LHCb is in an effort to reduce as much as possible the material in
every subdetector. The VELO is particularly concerned by this.
We can do two things to reduce the material in the secondary vacuum foil:
1) to make it thinner (still aluminium but thickness <250 micron)
2) to make it out of Beryllium.
In Beryllium there is no complex (corrugated, deformed)
structure possible like in Aluminium. So, the idea is to give up the
overlap between the two opposite detector halves (which was enforcing
the complex toblerone shape), and using thin Beryllium in a simple
shape for the secondary vacuum foil.
Beryllium secondary vacuum foil box:
Different Si design:
Click here for FEA study
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