hodo and micro

[Jim Stewart <jstewart@jlab.org>]

Hello Franz

Just a few thoughts here. We can discuss this with everyone in the meeting.

If the hodoscope comes first then the scintillators can be say 1cm away 
from
the flange which holds the window. The microscope will be then a few cm away
 from the hodoscope. This makes for a very compact arrangement. A cover 
plate
to protect the exit window would then also protect the hodoscope. As the
hodoscope is not in the way it is easy to work on or move the 
microscope. You
will need to add a piece of light guide to the scintillators to move the 
pmt's low enough
to not hit the window. Except for that I think there are a lot of 
mechanical simplifications
with this option.

You are of course correct in your mail that the deciding factor needs to 
be the resolution in
the microscope. Richard now has a 1mm aluminum exit window and claims 
not to see any
influence in the resolution from this. Based on this he expects changing 
the order to have
no measurable influence on the resolution.

Doing this will move the microscope ~60mm  away from the exit window
(your biggest scintillator is 30mm wide). This gives a drift distance of
60mm/sin18deg= 200mm. This is still rather small and the particle do have
a high energy. The effect of multiple scattering of a 4GeV particle on a 
250um kapton
window combined with a 200mm drift distance is an angular spread of 
0.1mrad which
gives a position spread on the microscope of 20um. The effect of 
multiple scattering of a
4GeV particle on a 5mm scintillator is an angular spread of 0.4mrad 
which combined with
a 130mm drift gives a 50um position spread. These numbers are all small 
compared to the
2000um square fibers.

If we take the beam emmitance of e_x=10 mm-urad and deltaX=1.6mm then the
angular spread of the beam at the radiator is 6urad. Over a 10m drift 
distance this
gives a spread of 60um. Therefore the spread given by multiple 
scattering is roughly the
same as the intrinsic spread in the beam. (the beam is a factor of 2 
better in y)

I think we need a MC study but my initial reaction is that it is worth 
looking into.

Jim


Franz Klein wrote:
>
> Richard and Tim,
>
> in the updated design the fixed array is 30cm displaced from the focal 
> plane. Last May I didn't have any drawings from Richard ... but that 
> has changed! It doesn't make sense to have the microscope (high 
> resolution device) behind the fixed array (low resolution device).
>
> Franz
>
>
> On Mon, 29 Sep 2008, Richard Jones wrote:
>
>> Tim, Franz,
>>
>> Another possibility would be to put the microscope BEHIND the fixed 
>> array. The fixed array counters are only 4mm thick, and the entrance 
>> window to the microscope could be located a few cm behind those 
>> counters.  The geometry of the microscope works equally well in the 
>> two positions.  On the other hand, pushing back the fixed array makes 
>> its support rail substantially longer (several meters) at the 
>> downstream end (9GeV electrons, 3GeV photons).  It also makes sense 
>> at the low-energy end of the focal plane (photon energies near the 
>> end-point) to have the fixed array counters right next to the exit 
>> window.  There the focal plane deviates substantially from a straight 
>> line and having 70 cm of extra flight path between the focal plane 
>> and the tagging counters down there is not ideal.  If we put them 
>> next to the exit window there then we might as well design it that 
>> way along the entire focal plane.
>>
>> In the present design, we are configuring the fixed array to be able 
>> to do endpoint tagging without the microscope, just using the fixed 
>> array.  With the latest microscope design, there is now the added 
>> possibility of reconfiguring the microscope for endpoint running and 
>> moving it down there behind the fixed array in case one wants the 
>> highest tagging resolution and rates at the highest energies.  Given 
>> the low profile of the fixed array and the extended support that is 
>> needed to hold it, I think it makes the most sense to put it next to 
>> the exit window and mount the microscope behind it on a movable table.
>>
>> Richard Jones
>>
>> Tim Whitlatch wrote:
>>> Hi Richard,
>>>
>>> We can move it back as far as you wish (within reason). When all the 
>>> numbers and locations are agreed upon, we will update the drawings 
>>> to reflect the changes.
>>>
>>> Cheers,
>>>
>>>       Tim
>>>
>>
>>
>
> ===============================================================
>                   Franz J. Klein, Associate Professor
>                   CUA, Department of Physics
>                   Washington, DC 20064
>   office: Hannan Hall 206          phone: 202-319-6190
>   or: Jefferson Lab,CC F-243       phone: 757-269-6671
> ---------------------------------------------------------------
>