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We propose a new way of quick and very efficient acceleration of protonsand/or electrons in relativistic bulk flows. The new mechanism takes advantageof conversion of particles from the charged state (protons orelectrons/positrons) into neutral state (neutrons or photons) and back. In mostcases, the conversion is photon-induced and requires presence of intenseradiation fields, but the converter acceleration mechanism may also operate viainelastic nucleon-nucleon collisions.  Like in the traditional model -- ``stochastic'' (or diffusive) acceleration,-- the acceleration cycle in our scenario consists of escape of particles fromthe relativistic flow followed by their return back after deflection from theambient magnetic field. The difference is that the charge-changing reactions,which occur during the cycle, allow accelerated particles to increase theirenergies in each cycle by a factor roughly equal to the bulk Lorentz factorsquared.  The emerging spectra of accelerated particles can be very hard and theircut-off energy in some cases is larger than in the standard mechanism. Thisdrastically reduces the required energy budget of the sources of thehighest-energy particles observed in cosmic rays. Also, the proposedacceleration mechanism may serve as an efficient means of transferring theenergy of bulk motion to gamma-radiation and, if the accelerated particles arenucleons, routinely produces high-energy neutrinos at $\sim 50 %$ relativeefficiency.Comment: extended version, 10 pages, submitted to Phys. Rev.

Particle acceleration through multiple conversions from a charged into a neutral state and back