Re: question on pair spectrometer shielding

[Alexander Somov <somov@jlab.org>]

Richard,

I don't understand completely your concerns:
- There is enough space after the chamber in the case
we decide to unscrew it and move aside to map the field
(we could probably make inserts in the chamber to measure
the field)
- The platform can hold heavier objects, should not be an
issue (the platform support should be beneath the blocks)

We can replace cheap iron blocks with a band of lead bricks;
(we will need  ~20/30 radiation lengths of shileding anyway).
that is what we were thinking of doing originally.
Let's discuss it during the meeting.

Alex



On Mon, 23 Mar 2009, Richard Jones wrote:

> Alex,
>
> Can we have a discussion of this at the working group meeting this week? 
> This iron will make mapping in the pair spectrometer more complicated because 
> the iron has to be present during the mapping and not allowed to move.  There 
> will be large horizontal forces in addition to the weight.  I don't remember 
> discussing this before.  I wonder what can be the shielding benefit from this 
> iron outside the region of the gap, since it is already shadowed by 2m of 
> yoke iron.
>
> -Richard Jones
>
>
> Alexander Somov wrote:
>> Richard,
>> 
>> There were two main reasons of using the wall after the pair
>> spectrometer:
>>
>>   - to reduce the size of a shielding lead wall after the pair
>>     spectrometer; lead bricks are more expensive (if purchase them)
>>     than this iron blocks. This blocks should even be available in
>>     Jlab. These blocks are also not that heavy (about 1 ton each,
>>     8 tons in total) so that they could be placed on the GlueX
>>     platform without aditional reinforcing it.
>> 
>> - the wall also reduced remaining bg from the cave. Sweeping magnets
>>    deflect particles in horizontal directions.
>> 
>> All in all, after some discussions we decided to use this blocks
>> given that they don't  (marginally) affect the cost. They also
>> don't hinder moving out detector components.
>> 
>> Cheers,
>>         Alex
>> 
>> 
>> 
>> 
>> On Mon, 23 Mar 2009, Richard Jones wrote:
>> 
>>> Alex,
>>> 
>>> I am going through the new geometry for the photon beamline, and have a 
>>> question.  What is the purpose of the large iron wall 17.8cm thick at the 
>>> downstream end of the pair spectrometer?  It seems that this will play no 
>>> role, since there is already much more iron that this in the yoke and 
>>> poles of the dipole itself.  Is this a crude description of the support 
>>> structure for the magnet?  If it is not needed to shield the counters then 
>>> can I get rid of it?  If you think we need to shield the counters, can we 
>>> somehow reduce its dimensions? -Richard Jones
>>> 
>> 
>
>