BLTWG Meeting 02/12/2009

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  • Time: 9:30 EST
  • Place: EVO and ESNET (with telephone bridge)
  • Connecting: instructions are here
  • Present: Alex S., R. Jones, I. Senderovich, J. Stewart, T. Whitlatch, E. Aschenaur, F. Barbosa, and G. Yang.


  1. Updated construction and installation schedule - Jim
  2. Progress on change request, communication within Jlab - Jim
  3. Two issues for discussion - Jim
  4. Updates on the tagger background simulations - Alex
  5. Question of CFD vs LE discriminators for tagging counters - Richard
  6. Updated interface drawings - Tim
  7. Dates and preparations for the Spring 2009 CHESS run - Richard
  8. Plans for beam tests - Richard

Notes - with TODOs in red

Construction and installation schedule

This week sent around an updated schedule for the construction and installation of the tagger and photon beam line components, including manpower estimate for each task. One highlight is that the schedule shows stands for several beam line components being constructed in FY09 using contributed university shop time. Time in the UConn shop is tight at the moment, with one machinist out for illness. He is expected to return within 6 weeks, at which point Richard will inquire with the shop about constructing stands for the photon beam line at UConn. It is conceivable that one of the UConn shop members might be willing to be certified as a welder so that the stands might pass safety requirements at Jlab. Major university groups in the experiment have been asked to buy a license for the scheduling software package FastTrack so that we can read and write these schedule files. The design and build contract for the tagger is supposed to go out for bids in January 2010, so getting the documentation together for that bid package is a high priority for the coming months. The sections on construction of the tagging detectors is not yes broken down at the level of detail that will eventually be required. UConn and Catholic U should begin soon to break out the hodoscope construction projects into tasks and show that they can be completed in the time available and that the required manpower is accounted for.

Progress on the GlueX Photon Beam change request

All of the documentation has been submitted to the management that is required to bring the P3E schedule into line with the plans presented at the Review last November. As far as we know, this has all been approved. New signs of progress have been seen in the direction of getting our Interface Document adopted somehow into the record of the 12 GeV project. We see this as an important step to help prevent communications problem such as the one we saw earlier in the week regarding the tagger quadrupole. One thread we started back in summer 2008 that we need to pick up again is the agreement about how the position information from the active collimator will be interfaced to the electron beam controls. Jim will contact members of the accelerator 12 GeV design team to set up a second meeting to discuss this.

Two issues for discussion

  1. Possible interference in the active collimator coming from stepping motor pulses on the primary collimator positioning system: stepping motors are controlled by pulse waveforms generated by a controller that might produce RF pickup in the pA current sensors of the active collimator. RJ reported that during active collimator beam tests in Hall B, the active collimator was mounted on the XY table of the TAC which was actually in motion during the measurement, and no evidence of pickup from the stepping motor control signal was seen. On the basis of that experience, this is not thought to be a problem.
  2. Tagger backgrounds coming from synchrotron radiation in the tagger: inside the tagger magnet the main beam emits an average of 5 MeV of synchrotron radiation in the form of soft X-rays. What do we know about the effects of this radiation on the performance and lifetime of the tagging counters? Synchrotron radiation is turned on in the Geant simulations, so SR effects are currently included in our background estimates, but only above the gamma energy cutoff currently set at 100 KeV. This cutoff could be lowered, but Geant results are not reliable for photons in the soft X-ray region because cross sections there are dominated by the detailed atomic physics of the photoelectric cross section with its characteristic emission peaks and absorption edges for each material. In any case, the SR directly strikes the vacuum chamber walls on the side facing the yoke, and only indirect radiation reaches the focal plane. RJ will look into a reliable way for estimating these effects.

Tagger background simulations

Alex reported on progress in his ongoing tagger background studies. After a careful review of his earlier background simulations, he discovered that backgrounds are underestimated if one uses the built-in coherent bremsstrahlung generator. This generator gives an accurate simulation of the electrons that produce hard bremsstrahlung, but much of the background actually comes from beam electrons that do not produce hard bremsstrahlung but interact in some other way in the diamond or boundary materials, eg. large-angle Mott scattering and Moller scattering in the diamond, often followed by showering in the poles of the magnet or the flanges and walls of the vacuum system. Alex is redoing the simulation using a Gaussian beam and using the internal Geant processes to simulate these interactions. He sees significantly higher (but not catastrophic) backgrounds, but there are normalization issues he needs to resolve before he can report numbers. Alex will complete these studies and report on them at a future working group meeting.

Discriminators for tagging counters

There was discussion at the January collaboration meeting whether we need constant fraction discriminators (CFDs) on all of the tagging counters and the pair spectrometer. The original electronics plan called for CFDs on all tagging and PS counters, but the 1/2009 document showed leading-edge (LE) discriminators on the fixed array. Franz says he requested this change because of the high rates in the 9.0-11.7 GeV tagger range, and because he is not convinced of the need for CFD's if both ADC and TDC data are collected on all of the counters. Fernando reported some numbers on the maximum rates for the two types of discriminators, as quoted by CAEN for their products, which he says are typical. The CFDs require a substantially lower bandwidth, with a rise time no faster than 5 ns and a maximum burst rate of 66 MHz. This maximum rate sounds high, but clearly the CFD time resolution relies on well-separated pulses and will degrade gradually as the rates approach the tens of MHz range. The affects the fixed array (6 MHz per channel at full intensity) the most, especially with talk of a goal of 1% in intensity normalization. On the other hand, we heard that in order to reach our design spec of 200 ps rms time resolution, an offline time-walk correction is needed even if CFDs are used. Hrachya argued that hardware time-walk correction, which is what a CFD does, is no longer very useful in modern experiments, where software offers a more flexible and low-cost approach. RJ will communicate with the electronics working group that we wish all of the tagger (microscope + fixed array) and PS channels to carry both ADCs and TDCs on each channel, and to replace CFDs with LE discriminators.

Tagger and Photon Beam Interface drawings

Tim has produced updated interface documents for the tagger. The upstream shift of the dipole has been implemented, to leave the main beam on the original path to the dump that was obtained with the two-dipole design. The location of the fixed array counters reflects the updates from Franz. The microscope is shown in its nominal location, with coordinates marked as "TBD". These drawings are now posted on this wiki on the photon beam engineering drawings page.

Spring 2009 CHESS run

The dates for the run at CHESS this spring have been set as April 30 - May 7. So far, RJ and FK have indicated their intention to be there for the run. Plans are to examine a type 3 CVD diamond on loan from BNL, and to measure rocking curves of several radiators from the Hall B inventory.

Plans for beam tests

The UConn group would like to test a prototype of the Hall D tagger microscope in Hall B as soon as tagged beam is next available in the Hall, currently foreseen for early 2010. They would also like to request that the active collimator be mounted again in the alcove for follow-up measurements, especially to determine the bandwidth-resolution curve for the device, and also to measure its response in a beam without narrow upstream collimation, if that can be arranged.