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The Hall A Compton

The Compton polarimeter is used to (non-invasively) monitor polarization of the incoming electron beam. It has three major components: an optics table with a laser that can lock to a Fabry-Perot cavity, an electron detector, and a photon detector to count scattered electrons and photons, respectively.

Last edit: LThorne 02:08, 24 February 2016 (UTC)

Shift Workers' Operation Guide

Start a Compton run every two hours.

The Compton DAQ is located in the back room in the Counting House. All commands are done from compton@compton.

If CODA is not already running:

Kill CODA first, then start it up again:
   $ kcoda
   $ startcoda
A few new Terminal windows and a GUI pop up.
LT CODAgui.png
Let "top" refer to the bar with named menu options near the top of the GUI (ex: Platform, Sessions, etc). Let "bottom" refer to the bar below "top" that contains a line of buttons (hover over them to reveal their labels; starting with "Configuration" on the left).
  1. In top, select: Platform > Connect
  2. In bottom, click on "Configuration" button.
    • If configuration listed in the "Run Parameters" box is not FADC_prod: in top, select Configurations > Cool and change it to FADC_Prod.
  3. In bottom, click on "Download" button
  4. In bottom, click on "Prestart" button
  5. When ready to start run, click "Start" button. Typical run duration: 2 hours. Type in any Start Of Run comments in the box. Click "OK" (even if you left comments blank!).
  6. When ready to stop run, click "End" button. Type in any End Of Run comments in the box. Click "OK" (even if you left comments blank!).

Information for Compton Experts


  1. HV on/off? -> Compton#High Voltage Controls
  2. Rebooting components: Compton Reboot
  3. [Experts only]: Correct parameters for run in integratingDAQ.flags and compton.params

Collimator Systems

What the jaws look like when placed at their lowest position, as seen on comptcam2. To view this camera follow this link and login with compton credentials.
  • Primary Lead Collimator
    • Presently left fully open at 20 mm diameter.
    • This is a 10 cm long collimator with inserts which can reduce aperture down to 10 mm or 5 mm in diameter.
  • Synchrotron Shields
    • For PREX-II a single 0.25 mm lead foil (painted) is taped to the front of the photon detector.
  • Compton Jaws
    • Remote access for experts can be found at [1]
    • Note, for the duration of the PREX-II running period, the Compton Tungsten Collimator System ("Compton Jaws") will not be used. It should be left it is's parked position of:
   aperture =   1.5 mm
   y center = -46.0 mm

Detector Positioning System

Date X [mm] Y [mm] Comment
2019-06-17 70 33.5 Visually Centered


High Voltage Controls

  • HV Control GUI
    • GUI can only be accessed once per person. We share this GUI with Moller Polarimeter.

LED System (Linearity Measurement System)

  • Will add info on MiniMegan soon

Taking an LED run:

  1. Find parameter file GUI:
   $ cd /home/compton/franklin/
   $ ComptonConfig
  1. Click "Pulser On, Pulser Trigger" on parameter file. Click "Apply", then "Done" to close GUI.
  2. Go to CODA. Start a new run to apply the settings. (This does not need to be a long run. You may end it immediately.)
  3. Start your LED run.
  4. When you go back to normal Compton runs: don't forget to turn the pulser off and do a short run to update the parameter file settings!

Laser System

Real time o-scope signals

Cavity lock camera

Make sure laser flipping is on

Turning off the laser

IOC reboot

Accessing the laser hutch in the hall

Laser and Locking troubleshooting

Software & Data Analysis

Relevant files, scripts, etc.

Run Compton analysis:

In compton@compton:

   $ cd /home/compton/online/CompMon

Run analysis software:

   $ ./compmon <run number, no brackets>

Open ROOT:

   $ root -l /data/cmuwork/rootfiles/Fall2016/compmon_<run number, no brackets>.root

Run desired C script located in CompMon directory.

Some useful analysis scripts:

  • For PREX we should use: /home/compton/online/CompMon/dataQualityCheck.C as our online macro!
    • As of now the way to run it is to navigate to the directory with the file and then running with root -l dataQualityCheck.C\(<run number>,<tests flag>\) We're looking into adding a simplified interface in the near future.

