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.
Hall A Compton polarimeter Wiki page
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  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.
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.
Webcams were added to monitor the :
- hacp2 EthernetBlaster
- hacp3 webcam trigger
- hacp4 webcam on Beam Safety module, Front End power supply
- hacp5 webcam on silicon strips detector current
- hacp6 APC
- plug 1
- plug 2
- plug 3
- plug 4
APC : hareboot4
*plug 2 webcam *plug 3 EPICS Edet *plug 4 trigger *plug 7 FADC crate