Re: PS target issues (fwd)

[Jim Stewart <jstewart@jlab.org>]

Hello Hrachya and Richard

The present suggested design has the converter ~2.5m upstream of the 
magnet. The converter is the last item in the
cave extension before the concrete wall.  We want the facility to 
install detectors in front of the pair spectrometer
magnet which means there needs to be some space to work.. At present 
there is also about 1m space between
the magnet and the concrete wall. If we have a better idea how large the 
converter will be we could check to
see if it would fit in front of the pair spectrometer magnet. I expect 
the space should be enough. If this is the case
then you could design the converter system so that you could put the 
harp on either side of the wall. I do not
expect this to cost very much as it is mainly a set of extension cables 
for the harp and a straight piece of vacuum pipe.
 

The gain in moving the converter closer is not great as I understand. If 
the radiator is 10^-3 the dominant uncertainty
comes from the spot size and not multiple scattering, final state 
bremsstrahlung, and the finite opening angle .

I think we should try to make it possible to put the converter (modified 
harp) in either position.

Other changes like an active target can be discussed when one thinks 
about the detector package for the asymmetry
polarimeter. Here changes in technology in the next 10 years could have 
a large impact.  I would still like to look at
pixels. There is a lot of work on x-ray pixel detectors. 10^-3 radiation 
lengths is 90um in Si which is close to what
people are building.

Jim

Hrachya.Hakobyan wrote:
> Richard,
>
> You are right, I've proposed the use of two inserts for needs of PS 
> optimal functioning. Quantitatively I may answer few days after.
> The structure and costs of inserts for the  changeable converter and 
> microstrip  detector are different and I believe they have to be
> constucted independently, not to be a universal single, that  will 
> complicate device.  I didn't understand the problem of Hall D schedule 
> disruption with case of two independent converters.
> I can't do a  correct evaluations for JLAB, you have a better 
> knowledge of course.  But I  may  imagine that the standard vacuum 
> pipes,vacuum flanges and cross-like inserta are available in catalogs.
>
> Hrachya
>
>
> On Thu, 11 Sep 2008, Richard Jones wrote:
>
>> Hrachya,
>>
>> Are you proposing that we move the position of the converter from its 
>> current position ~2m upsteam of the PS magnet, and then envision a 
>> second converter somewhere else when we get around to designing the 
>> polarimeter?  We should consider this possibility, but I am skeptical 
>> that it makes much difference. Can you tell us quantitatively what 
>> gain we get in final energy resolution by moving the converter 
>> downstream a couple of meters from its present position? At least at 
>> its present position it is possible to think about doing polarimetry 
>> with the same converter.  We are never going to find anything like 19 
>> m of space for doing polarimetry in the way we did at YERPHI.  To use 
>> that system, we would need to scale that distance up by another 
>> factor to account for the higher energies involved.  Whatever we do 
>> (and I have some ideas) we will need to do within a few meters space 
>> that we have between the second sweeping magnet and the PS.
>>
>> If we move the converter to the entrance of the PS magnet, we 
>> probably double the cost of adding a polarimeter at some point in the 
>> future, and certainly make installing it much more disruptive to the 
>> running schedule of the hall. We could do that, but we need a 
>> compelling reason for it.
>>
>> Richard Jones
>>