Except interaction with a lattice, kernels can also interact among themselves. This process is characterized by time of backs - spin interaction which is designated usually as T affect each magnetic moment of a kernel not only a constant magnetic field of H0, but also the weak local magnetic field a lok created by magnetic kernels. The magnetic dipole at distance of r creates the field /r
Most often apply the variation magnetic field directed perpendicular to a constant field to supervision of a magnetic resonance. If to designate amplitude of a variation field through H0x, the part of a full Hamiltonian leading to transitions will have an appearance
Process of backs - a reshetochny relaxation leads to broadening of the resonant line since the transitions induced by other degrees of freedom of a molecule do final kernel life time in this state. An order of size of the broadening caused by this process it is equal:
If the kernel having the kvadrupolny moment is in rather symmetric environment, the gradient of electric field has to be equal in the location of a kernel to zero, and,.. , effects of kvadrupolny interaction are excluded. It gives the chance to observe ranges of a magnetic resonance of the kernels having the considerable kvadrupolny moments in crystals of cubic symmetry and in ionic solutions.
In practice for creation of the magnetic field oscillating lengthways a certain direction, for example, along an axis x, on the coil which axis is perpendicular to a field 0 and it is directed along an axis x, pass alternating current. Tension with frequency attached to the coil creates the field equivalent to two fields rotating in opposite directions the size (H1cos t+H1sin t) and (H1cos t – H1sin t).
If corresponds to resonance frequency, the magnetic dipole absorbs energy of the field created by the coil owing to what the vector of the magnetic moment deviates in the direction to the plane and in the second (reception) coil located lengthways axes at is directed.. .
Values of I for concrete kernels cannot be predicted, however it was noticed that isotopes at which both the mass number, and atomic number even, have I = 0, and isotopes with odd mass numbers have half-integer values a back. Such situation when numbers of protons and neutrons even are also equal in a kernel (I =, it is possible to consider as a state with the "full pairing" similar to full pairing of electrons in a diamagnetic molecule.
If to pass to the system of coordinates rotating evenly with an angular speed - , in the absence of other magnetic fields the vector of the magnetic moment in this system of coordinates will remain invariable in size and the direction. In other words, in the rotating system of coordinates the constant magnetic field as though is absent.
However time of life of each of them at this power level decreases. As relative phases of kernels change during time (Δν)-1, the spin exchange requires an interval of time of the same order. This process causes further reduction of time of T2, i.e. the broadening of the resonant line (observed at fixed frequencies at an order size a lok. Both of these factors are considered in the size T2 which is defined as time of life of spin in a certain state and which represents size, the return to width of the spectral line: