I'm trying to understand the basic principles of Nuclear Magnetic Resonance reading this link but I have some doubts:
1) I have ever known that when protons aren't in a magnetic field, their spins are random oriented in the space. When the protons are in a magnetic field, their spin feel the effect of $ \tau= \mu \times B$ and are inclined to align themselves with the magnetic field. But they precede around B, they aren't oriented like B! Isn't it?
So I can't understand why in the link that I have reported, at the section "Spin Physics", paragraph "Energy Levels", there is written
When the proton is placed in an external magnetic field, the spin vector of the particle aligns itself with the external field,just like a magnet would. There is a low energy configuration or state where the poles are aligned N-S-N-S and a high energy state N-N-S-S.
(please, see the animation on the link)
It seems to me that the two interpretations are in contrast..
And, if I follow the interpretation that I have ever heard, I don't understand this step: (paragraph "$T_1$ process"))
At equilibrium, the net magnetization vector lies along the direction of the applied magnetic field Bo and is called the equilibrium magnetization Mo. In this configuration, the Z component of magnetization MZ equals Mo. MZ is referred to as the longitudinal magnetization."?
The only thing that came to my mind is: it is due to the fact that we are considering a lot of atoms and so the x,y components of the net magnetization vector are -on the average- equal to zero.. But I'm not sure that it could be right...
2) In the paragraph "Spin Relaxation" I dont't understand if the motions that influence $T_1$ are rotational motions at the Larmor frequency or any motion that causes a time varying field at the Larmor frequency..
And I'd like to understand if the loss of phase of the transverse magnetization is due to the action of the many molecules that rotates at a frequency less than and equal to the Larmor frequency..