Barrel Calorimeter Commissioning
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This document describes the commissioning process for the GlueX barrel calorimeter (BCAL).
Response to LED Monitoring System
The BCAL is equipped with an LED gain monitoring system, which will be used to check the functioning of each MPPC light sensor, as well as checking stability under various operating conditions. One LED is glued into every light guide, which guides the light to one MPPC. The light from each LED is visible by the MPPC connected to the light guide, but also by the opposite MPPC on the other side of the BCAL module. Therefore there is some redundancy in the measured response to each LED light pulse. The response to the LED light signals is used to check the functioning of each MPPC by turning on one LED string and one HV channel at a time. Systematic studies have been completed to check the MPPC response under various conditions. However, the relative gains of each channel cannot be determined using this system, as the geometrical collection of light varies considerably between channels.
Gain Normalization with Muons
The gain of the BCAL FADC channels should be roughly matched initially because the voltage of each MPPC light sensor is set to a fixed voltage above its breakdown voltage. Further refinement of the gain settings will be done using cosmic-ray muons (or other penetrating particles). A trigger has been set up to identify particles traversing the magnet and BCAL at a constant position along the beam (z-direction). This trigger runs at a rate of about 0.5 Hz. Between the middle of July and the middle of October, it should be possible to run these trigger on nights an weekends. A reasonable estimate is probably 50 hours per week of data collection, which yields about 50k events per week, assuming 50% trigger efficiency. We estimate we need approximately 200k events to collect sufficient statistics for one iteration of gain matching with through-going muons and thus a few iterations of gain settings can be accomplished before the fall run. The energy loss of minimum-ionizing particles passing through individual channels depends on the channel layer and on the orientation of the modules relative to the passing tracks. We expect about 22 MeV of energy deposition in the innermost BCAL layer. Outer layers correspond to progressively larger detector areas as well as hardware summing of 2, 3 or 4 sensors. The gain adjustment will take these factors into account by comparing measured signals to energy depositions simulated with a cosmic-ray Monte Carlo.
The following activities are expected to be done using cosmic events.
- Measure efficiency of channels along the track. This study can be carried out with about 50k events with good tracks in the BCAL.
- Measure the response to thru muons. Compare to Monte Carlo to eliminate geometrical effects. Determine gain factors and/or adjust
voltage settings to equalize the response to energy depositions in the BCAL.
Gain Normalization with Michel Electrons
There is a second class of cosmic-ray events which can provide a gain calibration for the BCAL modules. These are Michel electrons from muon decays. The energy spectrum peaks at the endpoint at 52 MeV. We plan to self-trigger the BCAL on cosmic-ray muons and then look for energy depositions in the same module at a later time. The exponential decay of muons will be a signature for the desired signal. The endpoint energy can be used as a calibration point for each BCAL channel. These events have the advantage that the response should be relatively independent of geometry or orientation. We will require approximately about 200k decays in the BCAL. The efficiency of the trigger and selection of muon decays is yet to be investigated. However, we can use muons stopping along the entire length of the BCAL, so they should be plentiful compared to the restrictive external cosmic-ray trigger used for calibration with through going muons.
Commissioning with Photon Beam
Initial beam into Hall D is expected in November 2014. The data collected during this running will be utilized to continue commissioning the BCAL.
Magnetic Field Studies
- Verify response of detector is insensitive to the magnetic field
- Measure rates of hits and particles in the BCAL with and without magnetic field.
- Check that pedestals are all at their nominal settings, else adjust them.
- Evaluate optimum values of Nsa and Nsb based on existing data. Conservative values are Nsa= and Nsb=55.
- Make sure there is a comparable (unbiased) data set triggered on the FCAL that can be used as a reference for BCAL with no trigger thresholds.
- Set the data thresholds to the lowest robust setting possible, currently at 110.
- Need Working BCAL trigger (needs to be setup by Sasha/Serguei)
- Fix global threshold, set Nsa_trigger= and Nsb_trigger=
- Preliminary scan changing thresholds during one run
- Record BCAL scalers and trigger rate as Flash threshold is changed.
- Pick a set of Flash thresholds for run scan (next), 110, 120, 130, 140, 150
- Pick a set of global thresholds for run scan (next), e.g. 10*60*110=66000 (assuming 10 cells x 60 samples x 110 threshold/sample).
- BCAL trigger studies
- Scan across Flash threshold. Record trigger rate. Take data at each threshold to analyze signal efficiency for that threshold. Use pi0 peak if it is identified. Otherwise use energy loss of negative hadrons.
- Scan across global threshold. Record trigger rate. Take data at each threshold to analyze signal efficiency for that threshold. Use pi0 peak if it is identified. Otherwise use energy loss of negative hadrons.
- Voltage bias scan. To set voltages see Barrel Calorimeter Expert
- Nominal voltage setting at the nominal Vover=1.2 V (upstream=downstream). (200k events.Take data with FCAL trigger for low bias and BCAL for high statistics).
- Take data at Vover =1.4 V (upstream), Vover=1.4 V (downstream). 200k events
- Take data at Vover=0.9 V (upstream), Vover=0.9 V (downstream). 200k events
- Take data at Vover =1.4 V (upstream), Vover=0.9 V (downstream). 200k events
- Take data at Vover =0.9 V (upstream), Vover=1.4 V (downstream). 200k events
- Take data at Vover=1.5 V (upstream), Vover=1.5 V (downstream). 200k events
- Temperature scan. To set voltages see Barrel Calorimeter Expert
- Take data at 12 and 24 deg C. (>2 hours each to accumulate statistics and also study stability. Take data with BCAL trigger)
- Note that this requires changing the temperature of the chiller, which requires an access. So three accesses are required for this scan.
Calibration and Detector Performance during Normal Running
- Select an event sample of neutrals in the BCAL
- Find a clean sample of pi0 events that can be used for gain matching
- Select a sample of pi0 events with one photon in the FCAL and one in the BCAL
- If FCAL has already been gain matched, assume their calibration is correct and adjust gains of BCAL elements
- After initial calibrations have been completed, search for a sample of eta events and repeat.
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