Scholarship Sewanee poster 2014
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Transcript of Scholarship Sewanee poster 2014
Spin Echoes & the Principles of NMR Frank Odom III
Department of Physics & Astronomy University of the South: Sewanee
Overview Nuclear MagneEc Resonance is
widely used in the fields of biology, chemistry, and medicine as a non-‐destrucEve imaging method. NMR can obtain valuable informaEon about a given sample by manipulaEng the magneEc moments of its nuclei. I have conducted NMR experiments using Earth’s magneEc field (about 39 μT) as my primary, external magneEc field.
Figure 2. FID signal.
Elemental ComposiEon We can easily obtain informaEon
about the elements within a sample from an FID signal. Any nucleus with a net magneEc moment will precess at a characterisEc frequency, called the Larmor frequency. Fourier analysis can easily idenEfy the consEtuent elements.
Figure 1. Equipment from TeachSpin
Spin Flips & Spin Echoes Our FID signal decays primarily due
to field inhomogeneiEes; however, we could revive the signal by “flipping” our magneEc moments (a spin flip), causing them to precess in the opposite direcEon. The revived signal is called a spin echo.
Figure 3. Spin Echo signal
Electronics We will try to excite a spin flip using
radio waves. So, our electronic circuit must be able to provide the following: • Time delay of about 0.5 s • Sine wave output pulse of easily
controlled magnitude and duraEon
Figure 4. Electronic circuit.
Quantum Mechanics Using Schrödinger’s equaEon as well
as Eme-‐dependent perturbaEon theory, the probability of a spin flip is:
0.5 1.0 1.5»Vosc» HVL
0.5
1.0
1.5
2.0
Spin Echo HVL
10 20 30 40Number of Cycles
0.5
1.0
1.5
2.0
2.5
Spin Echo HVL
1500 1600 1700 1800 1900 2000fosc HHzL
0.5
1.0
1.5
2.0
2.5
Spin Echo HVL
Carr-‐Purcell Method We can determine the characterisEc
relaxaEon Eme for a given sample. This is called the spin-‐spin relaxaEon Eme.
Figure 5. Carr-‐Purcell method.
Spin-‐Labce RelaxaEon Time AddiEonal valuable informaEon can
be obtained from the spin-‐labce relaxaEon Eme. This is more easily thought of as the “polarizaEon Eme” for a given sample.
5 10 15Time HsL
1
2
3
4
5
6
7
FID Signal HVL
Figure 6. Spin-‐Labce RelaxaEon Time.
Characterizing a Sample We can use these two methods of
imaging to determine the physical characterisEcs of samples– namely, ice and a mixture of corn starch and water.
1 2 3 4 5 6Time HsL
5
10
15
20
FID Signal HVL
Figure 7. Spin-‐Labce RelaxaEon Emes for ice and mixture of water and corn starch.
Acknowledgements • TeachSpin • Dr. Peterson • Sewanee Physics Dept.
The mixture of corn starch and water behaves much more like ice– a crystalline solid– than a liquid. Without any addiEonal informaEon, it would be much easier to disEnguish the mixture from water.