Just how long does a cluster of protons and neutrons have to stick together to count as an atomic nucleus? That’s the question raised by the observation of nitrogen-9, a fleeting nucleus that possesses seven protons and two neutrons, a ratio so lopsided that it fates the tiny knot of matter to fall apart almost instantly, in less than one-billionth of a nanosecond. Yet it still counts as a nucleus, physicists say, and explaining its existence and properties should help expand the horizons of nuclear theory and may even have deeper implications for quantum mechanics, the theory that governs the atomic world.
Scientists not involved in the work are impressed with the discovery. “Both experimentally and theoretically, this is just a flex,” says Kate Jones, a nuclear physicist at the University of Tennessee, Knoxville. “I mean, you shouldn’t be able to do this.” Alexander Volya, a theoretical nuclear physicist at Florida State University, says nitrogen-9 is an extreme example of an “unbound” nucleus, which takes no energy to be pulled apart. Such nuclei are pushing the boundaries of theory, he says. “This is totally new physics that needs to be addressed.”
No single theory can predict the existence, structure, and behavior of all atomic nuclei. In fact, nuclei are so complex that only the lightest ones can be described by models that take as their building blocks individual protons and neutrons and how they interact through the strong nuclear force. Even then, these models run into problems when describing light nuclei in which the protons dramatically outnumber the neutrons, or vice versa.
Nitrogen-9 is an extreme example. To make it, a team of physicists working at Michigan State University fired a beam of oxygen-13 nuclei through a beryllium target 1 millimeter thick. The collisions broke up some of the oxygen nuclei into fragments, and the researchers trawled through the wreckage in search of new nuclei. If nitrogen-9 was an ordinary nucleus, scientists could have filtered it out of the spray of fragments using a standard device called a mass spectrometer. But nitrogen-9 falls apart far too quickly to do that, explains Robert Charity, a nuclear scientist at Washington University in St. Louis who led the new study.
Instead, Charity and his colleagues spotted these fleeting particles by looking for their decay products. For each event, they captured all the nuclear fragments spewing from the target with a detector that measured their momenta and energies. They combed the data for events that contained precisely the fragments a nitrogen-9 should spawn: five protons and an alpha particle (two protons bound to two neutrons). From those particles’ momenta and energy, the physicists could then infer the mass of their parent nucleus—if there was one.
If the protons and alpha particle were just random debris, then over many events a plot of the parent mass would be featureless. If those particles came from the decay of nitrogen-9, however, the plot would show a distinct peak at the mass for nitrogen-9—which, because of interactions among the protons and neutrons, differs slightly from the simple sum of those particles’ masses. That’s exactly what Charity and colleagues saw. After firing 40 billion oxygen-13 nuclei through the target, they produced just a few hundred nitrogen-9 nuclei, they report in a paper in press at Physical Review Letters. The technique has been used widely in particle physics, Charity notes. “We use the same technique that they found the Higgs boson with.”
To make sure they weren’t seeing a spurious signal, the researchers even employed the technique multiple times in an iterative way. They assumed that nitrogen-9 would split into a proton and carbon-8, an unbound nucleus that decays to two protons and beryllium-6. That beryllium-6 is also an unbound nucleus, and it decays to two protons and an alpha particle. They verified that in their candidate events they also saw the telltale mass peak for each nucleus in that decay chain. Emerging one after another, those successively smaller nuclei vaguely resemble a set of nested Russian dolls, Charity says.
Nitrogen-9, the first nucleus known to spit out five protons, just barely qualifies as a nucleus, Jones says. Its constituents attract each other just enough to hold together for an instant and to affect the properties of the assemblage, such as giving it a definite mass. Discovering a nucleus with so many unbound protons is impressive, Jones says. “The idea of having a system that’s an alpha particle plus five protons, that’s just nuts.”
Such unbound nuclei also lie on the cutting edge of nuclear theory, Volya says. Mathematically, theorists can model a small bound nucleus in terms of well-defined quantum states that do not vary in time and are confined in space—much as a chemist can model a sodium atom by listing the orbital shells its electrons occupy. But those standard states do not suffice to model an inherently unstable nucleus like nitrogen-9, which has multiple unfettered protons, Volya says. So theorists must employ new tools to model them, he says. “This is a very big change also, in terms of methods.”
In fact, unbound nuclei are a prime example of an open quantum system that interacts strongly with its surroundings. Others include biochemical systems and even quantum computers, which manipulate quantum bits whose delicate states are easily disrupted by environmental noise. So insights gained in dealing with unbound nuclei such as nitrogen-9 could have broader implications for quantum mechanics, Volya says. “There is a lot of connection between this and many other areas of physics.”
