Polarizeing current

[Göran]

Polarizeing current

In all the descriptions of PPM:s that I have read on the net one uses a DC puls to polarize the protons. Why isn´t an alternating current at the resonant frequency used instead, as they do in Magnetic Resonance Imaging. Wouldn´t that be more effective?

[Willy Bayot]

Re: Polarizeing current

It is not the DC pulse which polarizes but the DC itself by the powerful magnetic field it generates through the sensor coil. This proton alignment process takes SOME times depending on the liquid put inside the sensor but for distilled water, it takes around 3 seconds. Then, the current (and the field) is cut short and we listen to the weak and short-lived sine wave signal generated by the proton precession.
However, there are indeed some commercial PPM's based on the principle that you describe (using the Overhauser principle with an adequate RF frequency polarization and a specially prepared liquid containing a small quantity of free radicals) but they are even much more delicate to build and to adjust than the traditional ones.
Willy

[Göran]

Re: Polarizeing current

What I meant was that the DC is turned on and off in a pulslike manner. Am I right in saying that it is the (current * time) that matters. A short "puls" with a lot of current does the same job as a longer "puls" with less current?
The process is a form af energy transfer to the proton. That energy should be easier and take less energy and less time to transfer if it is given at the resonance frequency.

[Willy Bayot]

Re: Polarizeing current

And when do you capture and measure the precession frequency generated by the release of the protons after polarization?

[Göran]

Re: Polarizeing current

After the exciteition during the FID or maybe after refocusing the spins with a spin echo.

[Göran]

Re: Polarizeing current

My experiance with magnetic resonance comes mainly from its medical use. We use strong (1,5T) superconductiv magnets with resonance frequency at 60MHz. The magnetic field is highly stable and the equipment needs only minor adjustment to know the exact precession frequency of the protons. The aim is to make a "picture" of the human body. To that end one "modulate" the static magnetic field with linear gradients to make the protons within the volume of interest have different frequency.
The protons are "magnetized" with a very short AC-puls. One doesn´t use the FID to collect data. Insted the precession is refocused to get an echo at witch the data is collected.

The problem is somewhat different with a magnetometer. Here the precession frequency is not known eccept for its nominal value.The task i not to make a "picture" or a spectrogram of the protons but to find the precession frequency. One goal of constructing a magnetometer could be to use as little power as possible. To that en the transfer of energy during "magnetization" should be as effektiv as possible.
So, my question was. Is it possible to make the magnetization with less power by using AC with a frequenzy at least near the expected precession frequency than with a DC puls?

[Willy Bayot]

Re: Polarizeing current

Could you please explain these sentences in more details? What is FID? What do you call a 'spin echo'?
Willy

[Willy Bayot]

Re: Polarizeing current

Actually, there are 4 different types of PPM's:

1. The traditional one with DC switch off only and pure proton-rich liquids
2. Polarization by DC switch off + AF excitation of free radicals based liquids.
3. Polarization by short impulses + continuous AF excitation of free radicals based liquids (GEM systems).
4. Continous AF polarization by whose frequency is controlled by feed-back of precession frequency.

I think that the one you have in mind would be the last one.
Willy

[Göran]

Re: Polarizeing current

FID = #Free Induction Decay" That is the signal you get emediately after you turn off the polarizing current. It is the relaxation process mainly depending on the T2* when the proton precessing loses its phase coherens. But as long as the magnetization has not totaly decayed you can recover the phase coherens by applying a new puls and get a signal echo. That process can be repeted several times. Thats how it is done at higher magnet strength. I dont know if the same applies to experiment at the lewel of the earth magnetic field

[Willy Bayot]

Re: Polarizeing current




That one seems to be the third one of my previous list.
The traditional DC approach requires a polarization field of 100 to 300 Oe while the pulse system only requires 0.2 to 1.5 Oe to get an effective value of 200 to 1500 Oe.

Willy

[Göran]

Re: Polarizeing current

What is the HF in your exampel 2-4? Is it proton nuclear precession frequence at the earth magnetic field or is it the electron spin reconance frequency??

[Willy Bayot]

Re: Polarizeing current

the electron spin resonance frequency.
Willy

[Göran]

Re: Polarizeing current

Thanks for your help and valuble information. I am planning to build a PP Magnetometer just for fun and experimentaition. I am sure I will come back and ask for more help. Yours Göran

[Hubert Dong]

Re: Polarizeing current

This figures come from the article writen by V. Sapunov, etl, Quantum Magnetometry Laboratory, Ural State Technical University ??
Can I make a overhauser senor ?
Where can I get the schematics about the senor
from a web ste like PPM ?
Thank you!

[Willy Bayot]

Re: Polarizeing current

Yes, indeed.
Yes, I guess the easiest system is shown in the second figure. It just requires that you mix a free radical powder with the proton-rich fluid and that you excite the protons with an RF frequency during the standard polarization of the coil.

I do not know of any detailed overhauser sensor winding and mounting procedure document available on the net. You would have to make some hit-and-miss experiments, but, it is what makes the fun of this activity, is it not?

Willy