Re: [Fwd: Collimator layout questions...]

[Jim Stewart <jstewart@jlab.org>]

Hello Richard

I have a comment concerning the outer diameter of the exit window in 
front of the active collimator. As the
diameter of the window increases you will also have to increase the 
thickness of the window. This will increase
the bremsstrahlung  from this material on the  target. I do not  know 
how big an effect this will be but it can be looked at in the MC.

Jim



Richard Jones wrote:
> 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
>