Difference between revisions of "NMR Measurement"

Revision as of 21:17, 25 October 2008

Principle

The target used in the upcoming GEn experiment, E02-013, in Hall A is the polarized 3He target. The 3He nuclei are polarized by the spin-exchange mechanism via optically polarized rubidium. (The detailed process of Rb polarization is discussed elsewhere.) There are two methods currently employed to determine the polarization of the helium viz. NMR (Nuclear Magnetic Resonance) and EPR (Electron Paramagnetic Resonance). Here, we shall talk about Frequency Sweep (FS) NMR polarimetry.

The basic principle of NMR relies on sweeping either the magnetic field, or the RF through resonance. In the case of frequency sweep NMR we sweep the RF while holding the magnetic field (H0)1 constant. The expression is symmetric with respect to these two parameters.

The way NMR was performed so far is as follows (field sweep NMR):

• Set the RF at 91kHz. (This gives the resonance at H0 ~ 28G for 3He (~ 21.5 G for water). These quantities are related through the formula: ω = γ H1 where γ is the gyromagnetic ratio (3.24 kHz/G for 3He; 4.26 kHz/G for water), and H1 is the RF field.)
• Sweep the holding field H0 from 25G to 32G (18G to 25G for water) and back; so we sweep through resonance twice (up and down sweeps).
• Plot the data and fit the resonance peak with the square-root of a Lorentzian; extract the peak heights which correspond to the polarization of the substance (after proper calibration of course).

See here [PDF] for a treatise on FS NMR. It follows Abragam in the initial stages. The AFP condition is derived.

For the RF sweep, we keep the holding field H0 constant at 28G (21.36G) and sweep the RF from 87kHz to 95kHz and back. The sweep rate is pretty much determined by the AFP condition, and we found that 4KHz/s works really well (in terms of polarization loss).

This method was tested at University of Kentucky by Dr. Wolfgang Korsch (my advisor) and at The College of William and Mary by Dr. Todd Averett.

Basic Instrumentation for NMR system

Procedure

 Field Sweep for 3He cell

1)  Open the NMR_fieldSweep_v?.vi (current version is v11)
2)  Click the run button on the left top of the window (under the menu bar of the window you just opened).
3)  On the right side of the window, make sure the option button is on the "Helium" position.
4)  Click "Load Default" button to load the default values of the parameters.
5)  In most cases, you do not need to change any default setting EXCEPT the phase in the "Lock-in Settings" part.
    To determine the phase, you can follow the steps below. You can also jump to the next step and come back later.

      What you need to do is try a random number in the range from 0 to 180 and do a sweep first. Then from the
      height of the peaks in "Lock-in X Channel" and "Lock-in Y Channel" you get get some idea that
      what the phase should be. Change it and do another sweep. Check the result and make sure all signals go
      into X chennel and there is almost nothing in Y channel compared with X channel. You can try several
      times til you get satisfied signal.

6)  Make sure the "Wait Time" is 20 minutes.
7)  If you have set the phase right, select enable the "Save Data" option button. If you haven't, disable it.
8)  Click "Start" button. Then you start the field sweep.

 Frequency Sweep

Parameter setting

Constants

• Gyromagnetic ratio for He3 = -32.434 MHz/T
• NMR Resoncance Freq @ 25.000G = 81.085 kHz

polarization quick check by NMR 3He signal

P_[he] = 0.018943416 * h /over n_(eff)