A Right Hand Rule in NMR and EPR?

[Narayanan KR]

TL;DR Summary

I noticed that in Nuclear Magnetic Resonance and in Electron Spin Resonance (a.k.a Electron Paramagnetic Resonance) the B Field of Excitation Signal (Radio wave in case of NMR, Microwave in case of EPR) is kept Orthogonal (perpendicular) to Main Magnetic Field, also the output in NMR is an oscillating B field Orthogonal to both the main Field and excitation field, Does it always has to be like this (like in Flemming's Right Hand Rule) or can the angles differ ?

 

[Dale]

The main magnetic field is typically in the 1.5 T to 3.0 T range. The RF field is in the 5 uT to 20 uT range. The RF field may accidentally have a small component in the longitudinal direction, but because it is so small compared to the main magnetic field it is completely negligible. It produces an undetectable difference in the main magnetic field. It is only the transverse component that can be distinguished from the main field. It is still minuscule compared to the main field, but since it points in a different direction it is detectable.

 

[Narayanan KR]

The main magnetic field is typically in the 1.5 T to 3.0 T range. The RF field is in the 5 uT to 20 uT range. The RF field may accidentally have a small component in the longitudinal direction, but because it is so small compared to the main magnetic field it is completely negligible. It produces an undetectable difference in the main magnetic field. It is only the transverse component that can be distinguished from the main field. It is still minuscule compared to the main field, but since it points in a different direction it is detectable.

so we can have the RF coil wound around the magnets, such that their fields are along same axis right ?

 

[Dale]

so we can have the RF coil wound around the magnets, such that their fields are along same axis right ?

Sure. But no MRI manufacturer wants to do that.

 

[hutchphd]

so we can have the RF coil wound around the magnets, such that their fields are along same axis right ?

No. The homogeneous static field (z-axis) causes the degenerate spin states to split according to $m_s$ along that axis. The oscillating perturbation B field must rotate in the x-y plane. In quantum language the photons must be circularly polarized to impart the required spin.
This is in many books and wikipedia

 

[f95toli]

Indeed, the direction matters
Note however that this does not mean that you can't get away with having components in other directions. EPR is done using microwave cavities/resonators and whereas you typically should do your best to align all the field directions so that it is "by the book" the real-world field distribution can be quite complicated. It will still work as long as you have some components in the "right" direction.

 

[Dale]

No. The homogeneous static field (z-axis) causes the degenerate spin states to split according to ms along that axis. The oscillating perturbation B field must rotate in the x-y plane. In quantum language the photons must be circularly polarized to impart the required spin.

Just to head off potential confusion for the OP due to conflicting answers. I was answering the question “if the static field is in the z direction is it physically possible to make an RF coil that also points in the z direction?” To which the answer is “yes” as I said. You answered the question “could such a RF coil excite the spins?” To which the answer is “no” as you said.

 

[hutchphd]

@Dale thanks for the clarification. For @Narayanan KR (the OP): in fact, in most older NMR machines there is a small coil coaxial to the main static magnet that is driven with a sawtooth or triangular current at very modest frequency. This is used to slightly sweep the value of the resonant frequency which is electronically simpler than changing the frequency of the excitation field. In the late 1940's it was the only way to do it.

 

[Narayanan KR]

Just to head off potential confusion for the OP due to conflicting answers. I was answering the question “if the static field is in the z direction is it physically possible to make an RF coil that also points in the z direction?” To which the answer is “yes” as I said. You answered the question “could such a RF coil excite the spins?” To which the answer is “no” as you said.

thanks for clarification...