Re: PS target issues

["Hrachya.Hakobyan" <hakopian@mail.yerphi.am>]

  Hello Jim,

I've promised to give a numbers for multiple scattering influence. In my 
recent mail, I've brought the following ones:

-------------------------------------------------------------------------
The  multiple scattering cone's spot size (horizontal projection angle's 
RMS times 3.5m converter distance from FSF and WSF arms  ) is ranged from:
- 0.2 to 0.62mm for thicknesses x=10^-3 to 10^-2 Xo and Pe=8GeV/c 
- 0.45 to 1.15mm for thicknesses x=10^-3 to 10^-2 Xo and Pe=3.5GeV/c
-----------------------------------------------------------------------

For comparison the coordinate resolutions for FSF and WSF arms in 24x6 
PS's  configuration are:

WSF arm - a) sigmaX= 0.72mm and 0.58mm for Pe=7 and 8GeV/c strips
           b) sigmaX= 1mm in case when both strips have a width
              app. 3.5mm equal to beam spot size        .

FSF arm - <sigmaX> = 1.7mm


Shift of  of the PS converter to 2.5m upstream increases the distance to 
detectors from 3.5 to 6m, introducing a scale factor 1.714 and modifying 
numbers as a following:

- 0.34 to 1.06mm for thicknesses x=10^-3  10^-2 Xo and Pe=8GeV/c
- 0.77 to 1.77mm for thicknesses x=10^-3 to10^-2 Xo and Pe=3.5GeV/c

As you may see an impact of multiple scattering is quite important and 
comparable with detectors coordinate resolution  and becomes even 
dominant at 6m base and thicknesses  x>4x10^-3Xo.

Hrachya



On Mon, 15 Sep 2008, Jim Stewart wrote:

> 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
>> > 
>
>