Technical Design Report Super Fragment Separator

DRAFT
Figure 2.4.116: Schematic view of the degrader systems, which will be installed at the focal planes PF2,
MF2, and MF11 of the Super-FRS. They consist of two rotating parts with wedge-shaped disks to generate
the wedge-angle α and two linear drives equipped with rectangular wedges for the continuous thickness
variation. Additional degrader plates may be added on an additional linear drive (not shown).
Detailed investigations [4,7,58,59] have been carried out to investigate the performance of a
fragment separator employing a degrader system in dependence of the degrader material. These
studies take into account nuclear absorption in the material, atomic processes like energy-loss
straggling, small-angle scattering and charge-exchange, and the transport of the ions through the
ion-optical system. Although for the lightest ions (A < 20) a very light degrader material would be
preferred, the overall optimum choice yielding high transmission for all nuclei is a medium-heavy
element, like e.g. aluminium. Because of practical reasons, also quartz will be investigated (see
below).
The energy spread additionally imposed on the fragments when penetrating through the degrader is
the limiting factor of the resolution for the subsequent magnetic rigidity analysis. Thus it needs to
be minimized and adjusted to the ion-optical resolution. Energy-loss straggling is unavoidable and
thus the ultimate limitation. Practically, the main limitation arises from inhomogeneities and imperfect
alignment of the degraders. The real shape of the degraders should match as good as possible
the ideal shape, thickness non uniformities arising from inhomogeneous material (density)
and/or surface inhomogeneities must be minimized. The tables given below show the tolerable
values such, that the inhomogeneities cause the same width at the final focus as does the energy-loss
straggling. These tolerances are of the order of few ten micrometers in order to preserve
the ion-optical resolution, so that the degrader plates and the mechanical holders and drives require
very high precision.
Because of the strong requirements, quality control is an important issue. Although several different
manufacturers promise to fulfil the specifications (shape tolerances, surface uniformity,
density homogeneity), it has turned out that quality control needs to be done in-house. In the past
years, the GSI target laboratory has developed tools to measure tolerances on a micrometer scale.
The best results have been obtained with quartz material, which showed sub-micron surface tolerances
and deviations from the ideal shape of very few microns only. Thus, from this point of view
quartz is the preferred material. However, due to the manufacturing process, the minimum thickness
is limited to values exceeding about 5 millimetres for each plate thus yielding at present only
rather thick plates. In the near future we will explore other possibilities together with industry.
According to the specifications, the prototype system described in section 2.4.11.2.3 already
reaches this precision. In an experiment we have tested Suprasil®-2 material (which is quartz-type
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