[en] A device is described featuring a chopper of the continuous thermal neutron flux, based on a single monocrystal with longitudinal oscillation. For double reflection in the strongly asymmetric diffraction position, the plate-like shape is cut from a Si, Ge or SiO₂ single crystal. The path of the primary reflected beam of thermal neutrons follows the direction of the longitudinal plate axis. Slot diaphragms are adjusted in the direction of the beam of impinging and reflected neutrons. A system of two plate-like single crystals, adjusted and synchronized to oscillate at the same resonance frequency has so far been used for thermal neutron beam pulsing. The benefit of the device newly designed is invariance of the diffraction planes given by the plate structure. The use of a single plate also eliminates the necessity of maintaining for a long time the phase difference in the oscillations of two single crystals. The device can successfully be used in nuclear physics and in solid state physics for measuring the time of flight. (Pu)