The following macros were maintained by people who no longer work on the compton so use with caution:

  • /home/compton/lthorne/CompMon/plotAsymSpectrum.C: Quick and dirty view of asymmetry histograms, cavity and BCM limits.
  • /home/compton/lthorne/CompMon/buildQuartets.C and buildQuartetsPlot.C: For constructing multi-run asymmetry plots.
    • Run instructions are given in the comments at the beginning fo the file.
  • /home/compton/lthorne/CompMon/BCMCal/calibration<runNumber>,C: BCM calibration scripts for specific runs.
  • /home/compton/lthorne/CompMon/plotAccQuartets.C: plots sum, difference histograms for beam off, laser on, laser off for Acc0, Acc4.
  • /home/compton/franklin/CompMon/plotPulser.C: peek at DAQ pulser channels, including info on 'variable' and 'delta' for channel 13.

Link to list of good Compton runs:

List of runs with comments, so know which ones were not junk.

DVCS Google Docs: Runlist (Email Juan Carlos if you want to granted write permission) PREX-CREX Google Docs: Runlist (Email Juan Carlos if you want to granted write permission)

Helicity controls

From within NewTools controls (see first few steps in Compton Photon Detector Compton GUI section):

  1. On far right of JTabs menu, there are "master" controls. Find and click on Tools Screen.
  2. Go to the helicity controls. (The default is delayed, so be careful of this.)

Current Scripts

As a brief check, run plotAsymSpectrum.C.


PREX-II/CREX Compton Analysis

We've reworked the analysis to streamline finding the most important plots. We've adopted the analysis GUI used by panguin and other online monitors for Hall A experiments. To run, follow the steps below.

Online Analysis Steps and Checks

  1. Using the directions at the top of the page, open CODA and start a run.
  2. Open a terminal and login using ssh -Y
  3. Enter command gocompton
  4. First run compmon to analyze the run. To this use command ./ -r <run number>
  5. Once the run is finished analyzing and errors are logged, enter source online.csh -r <run number>. This will create the plots in the GUI window.
  6. Look at the plots in the following tabs:
    • Snapshots: This window has one plot of snapshot peak height vs sum. Plot should look starkly linear. Plot also has the percentage of snapshots that pass an arbitrary selection criteria. I find healthy runs have about 80% of snapshots pass.
    • Spectra: Plots compton spectrum broken down by beam and laser state. Is the compton edge where we expect it to be? Are Brem events low when beam or laser is off?
    • Acc0: Plots of accumulator 0. Are they gaussian? Does the mean of the beam ON, laser ON plot differ from all the others?
    • Acc0 vs Time: Plots of accumulator 0 value vs mps number. Are the accumulator values drifting significantly with time?Are beam trips readily noticeable?
    • Asym and Polarization: Includes plots of the quartet sums, differences and asymmetry. Do they all look roughly gaussian? Are there no double-peaking effects? Does the calculated polarization yield some reasonable value?
    • Asyms vs Time: Plots of the above vs time. Is there any significant drift in any of the quantities vs time?
    • Asym Spectrum: As of 2019-07-31 this plot is not working. Background subtraction needs to be more carefully refined.
  7. Once you click "Exit GUI" the same plots will be generated and saved to the compton plots web page for easy review.

Making Custom Plots

There are macros which make other compton plots which have been useful in the past, but are not being maintained anymore. To be certain, you can make your own plots to slake your curiosity. The following is description of CompMon output, the branches, as well as certain variables which you may find useful.

