Minutes of July 18 2003 Qweak Tracking Group Teleconference ------------------------------------------------------------ Participants: David Armstrong, Mark Pitt, Klaus Grimm, Tony Forest, Roger Carlini, Juliette Mammei, Russell Mammei, Yongguang Liang, Allena Opper. A) MRI status (David) - the official notification of the release of funds to the Sponsored projects offices at each university will take a few more weeks. We can spend matching funds in advance, if needed. B) Region 1: Tony See: http://www.jlab.org/~tforest/Qweak/ForwardTracker/ForwardTracker.html for pictures of the cosmic test stand at LaTech. - The prototype GEM is mounted between a 1 meter quartz bar and lucite bar, with smaller trigger scintillators. Have gas (Ar/CO2 gas bottle) + flowmeter; gas connectors for the GEM to arrive next week. - have MWPC power supply (3 kV) for GEM. - planning to add 50 nA current monitor to test power supply overcurrent protection. (50 nA increase in current is expected characteristic of spark). - have CODA DAQ with ADC and trigger board; will add TDC and G0 scaler in next few months. - have 10 Helix chips at LaTech - are testing now; need couplers to attach board to GEM; these just arrived, Will then power up and start testing (next week). - Ultimately will need to wire bond pads on chip to detector; (50 um wide pads; wire bonding can do 25 um). LaTech will have such a wire binding machine by September. For interim, will use a pressure connection. - first tests will be GEM -> low-noise amp -> ADC (i.e. bypass the Helix chip entirely at first). C) Region II: Mark, Russell, Julliette See: http://www.phys.vt.edu/~rrmammei/ Working in simulations to answer question of where to locate Region II. Issues are 1) rate due to Mollers 2) low-energy photon rates 3) what is acceptable rate with still reasonable efficiency? Issue 1): at 1 nA beam current will have 5.4 kHz of elastic ep events and 1.8 MHz Moller electrons with no minitorus Tried a minitorus (max field= 1/3 of QTOR field) with two options: - minitorus with bending outward (essentially same as what was shown in last Collaboration meeting). Could get down to 0.77 kHz/nA at 40% field strength, which seems reasonable. - minitorus with bending inward: counter-intuitively, this seems not as good suppression of Mollers (still have about 13 kHz at 40%, and doesn't die away much event at 100% of max field. Why? Large tail in radial distribution of Mollers. If we can deduce the origin of this tail, perhaps it can be shielded away. Now have Neven's toroid field code - can try smaller minitorus. The other option: Wedge-shaped chambers in main QTOR field (inside first third of QTOR). Various detector locations along z were tried (detectors numbered 6-11 in the plots; each detector number 20 cm further downstream, radially located so as to get the full elastic stripe). To get the Moller rate down to the same level as the ep elastics (5 KHz) one needs to be at a location of essentially full field strength (0.5 T). Drift chambers can work in such fields, but one must take care of Lorentz angle effects on the drift-distance relations. Still evaluating the two options. The goal is a decision by the next Collaboration meeting. Issue 2) background from low-energy photons - Had originally found GEANT results that gave a huge (90 MHz) flux of 10-100 keV photons right after the 2nd coll; would require some shielding to keep chamber rates manageable - 0.1 mm Pb? This was discovered to have been just a bug; the photoelectric was turned off in the code, which made the absorption length of low-energy photons unrealistically long. Now all looks OK, with only ~200 kHz photons incident on chambers (with Ti vacuum window on target). Folding in the low detection efficiency, this should be fine. Project: Rate vs. tracking efficiency. Idea is to generate a hit pattern at the chambers using GEANT and the Hall C reconstruction code to see what occupancy leads to reconstruction problems. Roger: points out that there is a new primary collimator design (see Jim Birchall's slides: http://www.jlab.org/Hall-C/Qweak/papers/76.pdf ) which is a "scooped out" version: the collimator consists of two 3-in thick pieces separated by a gap of roughly 0.35 m - there could be space there for a mini-torus; maybe an attractive option? D) Tracking Software: Klaus Garfield Electrostatics code- the author has now made binaries available on Garfield WWW site (compiled under Linux RedHat 7.3); no source code. David's linux box (wmg0.jlab.org) has this older O/S, so ask David for an account if you want. Roger suggested asking Garfield's author if f we could pay him as a consultant to compile and build binaries on a more current version of Linux. Tracking - Have Qweak GEANT running, generating elastic ep tracks from a point-like target, within 1/2 collimator (so no background generated) - each 1 cm the coordinates x,y,z,p,theta, phi, Bfield are written to an ntuple. Options being explored for tracking algorithm: 1) Use a lookup table method, with interpolation. Need to check from the generated ntuples if there are simple one-to-one relations between input and output positions and vectors and the momentum and scattering angle. 