Using lasers, scientists in Germany have precisely measured the single-neutron halo of the isotope beryllium-11 for the first time, work that may help them gain a better understanding of the forces within the atomic nucleus that bind atoms together. Measuring halo nuclei is extremely difficult because they can be artificially created in only minute amounts. Also, most of these synthesized nuclei decay within milliseconds. Scientists at the Institute of Nuclear Chemistry of the Johannes Gutenberg University Mainz, in cooperation with colleagues from other institutes, accomplished their ultraprecise laser spectroscopic measurement by coupling a method developed 30 years ago at the university with an optical frequency comb, one of the most modern techniques for precise laser frequency measurement. But that combination alone was not sufficient to achieve the right level of precision, the researchers say, so the technique was expanded to include an additional laser system and then applied to beryllium isotopes at the Isotope Separator On Line Detector (ISOLDE) facility for radioactive ion beams at the European Organization for Nuclear Research (CERN) in Geneva. The ‘halo’ nucleus beryllium-11 (11Be) consists of a core of 10Be and loosely bound neutron. The neutron orbits at a mean distance of 7 fm from the center of mass. (Illustration: Dirk Tiedemann, Institute of Nuclear Chemistry.) They used the lasers to measure the nuclear charge radius in beryllium-11. This nucleus consists of a dense core with four protons and six neutrons and a single weakly bound neutron that forms the halo. The researchers evaluated the dimensions of the halo, or cloud, and found that the atomic nucleus of beryllium is three times as large as normal because of the halo effect. Atomic nuclei are normally compact structures defined by a sharp border. About 25 years ago, researchers at the University of California, Berkeley, discovered that there are exceptions: Certain exotic atomic nuclei contain particles that shear off from the central core and create a cloud, which surrounds the central core like a halo. “We intuitively imagine the atomic nucleus as a compact sphere consisting of positively charged protons and uncharged neutrons,” said Dr. Wilfried Nörtershäuser of the Institute of Nuclear Chemistry. “In fact, we have known since the 1980s that atomic nuclei of certain neutron-rich isotopes of the lightest elements– lithium, helium and beryllium – completely contradict this conception.” A halo consists mostly of neutrons that are very weakly bound to the nuclear core, “normally with only one-tenth of the usual binding energy of a neutron inside the core,” Nörtershäuser said. The discovery of these exotic atomic nuclei created a new area of research, which Nörtershäuser has pursued since 2005 as the head of a young investigators group funded by the German Helmholtz Association at the university in Mainz and at the GSI Helmholtz Center for Heavy Ion Research in Darmstadt. The measurements revealed that the average distance between the halo neutrons and the dense core of the nucleus is 7 fm. Thus, the halo neutron is about three times as far from the dense core as is the outermost proton because the core itself has a radius of only 2.5 fm. “This is an impressive direct demonstration of the halo character of this isotope. It is interesting that the halo neutron is thus much farther from the other nucleons than would be permissible according to the effective range of strong nuclear forces in the classical model,” Nörtershäuser said. The strong interaction that holds atoms together can extend to a distance of only between 2 and 3 fm. The puzzle as to how the halo neutron can exist at such a great distance from the core nucleus can be resolved only by means of the principles of quantum mechanics: In this model, the neutron must be characterized in terms of a so-called wave function. Because of the low binding energy, the wave function falls off very slowly with increasing distance from the core. Thus, it is highly likely that the neutron can expand into classically forbidden distances, thereby inducing the expansive halo, the researchers say. By studying neutron halos, scientists hope to gain further understanding of the forces within the atomic nucleus that bind atoms together, taking into account the fact that the degree of displacement of halo neutrons from the atomic nuclear core is incompatible with the concepts of classical nuclear physics. Their work was supported by the Helmholtz Association, the GSI Darmstadt and the Federal Ministry of Education and Research. A paper on the research appears in the Feb. 13 edition of Physical Review Letters.For more information, visit: www.uni-mainz.de/eng/