We shoot on a plain plate of tungsten to see what goes through.

Initial energy: 85MeV. Variation of the thickness of the plate of tungsten.
Fraction of the particles going through (left: all particles; right only with Pz>70MeV) at a given distance of the plate in a radius of 10mm around the axis.
A thicnkess of 3mm seems well suited.

The mechanical workshop believes it is difficult to drill holes of 0.1mm in 3mm of Tungsten. So the collimators will be made of two bits assembled with a shim of the desired thickness in the middle. The collimators will be screwed on an Aluminium plate. Once assembled , the shim will be removed. The images below show the bit and the full assembly, as well as their implementation in Geant 4.