Vladislav,
Here are some answers to questions that Igor forwarded to me regarding
tolerances in the collimator cave specs.
>
1. Do you have dimensions between the collimator parts that are
critical
> and should be kept with small or even without any changes?
Igor -- This, I understood, concerns their freedom to move the
objects and also round to English units.
- The first and most critical one is the aperture of the primary
collimator. That aperture is stated to be 3.4mm in diameter. The
inner wall of that opening should be at a radius of (1.7 +/- 0.05) mm
from a straight-line that defines the collimator axis. The outer
radius of the primary collimator is stated as 100mm. That is not a
critical number, can be adjusted by a few percent. Some means must be
used to keep the individual disks of the primary collimator in mutual
alignment. One way would be to set them in a common V-shaped mount.
This requires strict tolerances on the uniformity of the radius with
respect to the central hole axis. It might be easier to drill a couple
of precision through-holes lengthwise through the collimator disks
somewhere out near the outer radius (not a problem if a bit of tungsten
near the outer radius is replaced with steel) and insert rods through
them to provide the alignment between them.
- The active collimator at the front of the primary collimator is
already built, so the dimensions there are not adjustable. The only
critical one there, again, is the tolerance of the co-alignment between
the central axis of the active collimator and the axis of the primary
collimator. That number should be +/-125 microns.
- Downstream of the primary collimator, the apertures are less
critical. There the beam openings may be round or square, and
dimensions of the openings varied within +/-5%. If there is a good
reason to vary things even more than this, please talk with Sascha to
see how critical it is. He is currently our best expert on the layout
of the magnets and shielding downstream of the primary collimator.
> 2. What are the position tolerances (true positions) should be
kept
> between the axes of openings (holes) in the collimator parts?
Igor -- I think he was mainly worried about the precision to
which they can pour the concrete blocks and position the sweeping
magnets. Could we review the tolerances on all these beamline
components
- To see how these tolerances fit into a global plan for alignment
of the
entire photon beam line from the radiator to the GlueX detector, see my
alignment plan writeup on the web at
http://zeus.phys.uconn.edu/halld/civil/alignment/photonBeamAlignment.html
- The above global plan should set the scale for how much the beam
can move around, which gives an idea of how much extra buffer needs to
be left between the shadow of the beam through the primary collimator
and any material that might interact with the edges of the primary beam.
> 3. Is 100 mm diameter of exit window in front of an active
collimator
> OK? Maybe you have suggestions?
That would be a lower limit. I would rather see a diameter of 200mm,
which would match the diameter of the primary collimator face. One
reason for having such a large beam pipe is that there can be
considerable settling in the site over the first few years after
construction. Another reason would be to be able to sweep the beam
across the active collimator during setup to profile the photon beam,
and not run into backgrounds from the beam scraping on the exit
flange. Does this make sense?
>
What kind of material and thickness we should use for 1-st and 2-nd
> windows: Al foil or another?
Igor -- I really did not understand what he was worried about
regarding the flanges. Is he worried about the tails of the beam
hitting something bulkier than foil? Also, he asked whether we are back
in a vaccum once we reenter the pipe on the way out of the collimator
hall.
Aluminum should be fine. Yes, we are back in vacuum soon after the
primary collimator. The gap in the permanent magnet sweep after the
primary collimator is evacuated.
> 4. Is active collimator attached to the tungsten discs? If yes -
how did
> you plan do it?
Igor -- I thought I answered him on that one that we would
rather leave it along for fear of ground loops. Shall we insist on
leaving it the way it is and just ask for a precise enough radius of
tungsten collimator discs so that the holes align with active
collimator on the v-shaped platform?
The only surface foreseen for holding the primary collimator is the
outside cylindrical surface of the insulating support. This is for
electrical isolation of all of the conducting components. The entire
assembly (active + primary collimator) has to be mounted on a
remote-control translation table to be able to move the collimator in
and out of the beam in the horizontal plane. If a V-shaped holder is
used to hold the primary collimator in alignment then shims can be used
to hold the active collimator in proper registration against the
primary aperture. There is no rigid connection between the active
collimator and the tungsten disks.
> 5. 2-nd collimator: should we join the nickel discs between each
other
> or not? The same for tungsten discs?
Igor -- I told him that we segmented mainly for their
machining convenience, but he seemed to imply that fewer blocks may be
better. Also he wondered why nickel instead of having both tungsten
Tungsten is the best material for the primary collimator because of its
small Molliere radius -- it lets a minimum of the showers from
collimated photons leak back into the beam aperture. Nickel is best
for the second collimator because it has the lowest yield of neutrons
per GeV of absorbed energy of any reasonably dense solid. The best way
to reduce neutron backgrounds is not to make them in the first place.
Richard Jones
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