If atomic nuclei are known for anything, it’s being small and dense. Halo nuclei are an odd breed with the opposite reputation–oversized and fragile, their outermost neutrons form an extended “halo” that clings precariously to a compact core. In the 30 June PRL, researchers describe experiments that delicately nudge apart the most extreme halo nucleus in order to learn more about its behavior. Their data are the first to clearly show characteristics of the breakup in this critical, low-energy regime and provide the cleanest demonstration yet of the importance of the interaction between the halo neutrons.
Lithium-11 is the largest and most fragile halo nucleus, made of two halo neutrons tenuously bound to a lithium-9 core containing three protons and six neutrons. The halo makes this nucleus as large as lead-208. Although researchers know its basic ground-state structure, to fully understand lithium-11–and other halo nuclei–they would like a theory for how it will respond to external disturbances. It’s difficult to calculate the behavior of a three-body system (core plus two neutrons), and in this case, experiments aren’t easy, either. The usual technique of nuclear physics–smashing nuclei apart–has to be applied very gently to get at such a weakly bound system and see the neutrons’ behavior just as they are nudged out of reach of the core.
In several previous experiments physicists have directed halo nuclei at moderate energy onto a dense target, such as lead. In these experiments the beam energy is fixed, but the amount of energy absorbed by each halo nucleus before breakup varies because collisions with the target atoms are not all identical. By measuring the emerging fragments, researchers can plot the relative likelihood of collisions versus the amount of energy absorbed, and they have found tentative evidence for a low-energy peak in this plot. The disintegrating nucleus appears to have a preference for absorbing a certain amount of energy, somewhat less than 1 MeV in lithium-11. Only halo nuclei can show this peak, which results from the relative motion of the core and the neutrons, according to theory, but experiments in this low-energy regime are extremely difficult.
To get better data, Takashi Nakamura of the Tokyo Institute of Technology and his colleagues devised an experiment with two main improvements. First, they set up two “walls” of neutron detectors, one behind the other, which dramatically improved the chances of detecting both neutrons from a single nucleus. Second, they used a “coincidence” technique to rule out confusing events where a single neutron triggered signals in more than one detector element.
The data show a clear peak at 0.6 MeV, of a size and shape that agree well with a previous theoretical calculation. When modeling the breakup of lithium-11, theorists have been uncertain of the importance of the attraction between the two halo neutrons, via the strong nuclear force. But in the new calculation, the peak arose only when this interaction was included, suggesting that the neutron-neutron attraction is significant. Although discrepancies remain, the team believes their data will provide direction for theorists tackling the notoriously difficult three-body problem for halo nuclei.
“This is the best experiment on any halo nucleus,” says Henning Esbensen of Argonne National Laboratory in Illinois, one of the developers of the theory the Japanese team fit to their data. Ian Thompson of the University of Surrey in England is also impressed with the results and adds that they support arguments he and a colleague proposed 12 years ago concerning the angular momentum of the halo neutrons. Lithium-11 is the “best and most extreme example” of a halo nucleus, Thompson says, and the results of these difficult experiments provide a rigorous test of theorists’ ability to understand this simple yet forbidding nucleus.