July 13, 2015
Plutonium magnetism confirmed for the first time
Researchers from the Department of Energy's Los Alamos and Oak Ridge national laboratories have solved the decades-old mystery by confirming plutonium’s magnetism for the first time, a new study published recently in the journal Science Advances claims.First produced in 1940, plutonium has an unstable nucleus that allows it to undergo fission and which makes it usable in nuclear fuels and weapons. However, little was known about the cloud of electrons that surrounded its nucleus – a cloud making it the most electronically complex element in the periodic table, the study authors explained in a statement Friday.
Scientists have long theorized that plutonium had magnetism, but had never been able to observe it experimentally before now. Using a process known as neutron scattering, the team was able to directly measure one unique characteristic of the element’s fluctuating magnetism for the first time, finding that its magnetism is in a constant state of flux.
Lead investigator Marc Janoschek explained that plutonium exists in a state known as a quantum mechanical superposition, which involves two extremes in its electronic configuration. In one, its electrons are completely localized around the plutonium ion, during which time it is magnetic. In the other, those electrons delocalize and are no longer associated with the ion at all.
Using a different isotope, keeping hydrogen at bay were key
Janoschek and his colleagues used the ARCS instrument at ORNL's Spallation Neutron Source to make a series of neutron measurements, through which they were able to determine that these fluctuations have different numbers of electrons in plutonium’s outer valence shell. He noted that this observation explains unusual changes that occur in the element’s different phases.
“The fluctuations in plutonium happen on a specific time scale that no other method is sensitive to,” Janoschek said. “This is a big step forward, not only in terms of experiment but in theory as well. We successfully showed that dynamical mean field theory more or less predicted what we observed. It provides a natural explanation for plutonium's complex properties and in particular the large sensitivity of its volume to small changes in temperature or pressure.”
The research, which was part of a larger effort to conduct an in-depth analysis of plutonium, used the plutonium-242 isotope instead of plutonium-239, which is more widely available. The reason is that the latter is highly absorbent of neutrons, thus potentially obscuring the weak signal - what the team was searching for. They also used a special technique to keep plutonium from absorbing hydrogen, which generated spurious signals where the magnetic ones were believed to be.
The study “provides the best explanation to date as to why plutonium is so sensitive to all external perturbations – something that I have struggled to understand for 50 years now,” said former Los Alamos laboratory director and plutonium science expert, Siegfried Hecker.
Update: Inside the research with Marc Janoschek of Los Alamos
Following the initial publication of this article, redOrbit had the distinct pleasure to discuss the research with Marc Janoschek of the Los Alamos National Laboratory in New Mexico. The first thing we wanted to know, of course, was just how he and his colleagues were able to “solve” the decades-old mystery of plutonium’s supposedly missing magnetism.
“We used neutron scattering to demonstrate that the magnetism in plutonium is not missing but dynamic,” he explained. “Neutrons carry a small magnetic moment – a compass needle that is sensitive to magnetic fields on the atomic-scale. Directing a beam of neutrons onto the plutonium sample and measuring the angle of deflection allowed us to detect this dynamic magnetism.”
“The observed neutron intensity pattern further shows that this ‘dynamic’ magnetism is driven by the configuration of the electronic cloud that surrounds the plutonium nucleus,” he added. “Just as neutrons, electrons also carry a small magnetic moment, but depending on the exact electronic configuration, those magnetic moments can either cancel each other [making the atom non-magnetic]... or add up to a magnetic configuration.”
“It turns out that the electronic cloud of plutonium is constantly changing between three different electronic configurations, of which two are magnetic, and one is not, resulting in this dynamic magnetic state,” Janoschek continued. While he noted that neutron scattering measurements are “routinely carried out to measure the magnetic state of various materials,” the radioactivity and poisonous natures required extreme safety measures be taken during the experiments.
Finally, Janoschek told redOrbit that “in addition to solving plutonium’s magnetic conundrum,” his team’s work “provides a natural explanation” for the rather complex structural properties of plutonium, which has six different structural phases. The various configurations of the electronic cloud imply distinct sizes of the plutonium atom, with more electrons leading to a smaller atom, and this as a result makes plutonium unstable to structural changes, he concluded.