from the middle to the ground state is fast, the population of the high level can build up while the middle one is kept nearly empty. Even if this doesn’t happen in hafnium-178, there must be plenty of other isomers to try. “I’m sure the right nuclear system exists somewhere,” says Collins. “Unfortunately, predicting the properties of a many-body system like a nucleus is beyond current capabilities. This business is more an art than a science.”
A gamma-ray laser wouldn’t emit a steady beam. In a continuous-wave laser, mirrors bounce the light back and forth, building up its power and letting it leak out slowly. But no known material can act as a mirror for gamma rays. Instead, a gamma-ray laser would use a long column of material, so a trickle of photons starting at one end would have a chance to build up into a flood. “What you would get is a flash of light, as in a pulsed laser,” says Collins. After the pulse, the nuclear fuel would be spent, so it would have to be replaced. “It would be just like using an expendable chemical fuel such as coal or oil,” says Collins. “Such a laser won’t become a reality tomorrow,” adds Walker. “I think we’re looking at the 50-year timescale.”
Gamma-ray lasers could also be used for atomic holography, storing information on the scale of atoms and molecules. Or they might be able to simulate the extreme conditions in supernovas and gamma-ray bursts.
One frustration is that Collins’s team, from eight labs in five countries, only gets together for a month of frantic work twice a year. “There’s no telling how fast progress would be if we could work on the problem continuously,” he says. There are others who may have more resources at their disposal. The American military is already taking notice, and the Naval Research Laboratory in Washington DC has forbidden its researchers to talk publicly on the subject of gamma-ray lasers. “I am absolutely speechless at that news,” says Collins. And it’s not only Collins who is uneasy at the prospect of a gamma-ray laser in the wrong hands. “It’s frightening to think of the energy you could unleash on a tabletop,” says Walker.
It is, of course, understandable that there is military interest in the idea of a gamma-ray laser. In the 1980s, Edward Teller and others raised the possibility of space-based X-ray lasers that could destroy Soviet nuclear missiles shortly after they were launched. The device they envisaged would have needed a nuclear bomb to power it, and the idea came to nothing because of the technical problems involved. Perhaps the gamma-ray laser will prove more practical.
Or maybe none of this will happen and gamma-ray lasers will come to be used for some quite different application – one that nobody has yet dreamt of. “When the first laser came along, no one knew what to do with it,” says Rivlin. “And look what happened to that.”
- Further Reading: The gamma-ray laser idea is described at www.utdallas.edu/research/quantum/cqeseg3.htm#TOPGAM
- A technical review of nuclear isomers can be found in Energy traps in atomic nuclei by Philip Walker and George Dracoulis, Nature, vol 399 p 35 (1999)
