3. X-ray spectroscopy of exotic atoms
An original idea was proposed by A.I.Smirnov and was realized  in 1976 to use the same target placed in a proton beam for generation both of pions and of pionic atoms and so to use it simultaneously as a source of X-ray radiation of pionic atoms ("Gathina target"). This stimulated a new wide field of applications of the crystal diffraction devices. In particular, it allowed to use effectively the crystal diffraction spectrometers for studying X-ray spectra of mesonic and hyperonic atoms at the accelerators of PNPI (Gatchina) and of IHEP (Serpukhov) as well as at Swiss meson factory (PSI).
Using the specially designed and constructed for this purpose crystal diffraction Cauchois spectrometer, and target mounted at the proton beam of the PNPI accelerator the transition energies in pionic atoms of Ñà and Ti  were precisely measured. From these data the value of pion mass with the best at that time accu-racy 1.5·10-5 was obtained.
At PNPI accelerator and meson factory of PSI (Switzerland), using the created in PNPI spectrometers, the advanced investigations of spectra of pionic atoms [58-60] were carried out under the direction of A.I.Smirnov. The energies and widths of X-ray transitions were measured, and the values of widths of 2ð-levels and their shifts caused by the strong interaction of a pion with nucleus from carbon up to sulfur were received.
At the modernized crystal diffraction spectrometer (of Cauchois scheme) with the target mounted at the proton beam of the 70 GeV accelerator in Serpukhov, the energies of X-ray radiation of K -- and S --atoms of carbon [61,62] are measured. As a result the mass of K--meson was determined (relative error 1.2·10-5) as well as the mass of S --hyperon (3.3·10-5) with the best at that time accuracy.
Recently a new two flat crystal diffraction spectrometer is created by A.F.Mezentsev, K.E.Kiryanov et al. in collaboration with the Moscow meson factory for study of the lightest exotic atoms [63,64]. In the energy range from 3 keV to 100 keV the spectrometer allows to perform measurements with relative accuracy (1 - 3)·10-6 and relative energy resolution up to 10-5.
A wide program of investigations is proposed for this device . In particular the experiments are planned on new measurement of the negative pion mass (which is necessary for the muonic neutrino mass estimation), on the measurement of the strong interaction shifts and natural widths of the 1s-state in pionic hydrogen and deuterium atoms with a higher accuracy for more precise definition of the scattering pion-nucleon lenths. A feasibility of measurement of the hyperfine splitting of 2p-levels of the lightest pionic atoms due to spin-dependent pion-nucleon interaction is shown (K.E.Kiryanov ). Also the determination of the nuclear quadrupole moments from the measurements of the pionic X-ray spectra is proposed.