Polyolefin fibres prodn

A gating member for generator in which laser pulses bombard a target in an evacuated enclosure comprises two discs each with at least one through hole and mounted on respective spindles driven to rotate the discs at diff. speeds with the speed diff. is equal to the pulse frequency so that an outlet passage is periodically opened in synchronism with X-ray pulse generation. The discs are pref. adjacent to the X-ray outlet outside the enclosure, and the laser pulse width is equal to the opening time in which the two holes are superposed. Pulse width and repetition frequency are e.g. 0.1 ns and 10-1000 Hz. Each hole has e.g. an area of 1 cm2 and is spaced at 5-10 cm from the axis of rotation.