Right Hand Rule in NMR and EPR?

In summary, the main magnetic field in an MRI machine is typically in the 1.5 T to 3.0 T range, while the RF field is in the 5 uT to 20 uT range. The RF field may have a small component in the longitudinal direction, but it is negligible compared to the main magnetic field and does not affect its detection. Only the transverse component of the RF field is distinguishable from the main field, but it is still very small in comparison. Additionally, the direction of the RF field is important in exciting the spin states of the particles.
  • #1
Narayanan KR
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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 ?
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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.
 
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  • #3
Dale said:
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 ?
 
  • #4
Narayanan KR said:
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.
 
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Narayanan KR said:
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
 
  • #6
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.
 
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hutchphd said:
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.
 
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@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.
 
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  • #9
Dale said:
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...
 

1. What is the purpose of the Right Hand Rule in NMR and EPR?

The Right Hand Rule is used to determine the direction of the magnetic field in NMR (Nuclear Magnetic Resonance) and EPR (Electron Paramagnetic Resonance) experiments. It helps in understanding the orientation of the nuclei or electrons in the presence of an external magnetic field.

2. How does the Right Hand Rule work in NMR and EPR?

The Right Hand Rule states that if you point your thumb in the direction of the magnetic field, your fingers will curl in the direction of the current or spin of the nuclei or electrons. This allows scientists to determine the orientation of the particles and their response to the magnetic field.

3. Why is the Right Hand Rule important in NMR and EPR experiments?

The Right Hand Rule is important because it helps in interpreting the results of NMR and EPR experiments. By knowing the direction of the magnetic field and the orientation of the particles, scientists can determine the chemical and physical properties of molecules and materials.

4. Are there any variations of the Right Hand Rule in NMR and EPR?

Yes, there are variations of the Right Hand Rule in NMR and EPR experiments. One variation is the Larmor frequency rule, which describes the relationship between the frequency of the magnetic field and the energy of the nuclei or electrons. Another variation is the gyromagnetic ratio rule, which relates the magnetic moment of the particles to their angular momentum.

5. How can the Right Hand Rule be applied in other areas of science?

The Right Hand Rule is not only used in NMR and EPR, but it also has applications in other areas of science, such as electromagnetism, mechanics, and quantum mechanics. It is used to determine the direction of magnetic fields, electric currents, and forces in various systems. It is a fundamental concept in physics and has many practical applications in different fields.

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