Spin-Echo

In physics, the spin-echo is a phenomenon of quantum mechanics, (ISR ) is used in particular for the measurement of the magnetic relaxation in the field of nuclear magnetic resonance (NMR) and electronic spin resonance.

Some particles, such as electrons and certain atomic nuclei have, as a quantum mechanical property intrinsic angular momentum, which is called spin. Due to the electrical charge is associated with the spinning a magnetic dipole moment, so that the particles in a static magnetic field, depending on the setting angle have different energy. A macroscopic magnetization of the material sample arises by the fact that more particles along the field direction aligned than opposed. It lies parallel to the field, can be rotated away from the field direction by an additional alternating magnetic field of appropriate frequency, however, with its setting angle increases proportionally with longer exposure of the alternating field. The static field then acts with a torque which would, by itself, the magnetization parallel again, but causes the Larmor precession about the field direction in the presence of angular momentum of the particle. The component of the magnetization which is perpendicular to the axis of the static field, and induced to rotate about this axis in an additional coil, an AC voltage, as the rotating magnet in an electrical generator. The frequency of this alternating voltage is the Larmor frequency of static field and its amplitude is proportional to the strength of the rotating component of the magnetization. Since you have to give the alternating field of the same frequency, to twist the magnetization to the field direction, there is a resonance phenomenon.

For the spin-echo now the alternating field is switched on for so long that the magnetization is rotated by 90 ° ( pulse), that is perpendicular to the direction of the static magnetic field. In the subsequent Larmor precession, the magnetization increases, among other things therefore from, because the ( temporal) static magnetic field is somewhat inhomogeneous, ie spatially varied. Then the individual spins do not rotate all the same speed; one speaks of the dephasing of the spins. This increases the rotating component of the magnetization from virtually to zero. After a while the alternating field is now twice as long as before switched, so that the spins are rotated by 180 ° ( pulse). Then they are again perpendicular to the static field and put their Larmor precession continues in the same direction, but the slower among them now have the lead over the faster one. "Walk" through the same inhomogeneities of the static field, the spins back "together" ( rephasing ), so that, after the same period of time, between the -, are located pulse of the alternating field again in phase and produce a clear maximum of the induced signal - and the, the so-called spin-echo.

During this process, some of the spins are aligned, however, after the static magnetic field and thus no longer contribute to the transverse magnetization. The echo is therefore weaker than the initial magnetization. The time constant of this decay is the transverse relaxation time. It can either be determined in a number of experiments with different echo times, or by further pulses after the first spin-echo periodically applied so that a plurality of spin- echoes can be observed one after another from time to time the weaker fail. The latter method, the so-called Carr -Purcell pulse sequence, characterized not only by their speed by their insensitivity to diffusion processes.

The spin- echo takes place in the magnetic resonance spectroscopy and magnetic resonance imaging application, since different atomic nuclei, ie different isotopes, and even atoms in different compounds have different relaxation times. This compounds can be studied precisely or distinguish tissue types in tomography.

One very important and widely used application of the spin-echo experiment, the measurement of diffusion and flow movement by means of the field gradient NMR. The diffusive or coherent ( in flow ) of movement of particles in a magnetic field on the spin echo in Kernspinpräzessionsphase experiment is measured. Thus, one can also be physically and chemically identical species, eg distinguish certain water molecules in the water, and their diffusion, which is called in this case " self-diffusion " study.