The frequency associated with the EPR signal gives us the holding field, and the shift in frequency gives us the polarization. The frequency that we saw this morning was 132kHz lower that what we had seen in the past. In terms of EPR signals, this is a huge change, and corresponds to a magnetic field that changed by approximately 0.25G.
The central frequency has moved, but the size of the frequency shift was the same for both measurements. The difference in the size of these shifts was smaller than the noise associated with these measurements.
What had changed dramatically was the size of the NMR signal. It had increased by about 5%, with the EPR signal realtively unchanged. This indicates that the polarization did not change, but the signal size did change.
Bogdan told us that he had added a large metallic box to the outside of the magnet box, but near the window. We removed the box, and the EPR frequency returned to the location that corresponds to our nominal field settings. The EPR calibration that followed looked like the last calibration performed before the metallic box was added.
This metallic box will be used for the currently running elastic measurement. We replaced it and reproduced the effect.
At the conclusion of the elastic measurement, we will remove the box,
then take an NMR measurement, and resume production running. Right now,
I have a good idea of the size of the effect due to the box, but I will
need to perform NMR tests with the box after the experiment to truly
gauge the effect. In addition, it is clear that the box changed the
magnitude of the magnetic field, and a careful measurement is required to
see if the direction of the field was affected as well.
A copy of this log entry has been emailed to: tdaver@wm.edu, cates@virginia.edu, jpchen@jlab.org, bogdanw@jlab.org