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User name Jin Huang
Log entry time 01:49:53 on May 30, 2009
Entry number 277464
This entry is a followup to: 277450
keyword=Reducing Transverse Pickup Coil Pre-amplificationAs shown in Halog 277446, the transverse pickup coil is much more noisier than before. It's shown at the beginning of Transversity, if lock-in over loads (which means the auto offset ->110%, now is 107%), NMR measurements will not be reliable anymore.
Therefore, taking the opportunity of control access, I reduced the pre-amplification of transverse pick up coil from 5x to 2x. So, naively, the calibration constant would increase 2.5 times, from 3.32 to 8.29.
An NMR is performed after the control access as shown below. The NMR pol. measured after the gain changing is fairly consistent with last measurement (54.1% @ 12:35AM -> 53.9% @ 1:27AM).
It's also confirmed that, the lock-in is safe from overflow after the access. Current off set is ~42%. So NMR measurement should be reliable now; although the noise issue is not solved which would go into statistical error of NMR polarimetry.
Note1: A EPR calibration is necessary to give a real calibration number rather than the naive one using right now. But since the raw NMR data is recorded, we can always recalculate the polarization using a better calibration constant later.
Note2: Reducing pre-amplification will not solve the noise issue. I think
it's still necessary to adjust the pickup coil during next long access.
It would require opening target chamber and taking out the transverse
line oven mirror.
A copy of this log entry has been emailed to: yqiang, jpchen, doug, camsonne
NMR after controll access; NMR gain x2
NMR before controll access; NMR gain x5