2) Apply the method of Wind (used by the TRIUMF/CHAOS collaboration, suggested by Greg Smith), which uses quintic spline fits to the solutions of the equations of motion, then a lookup table to get the momentum etc. -> can get momentum to 1% accuracy. Had originally worried that the algorithm needed at least one chamber in the magnetic field, but this is apparently not so. 3) A matrix inversion technique such as used in the Hall A HRS or the Hall C HMS. Problem: what is the first order matrix element of a toroid look like? Would use GEANT to create simulated "sieve slit" data for second order matrix elements. Some other ideas were suggested: - find out what BLAST (MIT/Bates) has done. David has had a few brief discussions with Bill Hersman about this. Maybe we could even get code from them. - find out what CLAS does for their toroid. - look at the Kalman filter algorithm used by STAR (RHIC) described in http://arxiv.org/abs/nucl-ex/0307015 Klaus also noted that he attended the recent Hall A workshop on track reconstruction in which problems were reported with low ( ~ 50%!) reconstruction efficiency for the Hall A VDCs at low (why low??) rates. Problems were brought forward with the cluster finding and multiple track handling software. E) REGION III: Klaus Investigating the option of using HDCs instead of VDCs, specifically piggybacking on the design of the Hall C HKS HDCs (Tang et al.) - they are relatively cheap and simple have complete drawings, etc.... this design uses G10 frames, much less expensive than the Hall A VDC frame design. But: can one get 2 m long G10 frames built and Cu-cladded for the photo-etching, and for what cost (the HKS chambers are only 120 cm long)? Also: HDCs require about a factor of two more electronic channels than VDC option - cost? Discussion of TDC options: - old Hall C TDCs (LeCroy 1877). But: FASTBUS crates are now hard to come by, and the modules are no longer supported by LeCroy. - or Hall D/JLab new development - high resolution? This is what HKS are considering. HKS will make a decision of this at the end of this month. - new CAEN modules based on chip developed for CERN. no decision yet on TDCs. F) Trigger Scintillators: Allena Yongguang has been looking at light attenuation in scintillator, using the Guideit code (used by G0 in their FPD design). Find that 0.6% of produced photons get to PMTs; for 2 MeV energy deposition (1 cm thick) -> only 120 photons to PMT, (not including QE of PMT); including the QE we'd expect only about 25 photons, which seems uncomfortably low. This is due to the extreme aspect ratio of the scintillators (long and narrow). Will try thicker scintillator in model next.). The time distribution peak is about 0.5 ns wide. What is our time resolution requirement? 1 ns seems OK. Next - turn on backgrounds/secondaries -> can we make scintillator thicker? Also, do we need dummy scintillators? - how fast can we get trigger to GEM? With off-the shelf electronics could get 65 ns from electron arrival to final signal (with meantimer); could get 20-30 ns without meantimer (5ns slop). Maybe we can do both, in parallel, since Tony needs result in 100 ns for GEM readout. F) Mounting of Tracking system: Roger A new concept has been proposed for the shield house for the Cerenkov detectors. Only the front wall is specially made concrete with holes foe the tracks; after this is an enclosed room, made of ordinary (removable) blocks (concrete and/or Pb). This is a cheaper option; 0.5 - 0.75 m of concrete seems enough. Side walls could be removable to survey and service the detectors. This might allow the Region III Ferris Wheel and trigger scintillators to be be mounted *after* the concrete , i.e. right in front of the quartz bars. Could register chambers directly to the quartz. How much space in z would we need? Also, in the new "scooped out" collimator design, there might be space between the two parts of the primary collimator for the GEMs to be located - could register them directly to the primary collimator. Allena asked about plans to allow measurements with collimators plugged up. The current plan is to allow at least one sector to be filled, in order to look at backgrounds and cross talk between sectors. --------------------------------------------------------------------------- Next meeting: Friday August 1, 10:00 AM EDT