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The Compton polarimeter can be used to non-invasively monitor the beam polarization. It has three major components: an optics table with a laser that can lock to a Fabry-Perot cavity; an electron detector to count scattered electrons; and a photon detector to count scattered photons. At present, the electron detector is in commissioning, so only the photon detector is fully functional.

There is additional documentation at Hall A compton webpage

Standard Compton

Monitoring the beam polarization using the Compton polarimeter requires two steps:

  • Take Compton data. These can be long runs, but since you can only check the polarization after a run has ended, it is best to start a new run every hour or two. When you end a run, click "Submit" to dismiss the gray pop-up window.
  • After a Compton run has analyzed (this takes about 10% the length of the run), check the logbook [1] for the Compton asymmetry.

If runcontrol shows that it can't communicate with a ROC or if the data won't analyze, stop the current run and type "coda reboot" into a compton@compton terminal window to reboot the ROCs and CODA. If the problem is with ER1, you will need to type

    coda_er -n ER1

into a separate window.

See below for more details.


The Compton is run from the compton machine, which is the leftmost active monitor in the back room of the Hall A counting room. The login name is compton and the password is the same as the one for the adaq account.

To begin a new CODA instance, type

   coda start

To recover from a CODA crash, type

   coda reboot

To open a strip chart, type

   StripTool transversity.stp


To start a run, click the "Start" button in the runcontrol window. After a time, you should see the event count start to go up and the message "Transition Go Succeeded" at the bottom of the screen.

To setup the compton trigger parameters use the spyconf program :



One can select the different data taking modes in General Daq Setup.

On the right panel one can setup the photon detector parameters. We are using only the central crystal only.

On the bottom middle pannel, one can setup the electron detectors parameters.

number of planes needed to generate a trigger
coincidence time windows between planes
number of adjacent strips ored to be looked in the next plane for the trigger

When it is time to end a run, click the "End Run" button in the runcontrol window. A gray screen should pop up, giving you the opportunity to enter a message (say, if the beam was very trippy). Make sure you dismiss the window by clicking "Submit" -- not "Exit"! Hitting "Submit" sends the run to the analyzer, which is very important.

You may additionally get a pop-up window saying "End run failed!" on the runcontrol window. This is okay. Dismiss this popup window and click the "Reset" button on runcontrol. (Yes, despite the dire warning, you really do want to do this.) You should then be able to start the next run.

While running, you can monitor the beam current, photon rates, cavity status, etc using a strip chart. Here's what it might look like:

Transversity striptool.jpg

  • Purple line: Beam current (uA). In the figure you can see that it has tripped a few times.
  • Yellow line: Vertical beam position. This should remain roughly constant. A beam trip sends it to the top of the chart (position 0).
  • Red line: Cavity power. This should be a square wave: the cavity is locked in the right polarization state, then turns off, then locks in the left polarization state, then turns off, then repeats. The powers for the right and left states appear different but are actually the same. The power should stay comfortably above 400 W. (Cavity will not lock during extended beam trips.)
  • Blue line: Scattered photon rates in the detector. As in the figure, these should track the cavity on/off states; if you can tell from the blue line alone whether the cavity is on or off, then you are probably getting good Compton data.
    • If this is not the case, this may be due to a bad tune resulting from very low current, or the Compton chicane is off. If one of these is not the cause, contact a Compton expert.
    • If your blue line is zero or not displaying, there are three possible causes:
      • Beam trip
      • HV trip. Check the Beamline HV card 11. Channel 1 should be set to -1600 V for the GSO detector.
      • DAQ is not running. Start a CODA run and the blue line should come back. If CODA is already running, it may be in a strange state;

end the run and type "coda reboot" into a terminal to restart the ROCs and CODA, then start a new run.

Checking a Run

Compton analysis results are written to the Compton logbook, which can be accessed onsite at [1]

Find the run number (newest runs are at the top) and click the html link. (The pdf link does not work.) This will open a new window with the analysis results.

The full results will not display right away (because the analyzer does not finish immediately). It takes the analyzer about 10% of the runtime to complete. So an hourlong run will be analyzed after about 6 minutes.

The most important graph is about a third of the way down the page. You can find it quickly by searching the page for "Counting Rates". It's the graph titled Asym in ADC bins:

Compton asym graph.jpg

Here are the important things to look for:

  • The green line (cavity off) should be consistent with zero. This shows that there is no asymmetry in the background.
  • The red/blue lines (cavity on, right/left) should be roughly symmetric about zero. This reflects the fact that flipping the cavity polarization changes the sign of the asymmetry. (It's ok for there to be slight differences as above.)
  • At the extremes (highest/lowest point) we should see an asymmetry greater than about 5%. (Here, we see extremes of about 8% and just over 6% for right and left.) If the asymmetry drops below 3%, we worry. Call an expert.

CIP Scan

Getting good data sometimes requires a CIP (Compton interaction point) scan, where the beam tune is adjusted slightly in order to maximize its interaction with the photons in the cavity. Call a Compton expert to perform such a scan if:

  • There has been a new beam tune through the Compton chicane.
  • The beam has been shut down for longer than an eight-hour shift.
  • The photon counting rates drop drastically during steady running. (Compton rate (blue line) in strip chart -- see below)

Compton Upgrade

Photon detector

The new photon detector crystal is a cylinder of GSO with a diameter of 6 cm and a length of 15 cm. It was installed in Hall A in December 2008.

In October 2008, the crystal was tested at incident photon energies of 20 MeV, 22 MeV, 25 MeV, 30 MeV, and 40 MeV. We are working on extracting the response function.

PREx requires an integrating DAQ system, which will run on a Struck FADC. We are currently commissioning this DAQ with a prototype FADC.


We are commissioning a new FADC-based DAQ for the photon detector. See run information.

Electron detector

Detector new event structure

Webcams address

Webcams were added to monitor the :

APC : hareboot4

*plug 2 webcam
*plug 3 EPICS Edet
*plug 4 trigger
*plug 7 FADC crate

Compton Edet DAQ

Compton Experts

Expert Phone Pager
Sirish Nanda x7176 757-584-7176
Alexandre Camsonne x5064 757-584-5064
Abdurahim Rakhman x5626 757-584-5452
Diana Parno 202-821-3471 (cell)