Atomic nuclei are normally compact structures defined by a sharp border. About twenty-five years ago, it was discovered at the University of California in Berkeley that there are exceptions to this picture: Certain exotic atomic nuclei contain particles that shear off from the central core and create a cloud, which surrounds the central core like a 'heiligenschein' or halo.
The \'halo\' nucleus 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
An example of such a halo occurs in beryllium-11, a specific isotope of the metal beryllium. Here, the halo is made up of a single neutron. For the first time ever, scientists at the Institute of Nuclear Chemistry of the Johannes Gutenberg University Mainz in cooperation with colleagues from other institutes have succeeded in precisely measuring this one-neutron halo by means of a laser, and in evaluating the dimensions of the cloud. 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.
"We intuitively imagine the atomic nucleus as a compact sphere consisting of positively charged protons and uncharged neutrons," explains 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." These isotopes consist of a compact nuclear core and a cloud made of diluted nuclear material - called 'heiligenschein' or 'halo'. 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," explains Nörtershäuser.
The discovery of these exotic atomic nuclei created a new area of research, which Nörtershäuser as the head of a young investigators group funded by the German Helmholtz Association has pursued since 2005 at the University in Mainz and at the GSI Helmholtz Center for Heavy Ion Research in Darmstadt. Measuring halo nuclei is extremely difficult, since they can only be artificially created in minute amounts. In addition, these synthesized nuclei decay within seconds, mostly even in milliseconds.
Nörtershäuser's team has now succeeded for the first time in measuring the nuclear charge radius in beryllium-11. This nucleus consists of a dense core with 4 protons and 6 neutrons and a single weakly bound neutron that forms the halo. In order to accomplish this ultra-precise laser spectroscopic measurement, the scientists used a method developed 30 years ago at the University of Mainz, but combined it now for the first time with the most modern techniques for precise laser frequency measurement, i.e., by employing an optical frequency comb. This combination alone was not sufficient, though. Only by further expanding the method using an additional laser system it was possible to achieve the right level of precision. The technique was then applied to beryllium isotopes at the Isotope Separator On Line (ISOLDE) facility for radioactive ion beams at the European Organization for Nuclear Research (CERN) in Geneva. The professional journal Physical Review Letters published this work in its latest February 13 issue.
The measurements revealed that the average distance between the halo neutrons and the dense core of the nucleus is 7 femtometers. Thus, the halo neutron is about three times as far from the dense core as is the outermost proton, since the core itself has a radius of only 2.5 femtometers. "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," explains Nörtershäuser. The strong interaction that holds atoms together can only extend to a distance of between 2 to 3 femtometers. The riddle as to how the halo neutron can exist at such a great distance from the core nucleus can only be resolved 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 only 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 'heiligenschein'.
This work was supported by the Helmholtz Association, the GSI Darmstadt and the Federal Ministry of Education and Research (BMBF).Original publication:
Petra Giegerich | idw
Further reports about: > Atomic > Atomic Nucleus > Atomic nucleus of beryllium > Beryllium > GSI > HALO > Heiligenschein > Helium > Lithium > Nuclear > One-Neutron Halo > Physical > atomic nuclei > beryllium isotopes > classical nuclear physics > ion beam > isotopes > laser system > lasers > precise laser frequency measurement > ultra-precise laser spectroscopic measurement
APEX takes a glimpse into the heart of darkness
25.05.2018 | Max-Planck-Institut für Radioastronomie
First chip-scale broadband optical system that can sense molecules in the mid-IR
24.05.2018 | Columbia University School of Engineering and Applied Science
A research team led by physicists at the Technical University of Munich (TUM) has developed molecular nanoswitches that can be toggled between two structurally different states using an applied voltage. They can serve as the basis for a pioneering class of devices that could replace silicon-based components with organic molecules.
The development of new electronic technologies drives the incessant reduction of functional component sizes. In the context of an international collaborative...
At the LASYS 2018, from June 5th to 7th, the Laser Zentrum Hannover e.V. (LZH) will be showcasing processes for the laser material processing of tomorrow in hall 4 at stand 4E75. With blown bomb shells the LZH will present first results of a research project on civil security.
At this year's LASYS, the LZH will exhibit light-based processes such as cutting, welding, ablation and structuring as well as additive manufacturing for...
There are videos on the internet that can make one marvel at technology. For example, a smartphone is casually bent around the arm or a thin-film display is rolled in all directions and with almost every diameter. From the user's point of view, this looks fantastic. From a professional point of view, however, the question arises: Is that already possible?
At Display Week 2018, scientists from the Fraunhofer Institute for Applied Polymer Research IAP will be demonstrating today’s technological possibilities and...
So-called quantum many-body scars allow quantum systems to stay out of equilibrium much longer, explaining experiment | Study published in Nature Physics
Recently, researchers from Harvard and MIT succeeded in trapping a record 53 atoms and individually controlling their quantum state, realizing what is called a...
The historic first detection of gravitational waves from colliding black holes far outside our galaxy opened a new window to understanding the universe. A...
02.05.2018 | Event News
13.04.2018 | Event News
12.04.2018 | Event News
24.05.2018 | Ecology, The Environment and Conservation
24.05.2018 | Medical Engineering
24.05.2018 | Physics and Astronomy