Laser Spectroscopic Determination of the Nuclear Charge Radius of ^6^He

L.-B. Wang,^1,2^ P. Mueller,^1^ K. Bailey,^1^ G. W. F. Drake,^3^ J. P.
Greene,^1^ D.Henderson,^1^ R. J. Holt,^1^ R. V. F. Janssens,^1^ C. L.
Jiang,^1^ Z.-T. Lu,^1,4^ T. P. O'Connor,^1^ R. C. Pardo,^1^ K. E.
Rehm,^1^ J. P. Schiffer,^1,4^ X. D. Tang^1^

*^1^Physics Division, Argonne National Laboratory, Argonne, Illinois*

*^2^Physics Department, University of Illinois at Urbana-Champaign,
Urbana, Illinois*

*^3^Department of Physics, University of Windsor, Windsor, Ontario,
Canada*

*^4^Enrico Fermi Inst. and Dept. of Physics, University of Chicago,
Chicago, Illinois*

For physicists the nucleus ^6^He, 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 ^4^He. This observation was interpreted as a strong indication that
^6^He is a three-body \"halo\" nucleus, i.e., it can be best viewed as a
well bound ^4^He core and 2 neutrons orbiting this core at large
distances. Moreover, while these three constituents of ^6^He form a
bound system, the nuclear potential is not strong enough to bind any two
of them separately. For this reason, ^6^He 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).![](media/image1.jpeg){width="1.9972222222222222in"
height="1.9972222222222222in"}![](media/image2.jpeg){width="1.95in"
height="1.9972222222222222in"}

Because of its intriguing properties, ^6^He 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]{.underline}
(<http://dnp.nscl.msu.edu/current/exact.html>) 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 ^6^He has been determined for the first time by
measuring the atomic isotope shift between ^6^He and ^4^He using laser
spectroscopy. For this work, ^6^He atoms were produced at the
[ATLAS]{.underline} (<http://www.phy.anl.gov/atlas/index.html>)
accelerator facility at Argonne National Laboratory, and quickly
captured and cooled by an on-line laser trap. By applying laser
spectroscopy on the trapped ^6^He atoms as well as on their ^4^He
isotopic partner atoms, the charge radius of the ^6^He 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.![](media/image3.png){width="4.544444444444444in"
height="1.725in"}

**Method: Production and Laser Trapping of ^6^He Atoms**

In this work, ^6^He nuclei were produced via the ^12^C(^7^Li,
^6^He)^13^N reaction with a 100 pnA, 60 MeV beam of ^7^Li from the ATLAS
accelerator at Argonne National Laboratory. Neutral ^6^He 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 2^3^S~1~ metastable level was
accomplished by exciting the 2^3^S~1~ -- 2^3^P~2~ transition using laser
light with a wavelength of 1083 nm. ^6^He atoms were mixed with a
krypton carrier gas and sent through a discharge to be excited to the
2^3^S~1~ level. The metastable ^6^He 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]{.underline},
[Office of Science]{.underline}, [Office of Nuclear
Physics]{.underline}, under contract W-31-109-ENG-38.

**Reference**

-   Project homepage: <http://www-mep.phy.anl.gov/atta/>

-   *Laser Spectroscopic Determination of the ^6^He Nuclear Charge
    Radius.* L.-B. Wang *et al*., Phys. Rev. Lett. 93, 142501 (2004*)*
    [Preprint *(nucl-ex/0408008)*](http://arxiv.org/abs/nucl-ex/0408008)