  • General Variables: These variables may be found stored in multiple trees and should come in handy:
    • helicityStateReported: 0 for negative, 1 for positive
    • laserState: 0 for right-hand polarization and laser on, 1 for left-hand polarization and laser on, 2 for right hand polarization and laser off, 3 for left hand polarization and laser off, 4 for unknown state
    • beamState: 0 for beam off, 1 for beam on, 2 for unknown beam state.
    • cavPowerCalibrated: laser cavity power
    • bcm: calibrated BCM signal
  • mpswise: A tree of data stored per MPS
    • mpsCoda: MPS number
    • Acc#: Accumulator total for number #
    • NAcc#: Number of accumulator samples per MPS for accumulator number #
    • numTriggers: Number of triggered sums in current MPS
  • triggerwise: A tree of variables stored per trigger
    • sum: Total sum of trigger
    • sumIsRandom: 0 if sum is real, 1 if it is random
  • quartetwise: A tree of variables stored per quartet
    • PosHelAcc#: Sum of accumulator number # for positive helicity states in the quartet
    • PosHelNSamples#: Number of samples for accumulator number # for positive helicity states in the quartet
    • NegHelAcc#: Sum of accumulator number # for negative helicity states in the quartet
    • NegHelNSamples#: Number of samples for accumulator number # for negative helicity states in the quartet
    • quartetReportedHelicityPattern: Two different values, one for each valid quartet pattern
  • snapshots: A list of variables and snapshot readouts for each snapshot taken
    • numSamples: Number of samples in snapshots
    • snap: An array variable with numSamples entries that details the fADC readout of the snapshot
  • pulserwise: A tree of variables stored per pulser cycle. (SHOULD ONLY BE NEEDED BY EXPERTS)
  • runwise: A tree of variables stored per run. This tree only has one entry per leaf, and details info about pedestals.
  • epicswise: A tree of variables taken per EPICS event. These variables are also stored in other trees, so they can be accessed from there.

This is in addition to the inbuilt histograms produced in every CompMon output file. This should be more than enough to diagnose any compton DAQ problems.

Compton new counting DAQ

A new event based counting Compton DAQ is being developped.

It uses a VXS crate, the Intel CPU is acomp1.

It has 1 JLAB FADC250 in slot 10 VME address 0x500000, on SD and one VTP hostname hallcvtp in central slots. Currently there are 3 VETROCs in slot 13,14,15 respective addresses are 0x680000,0x700000,0x780000.

To look at the VETROC rates, there is a diagnostic program /home/compton/camsonne/Edet2019/fe_diaggui

 DiagGUI ./ROC_compton_halla.txt

Make sure Clk sources are all 4 from SWB Issue a reset and the boards should be displaying rates

one has to start the server on acomp1 in ~/linuxvme/diagguiserver : DiagGuiServer

Electron detector current webcam comptcam4

Compton Counting DAQ Software HOWTO

EDet DAQ Cable Maps

TI Signal
TS#6 Helicity
TS#1-5 VTP output
TRG Unknown
FADC Signal
Ch.1 Photon detector
Ch.2 MPS
Ch.3 Helicity
Ch.4 Pulser
Scaler Signal
Ch.1 MPS
Ch.2 Clock
Ch.3 2AY-
Ch.4 2AY+
Ch.5 2AX-
Ch.6 2AX+
Discriminators Cable VECTROC
A-Left-Top AL4 A-Left-Bottom
A-Left-Bottom AL3 A-Left-Top
A-Right-Top AR4 A-Right-Bottom
A-Right-Bottom AR3 A-Right-Top
B-Left-Top BL4 B-Left-Bottom
B-Left-Bottom BL3 B-Left-Top
B-Right-Top BR2 B-Right-Bottom
B-Right-Bottom BR1 B-Right-Top
C-Left-Top CL2 D-Left-Bottom
C-Left-Bottom CL3 D-Left-Top
C-Right-Top CR2 D-Right-Bottom
C-Right-Bottom CR1 D-Right-Top
D-Left-Top DL2 C-Left-Bottom
D-Left-Bottom DL1 C-Left-Top
D-Right-Top DR2 C-Right-Bottom
D-Right-Bottom DR1 C-Right-Top

Compton Meetings

See last list entry for most recent meeting.

Current Compton Experts

Expert Phone
Dave Gaskell x6092
Alexandre Camsonne x5064
Ciprian Gal x5324
Juan Carlos Cornejo (Photon Detector) x5282

Expert Information

For expert monitoring and troubleshooting information please visit Compton Expert Information.


Code can be found on github: JC says:

The Makefile is ancient (since it has to work on old systems too) but there is an easy "fix".  In the src/config.d directory, create a file similar to cornejo_compton_jlab.conf but modify the paths respectively. 
Then run ./config config.d/<your_config_file> 
It will update the silly Makefile accordingly

Some old data and examples can be found on


Old Compton page (pre-Spring 2016)

See Pre-Spring 2016 Compton

Last edit on 2020 02 04, by Jlzhang