Prepared by Robert V.F. Janssens and Zheng-Tian Lu for the DNP webpage
For physicists the nucleus 6He, with 2 protons and 4 neutrons, has been intriguing for quite some time. Measurements in the eighties and nineties have indicated that, when used as a beam, the probability for it to induce a nuclear reaction on any target is much larger than that for 4He. This observation was interpreted as a strong indication that 6He is a three-body "halo" nucleus, i.e., it can be best viewed as a well bound 4He core and 2 neutrons orbiting this core at large distances. Moreover, while these three constituents of 6He form a bound system, the nuclear potential is not strong enough to bind any two of them separately. For this reason, 6He is often referred to as "Borromean" (The name derives from the heraldic emblem of the medieval princes of Borromeo, three rings interlocked in such a way that the removal of any of the rings will cause the remaining two to fall apart).
Because of its intriguing properties, 6He has the potential to teach us about the fundamental forces among the constituent nucleons. Indeed, the halo character can be revealed by an accurate determination of the nuclear charge radius, since the motion of the core with regard to the center of mass reflects both the radial extent of the neutrons and the correlations between these particles. The result can in turn be compared with the most modern theories as recent advances in computational methods have made it possible to calculate the structure of few-nucleon systems from the basic interactions between the constituents.
The charge radius of 6He has been determined for the first time by measuring the atomic isotope shift between 6He and 4He using laser spectroscopy. For this work, 6He atoms were produced at the ATLAS accelerator facility at Argonne National Laboratory, and quickly captured and cooled by an on-line laser trap. By applying laser spectroscopy on the trapped 6He atoms as well as on their 4He isotopic partner atoms, the charge radius of the 6He nucleus was determined to be 2.054 ± 0.014 fermi, approximately two millionth of a nanometer. The measurement is of such accuracy that it distinguishes between the available theoretical predictions. The data offer new insight into the dependence of three-body interactions on neutron number, which in turn is essential to the understanding of the structure of all neutron-rich systems, including neutron stars.
In this work, 6He nuclei were produced via the 12C(7Li, 6He)13N reaction with a 100 pnA, 60 MeV beam of 7Li from the ATLAS accelerator at Argonne National Laboratory. Neutral 6He atoms diffused out of the hot graphite target and were transferred in vacuum to the nearby atomic beam assembly at a rate of approximately one million per second. Trapping helium atoms in the 23S1 metastable level was accomplished by exciting the 23S1 – 23P2 transition using laser light with a wavelength of 1083 nm. 6He atoms were mixed with a krypton carrier gas and sent through a discharge to be excited to the 23S1 level. The metastable 6He atoms were transversely cooled, decelerated with the Zeeman slowing technique, and then captured in a magneto-optical trap at a rate of approximately one atom per minute.
This work is supported by the U.S. Department of Energy, Office of Science, Office of Nuclear Physics, under contract W-31-109-ENG-38.
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