EPR during GEn

EPR during the GEn experiment will be performed by experts only. The following steps outline a typical polarization measurement using EPR.

Electronics Apparatus:

• HP E4400B : The EPR RF function generator
• SRS DS345 : Modulation function generator
• EG&G 7265 : Lock-in amplifier
• SRS SR620 : Universal Time Interval Counter (Frequency counter)
• Homemade EPR Feedback box NU-112 (with UK logo)
• HP 3324A : NMR RF generator for AFP spin flip

Doing a measurement:

1. Fill in the EPR Checklist after you perform the FM Sweep and before you run the AFP^* vi.
3. Check the dc output from the photo diode with a scope or multimeter. It should read around ±95 to ±115 mV.
4. Connect the output back to the lockin (7265) channel A.
5. Turn the EPR RF FG (E4400B) ON.
6. Run the GEN_FM_SWEEP.vi progam in the Target Experts folder. Set the lock-in sensitivity (SEN) to 100µV in the vi. Start the FM sweep over a ±200kHz range about the frequency at which the resonance occured during a previous EPR. You should see a shape that looks like this:
   
   that is, should go to negative values and cross over to positive ones. The peak positive and negative values should be approximately the same.
7. Plot the data, zoom in to the central linear region and do Fit Data. Then, if the signal and fit look good, hit Lock. You are now locked at the resonance frequency. Note the slope of the line fit (m_FM)
8. Now use the following formulae to get the Sensitivity on the lockin and the number of turns on the absolute gain of the UK PI feedback box:
   
   -½ = G_abs = (m_FM) { dev (in kHz) / S (in µV) } G'_abs
   
   G'_abs = {-5.12 * n - 1.17}
   
   where:
   m_FM: slope fom the FM sweep;
   dev: Deviation on the E4400B;
   S: Sensivity on the EPR lockin;
   n: number of turns on the Absolute Gain of the UK PI box;
   -1 ≤ G_abs ≤ 0.
   Note that the second equation above is specific to the UK PI box NU-112.
9. To set the number of turns correctly, plug in the modulation signal into the input of the PI box and plug the V'_out from its back into a multimeter. Then refer to the table on p.176 of the Target Logbook to see what the output should be and adjust the Absolute Gain till you see approximately that output. Connect the cables back to their initial ports.
10. Once you've picked the SENS and # of turns, run the EPR.vi LabView program (click on the white arrow). Then click on Download Parameters. The status should display the state. Once that is over (should be about 5 seconds), hit the Pause/Run button. Verify items that come up in the pop up window. Click OK.
11. Now, plug in the BNC cable from the rear of the lockin (7265) to the "input" of the PI feedback box. The frequency should change a little. Flip the switch to the "Integration ON" position. The lockin signal should quickly fall to zero.
12. Now, turn the Relative Gain knob clockwise till you see oscillations on the EPR counter signal. Turn back half a turn (or till you see no oscillations). This is your final setting.
13. Wait for the x axis to reach 30 or 40 counts after you turn Integration ON; then hit Start Sweep. Status should display "sweeping". While in automatic mode, the rest of the sweep should follow on its own. When status changes from sweeping to waiting ofr sweep trigger, you can pause it and stop the VI.
14. Finally, turn Integration OFF, unplug the input cable and the RF FG (E4400B). And that's it.
15. Troubles? Refer to the Troubleshooting section below.

Troubleshooting:

FAQ:

Q Why do we need to check the photo-diode signal?

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Q What is the structure in the figure above, and why?

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Q Why do we choose G_abs = - ½?

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