The neutron halo is one of the most spectacular phenomena that nuclear structure physicists have observed in their exploration of the isotopes at and close to the neutron dripaline (the curve on a plot of atomic number versus neutron number beyond which neutron-rich nuclei begin “leaking” neutrons) using exotic beam facilities. Atomic nuclei are usually uniformly dense objects with surfaces that are nearly well defined, having only a modest amount of diffuseness. However, in halo nuclei one or more nucleons have wave functions that extend far outside the nucleus so that the matter distribution has a long tail.
The first halo nucleus observed was 11Li [1], which has two correlated neutrons in its halo [2]. The experiment that resulted in the observation of the halo was performed at Lawrence Berkeley Laboratory with the Bevalac, the only relativistic heavy-ion accelerator in the world in the mid 1980s. Researchers slammed a high-energy beam of the stable isotope 20Ne into a “production target,” and then separated the short-lived products of that “fragmentation” reaction using magnets mounted on the beam line. The resulting short-lived (half-life 8.6 ms) 11Li nuclei were then steered onto targets of beryllium, carbon, and aluminum and the cross-sections for these resulting reactions measured.
The radius of a nucleus as a function of atomic mass number A can typically be calculated as (1.2 fm)A1/3, which for 11Li would give 2.7 fm. The initial analysis of the data of Tanihata et al. gave a large rms matter radius of 3.11±0.16 fm, but refinements to the reaction model eventually resulted in a conclusion that the rms radius is even larger (3.53±0.10 fm [3]). Weak binding is critical to the formation of a halo, and with a two-neutron separation energy of 300±19 keV, 11Li provided the prototypical example of this as well. Finally, 11Li essentially consists of three pieces—two neutrons and a 9Li “core”—that all must be present for the system to be bound since 10Li is unbound. Such a three-body quantum system where all three parts must be present for the existence of the system is called “Borromean” after the three interlocking rings on the 15th century coat of arms of the Borromeo family in northern Italy (Fig. 1). The three rings are connected in such a way that the cutting of one ring results in the separation of all three.
With advances in the production of exotic beams that have involved both improvements in the accelerators for the “primary” stable beams and the development of highly sophisticated magnetic spectrometers to separate the exotic products of the fragmentation reactions between the stable beams and the production targets, detailed studies of dripline isotopes of heavier elements have become possible. As K. Tanaka and colleagues from several institutions in Japan now report in Physical Review Letters, the Radioactive Isotope Beam Factory at RIKEN [4], which delivered its first beam in 2006, has now been used to measure a two-neutron halo in the dripline nucleus 22C [5].
The sensitivity of the measurement is impressive, having been performed with a beam rate of only ten 22C nuclei per hour. With twice as many protons and neutrons as 11Li, 22C is the heaviest Borromean nucleus yet observed. The rms matter radius of 5.4±0.9 fm deduced for 22C differs markedly from the “standard” nuclear radius of (1.2 fm)A1/3=3.4 fm. Furthermore, the cross section for the 22C breaks sharply from the trend exhibited by the measurements of lighter carbon isotopes that Tanaka et al. also performed.
Link in the title. This is really cool!
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