**Discussion of BTeV Pixel Vacuum Issues**

**Major Requirements/Parameters of Pixel system that relate to vacuum
design considerations**.

Low mass (small fraction of a radiation length) in the acceptance
regions (\~300 mRad half angle cones on each side)

Pressure in beam region (to minimize detector background from beam-gas
interaction \<1e-8 torr

Liquid cooling for 3000 watt heat load

Feed through required for \~100K signals

A large quantity (surface area and volume of non-UHV compatible
materials)

Precision alignment: a few µM (the intrinsic resolution of pixel \~ 7 µM
)

Movable pixel sensors to clear beam during injection and to provide a
dynamic aperture horizontal 17 mm, vertical 6 mm

Located in a the 1.6 T field of a dipole magnet

Collider detector like space constraints. No space on the sides, some
room above and below

Shielding structure required: to reduce noise to pixel electronics,
minimize beam impedance and carry beam image current

**Vacuum System Design Concept Description:**

Membrane:

All non-UHV materials will be separated from the beam region by a thin
(0.15 mm) thick corrugated aluminum foil. Large pumping (\~1000 L/s
effective) will be provided on the high gas load side of the membrane. A
small amount of pumping will be provided on the UHV side of the membrane
to handle the outgassing of the membrane, beam pipe and gas passing
through restrictive apertures from the high gas load side of the
membrane. The membrane is not a sealed barrier. Long mean-free path
barriers will restrict any aperture between the high gas load side and
UHV side. We also hope to create a linkage between the high gas load
side and the UHV sides of the membrane that would not be restrictive in
the viscous flow regime. We want to prevent a significant differential
pressure across the membrane that would distort the membrane into the
beam or crush the pixel detectors. The ideal would be to do this
passively perhaps by having apertures that are restrictive to molecular
flow but not to viscous flow.

If we can come up with a way to make the membrane a sealed barrier we
would prefer that option. However given all the requirements we think
that is not likely to succeed or not likely to succeed without large
compromises of the other requirements.

Signal feed through:

6 multilayer printed boards will be mounted into frames on either side
of the vacuum vessel. O-rings will seal to the face of the board. The
signals will feed from the vacuum to air side via traces in the
multiplayer board.

Pumping:

Pumping on the large gas load side would be provided by cryopumps
(likely with special features). The pumping will be mounted on the
vessel with gate valves to allow regeneration. (Backing pumps located
outside the magnet)

Non-evaporable getter and possibly special ion pumps configured to work
with the dipole field might provide the UHV pumping.

**Development Work in progress: (most being done at PAB)**

Leak testing of printed circuit board feed through (checking seal on
face of board and of traces running to edge of multilayer board)

Measuring specific outgassing rate (using rate of rise method) for
materials in Pixel vessel

Measuring capacity of movable shield to maintain a pressure differential
in high vacuum regime.

Measuring conductance of various apertures in a molecular flow regime.

In the future: demonstrate proof of concept manufacturability of thin
aluminum pixel membrane