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

[Tim Whitlatch <whitey@jlab.org>]

Hi Richard,

A quick calculation of a .5mm Kapton window with a 20cm diameter will 
exceed the yield strength of the material and deflect over .5 inches at 
the center under full vacuum. In order to stay under yield stress, it 
will need to be greater than 1mm thick.

Tim

Richard Jones wrote:
> 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
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