346 reputation
113
bio website
location
age
visits member for 2 years
seen Oct 13 at 20:14

Oct
16
awarded  Popular Question
Oct
13
accepted Have $2s_{1/2}$ and $2p_{1/2}$ the same energy?
Oct
11
comment Have $2s_{1/2}$ and $2p_{1/2}$ the same energy?
@JohnRennie from lecture notes.. it is referring to the hydrogen...
Oct
11
asked Have $2s_{1/2}$ and $2p_{1/2}$ the same energy?
Oct
10
comment About Nuclear magnetic resonance
and what about frequency of the pulse? can it have any value in the range of RF?
Oct
10
comment About Nuclear magnetic resonance
(...continue) must be applied after spins relaxation... I have understood that the spins relaxation starts when I turn off the pulse... And that the pulse is so strong that forces all the nuclei to have the same larmor frequency... isn't it? The only thing that the pulse does is to move the net magnetization away from z-axes and allow some nuclei to make the transition?
Oct
10
comment About Nuclear magnetic resonance
Mmmh... wait! I'm loosing! :D I have understand that I apply a static magnetic field to polarize the spins. Then I want the NMR signal, so I have to perturb the system. I employ an RF pulse very intense and for a very small time interval. Some nuclei make the transition and the net magnetization feels 1) the static field and 2)this new field. The result is a spiral motion that leaves the z-axes. The net magnetization stops its motion at the flip angle and I have the transverse magnetization that I needed. (please, tell me I have made mistakes). Now, I haven't understand why the pulse..
Oct
10
comment About Nuclear magnetic resonance
thanks George. It is clear now! Talking with Floris, a new doubt comes out: does the pulse modify the larmor frequency of the nuclei or not? If not.. how can the pulse force all the atoms to be in phase? Many thanks again!
Oct
10
comment About Nuclear magnetic resonance
Let us continue this discussion in chat.
Oct
10
comment About Nuclear magnetic resonance
Sure! But.. Considering that when the pulse is on, the nuclei experiment two magnetic fields... Can I say that the larmor frequency is given by $\nu= 42 MHz*(B_1+B_2)$ where B_1 is the polarizer field and B_2 is the pulsed magnetic field?
Oct
10
comment About Nuclear magnetic resonance
Probably I haven't completely understood how the pulse works... I have understand that the pulse causes some nuclei transitions (from s=1/2 to s=-1/2) and, at the same time, it causes transverse magnetization. I have also understood that when the pulse is "on", the net magnetization experiments the first magnetic field (oriented like z-axes) and the new field (pulse). Now, considering that the pulse is very strong, what about the Larmor frequency of the nuclei when the pulse is "on"?
Oct
10
comment About Nuclear magnetic resonance
yes, sure. But the pulse forces protons to be all in phase, right? So I thought that the Larmor frequency (during the pulse) must be the same for all protons and proportional to the amplitude of the pulse...
Oct
10
comment About Nuclear magnetic resonance
Hi Floris, I have a doubt... when in your answer you said "transverse magnetization happens when the protons are all in phase, which you force by the RF pulse.", the Larmor frequency that you are referring is proportional to the amplitude of the RF pulse?
Oct
9
comment About Nuclear magnetic resonance
(I can't understand why the magnetization induces current)
Oct
9
comment About Nuclear magnetic resonance
Hi George, I have a question (I ask you because you work with NMR apparatus.. I hope not to disturb you!). I haven't understood why "As transverse magnetization rotates about the Z axis, it will induce a current in a coil of wire located around the X axis".. could you help me? Many thanks again!
Oct
2
comment About Nuclear magnetic resonance
Very interesting! Thanks!
Oct
2
comment About Nuclear magnetic resonance
Thanks for you answer! But I'm not sure to have correctly understood what you said about T1. I have understood that after a time T1, the Z-component of the net magnetization, is equal to 63% of its value at the equilibrium. Does it indicate also the time that the spins take to align with the external field?
Oct
2
accepted About Nuclear magnetic resonance
Oct
2
comment About Nuclear magnetic resonance
Wow!! :) Thanks a lot for your kindness!!
Oct
2
comment About Nuclear magnetic resonance
I have understood that the field experimented by a single nucleus is given by the vectorial sum of the external field applied B_0 and the field generated by near nuclei. So each nucleus experiments a slightly different value of B, and so it has a different value of Larmor frequency. These differences in the speed of precession are the cause of the dephasing.. and, finally, the trasverse magnetization goes to zero..Is it wrong?