Technical Design Report Super Fragment Separator

DRAFT
commissioned in February 2005 and characterized with relativistic 58 Ni ions at the FRS [61], see
Figure 2.4.119. During the on-line test, an overall efficiency of (1.8 ± 0.3) % has been achieved at
a gas cell pressure of 100 mbar helium, which can be divided into a stopping efficiency of
(5.0 ± 1.1) % and an extraction and transport efficiency of (35 ± 9) %. The overall efficiency is
hence limited mostly by the stopping efficiency, which could be increased in the future by operating
the stopping cell at higher pressures. From extraction time measurements of polyatomic ions
formed in the gas cell extraction times of atomic ions of 20 ms to 50 ms can be derived. In these
tests, only beam intensities of 2 ⋅ 10 5 ions per spill every ~ 8 s were used. However, on-line tests
with a similar gas cell, operated at the MLL tandem accelerator at Munich, have shown the
promising result that the overall efficiency of the stopping/extraction system stays constant up to
intensities of a few 10 8 107 Ag-ions per second [62]. To circumvent problems with molecular
contamination created in the gas cell in the future, a cryogenic stopping cell cooled with liquid
nitrogen is under development in collaboration with KVI Groningen. This stopping cell will also
allow reduced diffusion losses to the walls of the device.
As an alternative approach, groups of JYFL Jyväskylä and KVI Groningen are developing and
investigating the possibility to stop and extract exotic nuclei in/from superfluid liquid helium[63].
Similar extraction times and efficiencies as in the case of gaseous helium are expected, but the
approximately 800 times higher density (as compared to gaseous helium at standard temperature
and pressure) will allow for a much more compact apparatus. After thermalization in superfluid
helium, positive ions form “snowballs” – clusters of helium atoms that form around positive ions
due to electrostriction. Applying electrical drag fields, the snowballs drift to the superfluid helium
surface and escape to the vapour region with high efficiency. In the near future further tests with
improved setups are planned: off-line tests at KVI and on-line tests at JYFL. These studies will be
further pursued as part of the European DIRAC design study.
Figure 2.4.118: Off-line setup of a high-pressure helium-filled stopping system for relativistic heavy ions.
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