Active collimator for Hall D notes

[Elton Smith <elton@jlab.org>]

10/21 Afternoon
---------------
Elton Smith, Yuri Sharabian, Arne Freyberger, Phil Cole, Richard Jones and 
Chris Gauthier

Any participants, please comment/add to these notes.

We met by phone this afternoon to review the proposal for a prototype 
active collimator for Hall D which has been simulated by Chris Gauthier 
(UConn student working with Richard Jones). A document which simulates the 
response of Position-sensitive Tungsten Pin-Cushion Detector (soon to be a 
Hall D note) was used as a basis for discussions. The design is based on a 
SLAC design (Miller and Walz, NIM 117 (1974) 33-37).

We discussed methods for building the pin-cushion detector. Richard and 
Chris have talked to the authors and hand fabrication with W wire has 
initially tried and considered too labor-intensive. A suggestion is to use 
EDM (Electrostatic discharge machining, or simply electro-erosion) to 
machine pins produced using orthogonal slots into a solid piece of W. The 
JLab machine shop has experience using local shops to do similar work. It 
was agreed that Chris and Richard will make the drawings available on the 
web, so that Youri can obtain cost estimates for such work.

Prototype Suggestions:
- Build two small slices (instead of 8 necessary for the final design)
- Check conditions in Hall B which could be used for testing the 
prototype.
- Prototype would be connected to a Labview DAQ system for testing
- Suggestion was made to purchase a PCI-based 12-bit ADC and two preamps 
for the two slices. Richard and Chris will send us specific suggestions of 
hardware which could be purchased by JLab and lent to UConn for the 
prototyping tests.
- Boron nitrate material would be needed for the prototype.

Electronics:
- Arne suggested that the electronics be designed witha built in 
calibration loop which injects a known current onto the plates for 
calibration.
- Final electronics would be revisited and interfaced to EPICS for control 
in the Hall D beamline.

Testing:
- Calculations to date were performed assuming a 1 micro-A beam on a 
10**-4 radiator. 
- Typical running conditions in Hall B are 10 nA beam on a 10**-4 
radiator, but we have run RadPhi with 120 nA on a 5*10**-3 radiator. 
Without operating CLAS or the tagger, one can certainly produced a photon 
beam with 100 nA and 10**-3 radiator.
- Parasitic running in Hall B could be accomplished in the downstream 
alcove, but the beam spot will be fairly broad. Simulations should be done 
to see how the sensitivity of the pin-cushion detector changes with the 
size of the beam.




-- 
Elton Smith
Jefferson Lab
elton@jlab.org
(757) 269-7625