Re: background simulation

[Alexander Somov <somov@jlab.org>]

Hello Richard,

I think that option 2 or 3 would be fine.
I don't think that we will need to replace the
secondary collimators often. We will probably
start with a 10 mm collimator and make it smaller
later on. Jim actually suggested to make a 10 mm
collimator hole and insert there a tube with a smaller
inner radius. It should be relatively easy to change
and should not cost much.

I will check how much exactly we gain by decreasing
the size of the 2nd collimator. There are two effects
here: we reduce the number of beam-hallo photons
that enter to the detector region, and reduce the
number of interactions with the beam pipe before
the detector.

> 2. It might be worth taking Fig. 16, panel 3 to the electronics guys and
>  ask if that high flux of low-energy gammas in the area of the
>  electronics racks is a concern to them.  It is probably not enough
>  for them to care, but it is 5 orders of magnitude above cosmic levels
>  so I don't want to assume anything.

I think that it's a good idea. Actually the photon/electron fluxes
are not 5 orders of magnitude above cosmic level. The plot/caption
might be a bit confusing. What I plot here is the rate integrated over
10 cm radius bins, i.e., for R = 200 cm for photons we get

Rate (cm-2 sec-1) ~ 6 10^4 Hz / ( 2 pi R dR) ~ 5 cm-2 sec-1
<En photons> ~ 6 MeV

For electrons we have
Rate ~ 0.5 cm-2 sec-1
<En el> ~ 23 MeV

The rate of cosmic muons is 1 cm-2 min-1 ~ 0.02 cm-2 sec-1, which
is about 300 and 30 times smaller than that for low-energy photons
and electrons, respectively (I would say that the rates are negligibly
small for electronics racks, but we have to check).

In general, we can extend shielding after the pair spectrometer a little
bit further in a radial direction from the beamline and completely
eliminate this bg.

Cheers,
       Sascha



On Mon, 14 Jul 2008, Richard Jones wrote:

> Sascha,
>
> Thank you for this thorough study that you have done, and the excellent
> report on your results.  I have the following comments.
>  1. You have decreased the diameter of the secondary collimator from 10mm
>     to 6mm.  This decision means that either we must make the secondary
>     collimator exchangeable, or we give up considerable flexibility in
>     the ability to open the beam aperture.  It is useful to have the
>     ability to do that to check alignment during beamline setup and
>     commissioning.  It also has an impact on our ability to run at higher
>     tagging ratios for efficient unpolarized running.  So my question to
>     you is, how much do we gain by that reduction?  I know you want to
>     shadow the support structure of the Liq. H2 target.  Is it worth
>     increasing the cost and complexity of the beamline by making the
>     secondary collimator removable?  If we need a 6mm secondary to run
>     GlueX under optimum bg conditions, I see three options.  Obviously I
>     prefer option 3, but we need to justify the cost.
>      +  reduce the aperture to 6mm and design our alignment and run plan
>         around that constraint
>      +  reduce the aperture to 6mm, but plan to go into the cave, open
>         the vacuum system and manually remove or replace the collimator
>         with a different one when the need arises.
>      +  install an in-vacuum collimator ladder at the secondary
>         collimator station, for maximum flexibility.
>  2. It might be worth taking Fig. 16, panel 3 to the electronics guys and
>     ask if that high flux of low-energy gammas in the area of the
>     electronics racks is a concern to them.  It is probably not enough
>     for them to care, but it is 5 orders of magnitude above cosmic levels
>     so I don't want to assume anything.
> Richard Jones
>
>
>
> Alexander Somov wrote:
>
>  Dear colleagues,
>
> I put a draft of the document describing simulation of collimator
> cave and pair spectrometer background to
>
> http://www.jlab.org/Hall-D/software/wiki/index.php/Collimator_cave_background
>
> (I will place it to the DocDB later)
>
>
> Cheers,
>        Sascha
>
>
>
>
>