Tài liệu Magnetic resonance potx

Magnetic resonance is a phenomenon found in magnetic systems that possess both magnetic moments and angular momentum.. Magnetic resonance is an analytical technique based on a property

Contents :

Introduction

Theory basis

The basis of experiments

Summary

Magnetic resonance is a phenomenon found in

magnetic systems that

possess both magnetic moments and angular

momentum

Magnetic resonance is an analytical technique

based on a property of

matter called spin

Magnetic resonance techniques include:

Magnetic resonance imaging (MRI),

Nuclear magnetic resonance (NMR),

Electron spin resonance (ESR),

Electron paramagnetic resonance (EPR)

Magnetic resonance techniques are generally

non-invasive and non-destructive

MRI is used by clinicians to produce

tomographic images of

the inside of the human body MRI is also

used by scientists

to study materials as it is a non-destructive

imaging technique

NMR is used by scientists to study the

structure and dynamics

of molecules

ESR and EPR are used by scientists to study

structure and reactions of free radicals

Magnetic resonance technique applications

 A system such as a nucleus may consist of many particles coupled together so that in any given state, the nucleus possesses a total magnetic moment  and a total angular momentum J Two vectors

may be taken as parallel

 =  J

where  is a scalar called the “gyromagnetic ratio”

 In quantum theory, we have

J =  I

where I stands for a dimensionless angular momentum operator.

 As an external magnetic field is applied, this field produces an

interaction energy of nucleus of amount -  H.

The Hamiltonian: H = -  H

H = -  H0Iz in the z-direction,

and E =  H0m with m = I, I-1, ….-I (2I + 1 values)

 To satisfy the conservation of energy,

 =  E

 E is the energy difference between the initial and final

nuclear Zeeman energies.

  is an angular frequency

The energy of the four sublevels of a nucleus with

spin I=3/2 when placed in a magnetic field H0

0



 1 2

3 2 3

-3/2 -1/2 +1/2 +3/2

m

H0













0

2

3 2

3

H















0

2

3 2

1 H















 0 2

3 2

1

H















 0 2

3 2

3

H



For producing magnetic resonance, an

perpendicularly to the static magnetic field,

and is written by

H pert = -H 0

x I x cost

Consequently, the allowed transitions are

between levels adjacent in energy, giving

 = E = H 0

or  = H 0

and charge e moving in a circular path of

radius r with period T

J = mvr = m.2r 2 /T

because i = (e/c)(1/T),

H0

H0 x

dH

dz dH

2

1





m

2

1





m

2

1





m

B C

A

0



dz

dH

Slit The atomic beam arrangement of I Rabi and collaborators used to

detect magnetic resonance transitions in atomic energy levels

Experiments

610A

Magic T

Dummy load

X – Y

recorder

Y -in

X -in

B

D

Crystal

C

A

Cavity and sample

Sync in Microwave Oscillator And power supply

Magnet Power supply

Electromagnet pole faces

Schematic arrangement of apparatus for an electron

paramagnetic resonance experiment in the

microwave region.

With a large masses have low  ’s, a factor of 1000 s, a factor of 1000

lower  for nuclei than for electrons.

We can change  by changing Ho, but in most

cases it is advantageous to use as large a magnetic

field as possible.

The electronic systems have a resonance in the

microwave frequency region.

The nuclear systems have a resonance in the radio

frequency region.

Thanks for your attention!