X-ray spectroscopy with the TITAN EBIT on stable (and unstable) nuclei

The TITAN EBIT will allow for charge breeding of the injected ions to high charge states, for all elements at least to a Li-like configuration, while operating with an electron energy of up to 60 keV. The isotopes from the on-line separator or an off-line ion source will be injected into the EBIT, charge-bred and extracted to the following experiment or application. These operating conditions define the necessities for basic spectroscopy. In a latter stage more sophisticated measures are clearly possible and desired to make full use of the unique potential of the EBIT coupled to an on-line source of exotic nuclei. In the following the first step of x-ray spectroscopy is described.

The standard EBIT equipment for monitoring the composition and charge of the trapped ion cloud is x-ray spectroscopy using an intrinsic Ge detector. This device combines ease of operation with moderate spectral resolution and a large solid angle of detection while converting the full range of x-ray energies that is expected from the ions that can be produced in an electron beam ions trap like the proposed one. The device would be coupled to the optical excess port which is foreseen radial in the EBIT system. Considering that the charge state balance in an EBIT depends on the operating conditions including the background vacuum, a performance as designed is not necessarily achieved all the time. Thus it is of paramount importance to ascertain that the device actually is clean enough so that the anticipated fast charge breeding cycles meet their goal. Experience at Livermore EBIT [42] and the MPI EBIT group [43] in Heidelberg demonstrate quite obviously that external measurements are not good enough to tell this; only experiments can prove the validity of the operational setting.

In most electron beam ion trap laboratories this type of x-ray detector is used for time-integrated studies only. The performance parameter of interest at the TITAN EBIT, however, is not the average status of the ion cloud, but the result of a rapid development process. This time-specific diagnostic can be achieved by the time-resolved spectroscopy, evaluating x-ray spectra that have been accumulated in predetermined time bins. Obviously, one would not want to spend costly exotic isotopes on such tasks, but rather employ a conventional ion source, where ions in the same mass range of the isotopes of interest can be produced.

Atomic spectroscopic studies of radioactive ions have been until recently extremely difficult. Yet precisely these studies can deliver important data not only on the electronic structure of the ions, but also on their nuclear properties. For highly charged ions, the spectroscopic data wealth is increased (as the number of charge states), and their sensitivity to nuclear properties is greatly enhanced due to scaling laws. From the observation of isotopic shifts in the x-ray region, for instance, nuclear charge radii have been accurately determined for different highly charged ions of uranium isotopes.

Information on the Bohr-Weißkopf effect can also be extracted from the analysis of the spectral data in the different charge states, and this technique has delivered the most accurate data on the magnetization distribution of any nuclei by using hydrogenic ions. The EBIT device uniquely allows to collect useful x-ray (or UV or visible) radiation with samples as small as 10 ions, although typically the number of ions trapped is in the range 104 to 106. The continuous excitation by the electron beam makes the most efficient use of the trapped ions.

The possibilities opened with an EBIT, and its ability to obtain that kind of data as it performs its intended task of breeding higher charge states makes uniquely appropriate to deepen our understanding of the properties of such exotic nuclei. For this specific part of x-ray spectroscopy, a dedicated spectrometer would need to be built. This device will not be part of this application. The potential use of the TITAN EBIT for such spectroscopy is, however obvious, and will be perused in a later stage of the project.