after my colleague’s work on yesterday on fighting the noise, the noise level of the NMR system reduced to 25mV. and but water NMR signal also same magnitude, due to impedance mismatch. thus. i tuned the impedance and observed water NMR signal to be 330 mV. signal to noise ratio is around 10.
then i insert the sample and start polarization. i pick the data from 15 us to 25 us after the pulse, since there has a peak, which gives the FID amplitude different from BG significantly. i.e. BG FID amplitude is around 6 unit and polarized sample has FID amplitude around 20.
however, i cannot obtains a consistent data. say, if i set the crystal angle be 90 degrees, then on 95 degree. then i do again 90 degree, the 2nd time, the FID amplitude may change alot. even though i took 3 times on each angle.
the sample space has fluctuation due to the noise. thus, we need to take several data on each setting to get a reliable result. but each data need 300 seconds, thus the data taking is quite frustrated, due to inconsistent data.
the possible reasons for inconsistent data is due to NMR coil position and connection. since it is the only thing changing on experiment. each time, we have to unplug and plug in the coil to get the NMR date. even a tiny change of the coil position will change the NMR signal strength ( this was tested on water sample).
also, the reflected NMR pulse is about half of the input pulse. even though we matched impedance by smith chart. How is this possible??
- fix the NMR coil position and get consistent and reproducible data.
- find the maximum Crystal orientation
- find the laser polarization dependency