(Beryllium Trap for the Investigation of Nuclear charge radii)
Halo-nuclei have a very exotic nuclear structure because they do not exhibit a constant
density throughout the nucleus with a sharp borderline as we expect it from "classical"
nuclear physics. Rather they consist of a compact core of "usual" nuclear matter density
which is surrounded by one or more weakly bound neutrons orbiting at a rather large
distance (see group emblem). The mean distance is so large that they spend more time
outside of the attractive nuclear interaction than inside. Hence, from a classical point
of view, they should not be bound at all.
Halo-nuclei of the lightest elements (He, Li, Be) have been discovered in the 1980s by
Tanihata et al. and since then they have been subject of many experiments. However, some
elementary questions have still not been answered completely, like those about the
correlations between the halo-neutrons and the influence of the halo neutrons onto the
structure of the nuclear core.
As the charge radius of the nucleus is defined exclusively by the protons in the inner
core, it offers an ideal approach to study the inner core decoupled from the outer
halo-neutrons. A measurement of the isotopic shift, i.e. the change of the transition
frequency in an electronic transition between different isotopes, provides the only
model-independent access to this parameter. However, these measurements are challenging
since the halo isotopes are very short-lived - they decay within seconds, in most cases
even milliseconds - and they have to be performed with very high accuracy because the
effect that has to be measured is just a 10-9 contribution to the total transition
frequency. So far, only two groups succeeded in such measurements on halo isotopes: One
group from Argonne National Laboratory in USA determined nuclear charge radii for 6He
and 8He (see website) and our group
obtained the charge radius of 11Li (see ToPLiS experiment).
These measurements are based on very accurate atomic structure calculations that have to
be performed with spectroscopic accuracy. Up to now, such calculations are only possible
for atoms with at most three electrons. Hence, for measurements on beryllium isotopes,
with the goal to determine the nuclear charge radius of the one-neutron halo nucleus
Be-11, spectroscopy must be performed on Be+ ions instead of neutral atoms.
Figure 1: Simplified experimental setup.
Two dye lasers, operating at 626 nm, are frequency stabilized to an iodine line and a
frequency comb, respectively. The laser light is then frequency doubled to reach 313 nm
and superimposed with an ion beam in the COLLAPS beam-line. The fluorescence signal is
detected with two photomultipliers. - click for big version
A standard technique for isotope shift measurements on short-lived ions is collinear
laser spectroscopy (see COLLAPS website). However, for light elements like lithium and beryllium
it cannot be used in the conventional way because the uncertainty of the exact
acceleration voltage introduces uncertainties that are larger than the effect to be
measured. We have developed a modified approach, based on simultaneous collinear and
anti-collinear excitation and accurate frequency determination with a frequency comb
The setup for this technique is schematically shown in Fig. 1. The laser system consists
of two dye lasers at 626 nm, a frequency comb, and two frequency doublers
(SHG=second harmonic generation). Laser frequencies are stabilized to hyperfine structure
transition in molecular iodine and the frequency comb, respectively.
Figure 2: Part of
the laser setup used in experiment. Two cw ring dye-lasers pumped with Nd:YAG
lasers. On the left side is part of the iodine-lock system. - click for big version
The laser system was developed at Mainz University and then transported to the European
Nuclear Research Center CERN at Geneva, where the radioactive beryllium ions were
produced at the on-line facility ISOLDE. Beryllium ions were produced in the radioactive
ion source by bombarding a uranium carbide target with 1.5 GeV protons. After resonant
laser excitation and ionization the Be+ ions are extracted at 50 keV, mass separated and
delivered to the COLLAPS (Collinear Laser Spectroscopy) beam-line where fluorescence
spectroscopy on the D1 line (2s1/2 -> 2p1/2) was performed.
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.
- click for big version
From the measured absolute transition frequencies of the different beryllium isotopes, we
could extract the isotope shifts and combine those with the latest calculations of the
so-called mass-dependent isotope shift (this is the uninteresting part of the isotope
shift that is merely caused by the change of the nuclear mass between the isotopes). The
difference between the measured and the calculated value is proportional to the change in
the (mean-square) charge radius. Hence, we obtained for the first time charge radii for
the isotopes Be-7, Be-10 and Be-11. The charge radius of 11Be was determined to be
Interpreted in a quite simple two-body model, the change in the charge radius between
Be-10 and Be-11 can be accounted to the change in the center-of.-mass caused by the halo
neutron. This assumption leads to a distance between the halo neutron and the
center-of-mass of Be-11 of 7 fm, which is almost three times as large as the "classical"
binding length of 2.5 fm of the nuclear force. This is depicted in Fig. 3
Measurements of nuclear charge radii in beryllium isotopic chain are not yet finished.
Two isotopes, Be-12 and Be-14 are even more exotic isotopes. But there production rates
are one-thousand times smaller (for Be-14 even a million times smaller) than for Be-11.
Moreover, they live only a few milliseconds compared to the 3.6-s isotope Be-11. For
Be-12 the problem might be overcome with the pre-cooling and bunching of the produced ions
with the ISCOOL set-up available at ISOLDE/CERN. Activities in this direction have been
started. For Be-14, however, the production rate must be considerably increased before
high-resolution measurements become possible. Additionally, the accuracy can be improved
with measurements on laser-cooled ions in a Paul-trap, which is being built up here in
the Institute for Nuclear Chemistry, Mainz, Germany. Students (diploma, phd) who would
be interested in one of the experiments can find more about available projects here or