Spin Flip/How to Measure Spin

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This article discuss how to determine the target spin direction during an Adiabatic Fast Pass (AFP) spin flip, which is an important measure to make sure the system have the correct spin sign.


Target schematics

As shown in Target schematics, target cell containing polarized He3 gas sit inside a holding magnetic field generated by pairs of Helmholtz Coils. 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 sweepned from one side of resonance to the other (Adiabatic Fast Pass). So there are basically two way to carry out the sweep, which is equivalent to each other to the first order:

  • Field Sweep : Fix driven RF frequency, and change strength of magnetic field
  • Frequency Sweep : Fix holding field, and change RF frequency

During [[transveristy|experiment], Frequency Sweep will be used for spin flip, while NMR Measurement prefers Field Sweep.

The phase of induced NMR signal will differ by π, if any of following sweep conditions changes:

  • Sweep type (Field Sweep/ Frequency Sweep)
  • Sweep direction (Low to High/High to Low)
  • Spin State (Spin up/Spin Down)


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 amplifier is setup, so that NMR signal is mainly on the Lock In X channel. Then 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

TBC ...