Compton Photon Detector

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The Compton Photon detector consists primarily of a scintillator crystal and a PMT which receives the backscattered compton photons from the chicane. In addition to the main detector there are also two finger PMTs arranged perpendicularly to each other which aid in centering. Both are mounted atop a unistrut frame on top of a movable table. The operation of the detector and table is laid out on this page:

The Compton Photon Detector table.

Compton Controls GUI:

Open using NewTools:

  1. Login using ssh compton@compton
  2. Enter command NewTools
  3. A window with "OPS Menu" will pop up. Click on it. Click on "JMenu"
  4. In JMenu go to System Expert-->Hall A-->Compton->Controls
    • You can also find this window by searching "Hall A Compton Controls" in the search bar
  5. On the right side of the window will be a button labeled "Calorimeter." To open the table controls click that button then click "Table" on the drop-down menu that pops up.
  6. Enter the position you'd like to move the table to in both x and y.

Usually, it'll only be Y that needs to be moved. To move the table, check that:

  • "ERR ACK" (error acknowledge) for x,y is green.
  • The y button in the middle box is green.

Note: the vertical max is 39.99 mm!! The nominal X, Y: 69.9mm, 24.4mm.

Troubleshooting Table Motion

The lockout position is the same as the home position (poor design).

To jog it by hand away from the home position:

  1. Before doing anything please ensure, if the hall is in restricted or controlled access that nobody is working on the table that moving it would physically endanger them. Use the cameras to make sure of this.
  2. Remove the floor tile against the wall next to the Compton computer. Find the Compton control: small beige box with controls. (other box is a killswitch and says so; do not touch!).
  3. Activate the box (turn it on: bottom right switch), or it won't do anything. The red light may or may not come on when you do this (LED is dying).
    • You can activate the "shift" switch to use functions with a preceding "s".
  4. Toggle the table up just barely by flicking the switch up, then right back down.
    • Theoretically, the JTabs controls should be working. If not, check that there are no errors in the GUI (or at least 'acknowledge' them). You can also use the control to manually bring the table back to its home position, if you suspect it isn't already. In either case, the table won't move unless the middle set of buttons are green (for the direction you wish to move it in).
    • If toggling the table does not work at first, make sure to send the detector table to home before trying to clear any errors. You will definitely need to do this after power failures in the hall.
  5. Make sure to switch the box back off when you are finished, and replace with controls' switch face up so there's no accidental toggling.

Centering on Beam on Laser

Important EPICS Variables

Variable Name Summary
COMPTON_PW1PCAV_ca Laser power in the cavity
IBC1H04CRCUR2 Readout of analog BCM (could also use hac_bcm_average)
ComptonCentralRate Rate in the main PMT
ComptonCentralRateHigh Rate in the main PMT with the high threshold
ComptonHfingerRate Rate in the horizontal (top) finger PMT
ComptonVfingerRate Rate in the vertical (side) finger PMT
ComptonUSbg1 Rate in laser table background scintillator, first upstream
ComptonUSbg2 Rate in laser table background scintillator, second upstream (as of 2019-08-05 does not work)
ComptonDSbg1 Rate in laser table background scintillator, first downstream
ComptonDSbg2 Rate in laser table background scintillator, second downstream
COM_DEPTH_XCPOS_ai Detector table x position
COM_DEPTH_YCPOS_ai Detector table y position
IPM1P02A.XPOS X position in US BPM
IPM1P02A.YPOS Y position in US BPM
IPM1P02B.XPOS X position in DS BPM
IPM1P02B.YPOS Y position in DS BPM

Before running we must first show that we are receiving collisions in the photon detector. To do this MCC must thread beam through the chicane and then do a beam position scan until we see rats spike in the photon detector. The steps below detail this process:

  1. Turn off photon detector high voltage. Ensure table and finger scintillators are on.
  2. On the compton computer open a StripTool instance. Load the following EPICS variables onto one (or more) charts:
    • COMPTON_PW1PCAV_ca, IBC1H04CRCUR2, ComptonCentralRate, ComptonCentralRateHigh, ComptonUSbg1, ComptonUSbg2 (only if functional!!!), ComptonDSbg1, ComptonDSbg2, ComptonHfingerRate, ComptonVfingerRate, COM_DEPTH_XCPOS_ai, COM_DEPTH_YCPOS_ai, IPM1P02A.XPOS, IPM1P02A.YPOS, IPM1P02B.XPOS, IPM1P02B.YPOS
    • Alternatively once on the compton machine you can enter StripTool ~/compton/ajz_rates.stp and in another terminal enter StripTool ~/compton/ajz_BPMs_centering.stp to open up ready-made strip charts with all the above variables loaded.
  3. Wait for MCC to get stable beam through the chicane. Optionally take a run to observe backgrounds.
  4. Once beam is up and backgrounds on the table are suitably low, contact MCC to begin searching for collisions.
  5. Turn on the photon detector PMT. Ensure finger scintillators are still on.
  6. Start a run, for calibration purposes.
  7. Observe the BPM readbacks. Look for beam motion.
  8. Once you see the beam moving, check the photon detector rates. Look for spikes. Once rate looks maximized, record the position readback in both BPMs.

Centering Detector

After finding collisions, ask MCC to lock the position in the table BPMs. Then begin the detector-centering procedure:

  1. Ensure the finger scintillators and compton detector are both still on.
  2. Run StripTool ~/compton/ajz_BPMs_centering.stp or alternatively StripTool ~/compton/compton_detector_centering.stp
  3. Stop the current run. Look up what the visually centered table position is, and then set the x position to that center, and the y position to 3 cm below visual center (or as low as you can go).
    • (i.e., if your detector looks centered at x=70 mm and y=33.5 mm, move the table to x=70 mm, and y=3.5 mm)
  4. Start a new run
  5. Begin moving the table in 0.5 mm increments 2-3 seconds at a time. Watch the ComptonHfingerRate trace. Look for the y-position where the rate is clearly maximized.
  6. Once you have a rough idea of the center position, begin looking in smaller and smaller position increments to find the true maximum rate position. Once you know it, record it.
  7. Now do the same for the horizontal finger. Find the visual horizontal center and send the detector 3 cm behind it.
    • (i.e., if your detector looks centered at x=70 mm and y=33.5 mm, move the table to x=40 mm, and y=33.5 mm)
  8. Start moving the table in x ever 2-3 seconds in 0.5 mm increments.
    • NOTE: the table moves much faster in x than it does in y, monitor the table positions closely!
  9. This time monitor the ComptonVfingerRate variable. Find the x-position where it's maximized.
  10. Once you have a general sense of where it is, start scanning in smaller and smaller increments to find the true x-center. Once ComptonVfingerRate is maximized, record the x-position.
  11. Finally, set the detector table to be x=<x center position you just found> + 20 mm, and y=<y center position you just found> + 20 mm.
    • (i.e., if you found ComptonHfingerRate maximized at y=13.5 mm and found ComptonVFingeRate maximized at x=49 mm, set the detector to x=69 mm and y=33.5 mm)
  12. Stop the current run. Your detector is centered.

Rebooting From Upstairs

We have several power strips under our control.


Outlet Component Comment
3 MiniMegan
4 Compton IOC
8 Compton VME Crate Photon Detector Integrating DAQ


Plugged into outlet eight of this power strip is the power strip for the photon detector table. This includes the power supplies for the jaws, and pulser controls. Can switch off in emergencies.

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