PHY40058 Advanced Topics. Part 1 (October 2006).

Magnetic Resonance Spectrometry and Imaging (Professor J J Davies).

Magnetic resonance is one of the most important investigative methods in the physical and life sciences. It relies on the fact that electrons and protons (and many other nuclei) possess magnetic moments (associated with their spin) and can thus be aligned in a magnetic field. Measurement of the energy required to reverse the spin direction enables one to determine the strength of the magnetic field with very high accuracy. The importance of magnetic resonance is that the exact value of the field is extremely sensitive to the local atomic or molecular environment of the electron or proton, so that the technique provides very detailed information about local atomic or molecular structure and can be used as a probe for condensed matter. The energy required for the spin reversal is determined by irradiating the specimen with photons, which are absorbed when their energy hn coincides with the spin-flip energy. The photons and the spin system are then “in resonance”, which is how the technique obtained its name.

In the case of protons, the required frequency is typically in the 100-500 MHz region and we refer to the technique as nuclear magnetic resonance (NMR). NMR is used extensively in chemistry and the life sciences to identify molecular structure and most universities and research establishments possess several instruments. In medicine, the technique is used for imaging, since the NMR signals depend sensitively on the local environment; here the method is referred to simply as magnetic resonance imaging (MRI) and most large hospitals now possess MRI facilities.

Electrons have a magnetic moment which is about 2000 times greater than that of the proton, so that electron magnetic resonance (referred to as electron spin resonance, ESR or electron paramagnetic resonance, EPR) is typically carried out at frequencies in the range 10 to 100 GHz, which is in the microwave region. ESR spectrometers are very common and are used extensively for investigations of condensed matter. In physics, they are used to study dopants and defects in semiconductors and insulators and in chemistry and the life sciences they are used for investigations of molecular structure and function.

The course provides an introduction to the principles of magnetic resonance and to the very wide range of applications. Emphasis will be placed on the physics that underlies the experiments (rather than on the technological details) and on the information that NMR and ESR can provide.

JJD/28/09/06

Some reference sources:

General principles of magnetic resonance:

Electricity and Magnetism, Bleaney and Bleaney (OUP)

Introduction to Solid State Physics, Kittel (Wiley)

NMR

Modern NMR spectroscopy, Sanders and Hunter, (OUP)

ESR

Electron Spin Resonance, Wertz and Bolton (McGraw-Hill)

	PHY40058 Advanced Topics. Part 1 (October 2006). Magnetic Resonance Spectrometry and Imaging Proposed course structure
1	What is Magnetic resonance? Why is it important? Simple QM 2-level viewpoint, hn=gmBB, where g depends on the environment. NMR: protons. Electrons. Huge range of applications. How is it detected? NMR case: marginal oscillator ; simple ESR set-up (better methods later)
2,3	Rate equation approach. T1. Boltzmann distribution. Semiclassical approach. Precession of magnetisation in a field. Free induction decay. De-phasing. T2. The Larmor theorem. Resonance viewed from the rotating frame. Pulsed techniques Detection methods for nmr.
4,5	Proton magnetometry. NMR spectra in the chemistry and the life sciences. Interpretation of NMR spectra. Chemical shifts. Proton-proton coupling. Other nuclei.
6,7	Medical imaging.
8	ESR. Microwave systems. Simple and advanced spectrometers. Spin of ½ spectra. The g-value. Hyperfine interactions.
9	Values of S > ½. Relation to the atomic Landé factor. Crystal fields. Low temperatures.
10,11	ESR studies of defects in crystals ESR in the life sciences

JJD/September 2006