BLTWG Meeting 11/11/2013
Organization
- Time: 11:30 EST
- Place: ESNET and Seevogh (Jlab community)
- Connecting: instructions are here
- Present: Richard J., Alex B., Brendan P., Ann Marie C., Franz K., Mike D., Alex S., Natalie W., Yi Q., Nathan S., Jarrod F., Todd S., Ivan T., Tim W., Elton S.
Agenda
- Announcements
- Update from Accelerator Liaison - Todd
- 12-GeV Software Review
- GlueX Computing Document
- Section 11.2 paragraph Beamline (lines 1348-1354) Please comment on validity
- Racks location in the Tagger Hall - Fernando/Tim/Alex
- Beam Commissioning Plan - Elton
- Update on the fixed tagger hodoscope - Franz
- Update on mapping the tagger and PS - Tim
- Update on the microscope electronics - Alex B.
- Status of the microscope construction - Ann Marie
- Update on pair spectrometer detector - Ivan
- any other business
Notes
Original text from GlueX-doc-2350
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Members of the GlueX collaboration developed the beam-line calibration procedures for photon running in Hall-B during the 6-GeV era. This included tools to determine photon
flux, photon energy and the degree of photon polarization. While we feel that it will be relatively straightforward to implement the older Hall-B procedures in GlueX, the relevant individuals have been focussed on improving hardware to eliminate some of the problems that hindered this work in Hall-B
Draft of replacement text
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Calibration of the photon beam for GlueX consists of measuring the photon energy boundaries and relative time offsets for the tagging counters, the energy boundaries and time offsets for the pair spectrometer counters, the absolute tagging efficiency of each of the tagging counters, and the relative normalization of the coincidence rates in the pair spectrometer to the rates in the tagger. The order of the photon beam energy calibration is as follows. First, the coincidence rate spectrum in the pair spectrometer (PS) is taken and fitted in the end-point region to an empirical function to locate the end-point. This information is used, together with the electron beam energy provided by the accelerator, to set the absolute energy scale in the PS. A look-up table based on the field map of the spectrometer and the known positions of the PS counters is used to transfer this absolute calibration to the entire photon spectrum covered by the PS. A similar look-up table will be available for the tagging counters as well, providing a well-established relative energy calibration within the tagger arrays. Coincidences between the pair spectrometer and the tagger will be used to transfer the absolute energy calibration from the pair spectrometer to the tagger arrays.
The order of relative timing calibration within the tagger and PS counters is as follows. The PS is equipped with a second array of low-segmentation counters which are designed for precise timing of pair tracks. The relative time offsets between these counters will be calibrated by taking coincidences between all possible pairs of left-right counters, and adjusting the timing so that all of the offsets are aligned with one of them. Once this is done, the time measured in the pair spectrometer provides a single reference against which all of the counters in the tagger arrays will be calibrated. The same calibration dataset which is used to establish the timing calibration will also be used to measure the relative beam intensity normalization between each counter in the tagger and the corresponding coincidence rate in the PS. Absolute calibration of the photon beam rate in terms of the rates in the individual tagger counters will be performed using special runs at very low beam intensity, in which a total absorption counter is placed in the beam.
A data needed to perform this calibration procedure are contained within the standard GlueX data stream. Software to analyze the data and store calibration constants in the the calibration database will be written in the form of a standard consumer within the GlueX offline framework. This software has not been written yet, but will be simple to implement and debug, taking advantage of cross-checks between the tagger and the PS. Calibration of the gains and offsets for the real-time measurement of the photon beam centroid in the active collimator will be performed in special scans to be carried out at the start of a run, and at regular intervals throughout a run period as required. Special software for calibration and monitoring of the beam position signal from the active collimator has been prototyped using test data collected with this device in the Hall B photon beam, with the final version currently under development.
