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

[Richard Jones <richard.t.jones@uconn.edu>]

Jim,

Bremsstrahlung should be very small because the beam is essentially 100% 
gammas when it reaches the exit of the big steel pipe.  If material 
interactions are a concern, I suggest simulating with 0.5mm of kapton, 
which could hold vacuum across a 20cm diameter round aperture no 
problem.  A couple microns of aluminum coating is sufficient to make it 
vacuum-tight.  A rough calculation indicates loss of 0.2% tagging 
efficiency, and additional bg beam flux at the GlueX target of about a 
factor 1e-4, mostly low-energy. Simulation should show that secondary 
gammas inside the beam envelope are still dominated by scattering from 
the interior edge of the primary collimator.

I agree that it should be checked.  I do think it is important that we 
be able to characterize the complete beam shape at the collimator 
without being confused by bg from scraping in the delivery pipe exit 
flange.  Remember that these interactions, even if it is only with beam 
halo, will be greatly amplified by showering and will distort the 
information about the beam location and shape seen by the active 
collimator and any other gamma profile monitor that we may install in 
the collimator region to characterize the photon beam.  There cannot be 
any pre-collimator in front of the primary collimator, because of this.

Richard Jones


Jim Stewart wrote:
> 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