Spin Flip/How to Measure Spin
As shown in Target schematics, target cell containing polarized He3 gas sit inside a holding magnetic field generated by pairs of Helmholtz Coil. The FR drive coil is connected to an function generator, which bring He3 to NMR resonance during an flip. Meanwhile, He3 nuclei will reradiates a RF (NMR signal) with same frequency during the resonance. This signal could be observed through a pair of pick up coil perpendicular to the driving coil (For a well aligned system, the drive RF will be blind to Pickup coil). To separate tiny signal from noise, the NMR signal will then be amplified by a Lock-In amplifier, which is sensitive to both amplitude and phase of incoming signal.
To achieve a spin flip, usually one of the condition of target NMR resonance (RF Frequency & Holding field strength) is smoothly scanned from one side of resonance to the other (Adiabatic Fast Pass). So there are basically two way to carry out the scan, which is equivalent to each other to the first order:
- Field Scan : Fix driven RF frequency, and change strength of magnetic field
- Frequency Scan : Fix holding field, and change RF frequency
The phase of induced NMR signal will differ by π, if either the initial spin state or scan direction changes. Assuming NMR signal is WHY THIS WIKI DO NOT SUPPORT FORMULAR??!!
Determine Spin Sign
Therefore, there are two straight forward methods to determine spin sign.
From Peak Sign of Lock-In X Channel
The sign of spin state is equal to (or inverse to) the sign of NMR resonance peak.
To make it working, the initial phase of lock in is setup, so that the as much as NMR signal is on the Lock In X channel. And the initial spin direction is calibrated (by EPR measurement).
The advantage of this method is its simplicity. Therefore it's used in target Logic Electronics. Besides, on the system status window, both Spin from NMR Amp. and Spin from EPICS (a copy of Logic Electronics output) are based on this principal.
By Extracting NMR Signal Relative Phase
The sign of spin is depending on range of relative phase of NMR signal at resonance peak