B e T I N a  -  Be Halo Spectroscopy

(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 [1]. 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 (2s_1/2 -> 2p_1/2) was performed.

Figure 3: 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 2.463(16) fm.

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 contact us:

References

M. Zakova

D. Tiedemann