Process for producing a single crystal of silicon, comprising: the installing of a feed rod of silicon in a float-zone apparatus, the feed rod having a diameter of not less than 230 mm and not more than 270 mm, the installing of a first hollow cylinder having a bottom edge and an internal diameter which is larger by not less than 30 mm and not more than 50 mm than the diameter of the feed rod, so that during pulling, the hollow cylinder surrounds the feed rod and the longitudinal axis of the first hollow cylinder and the longitudinal axis of the feed rod lie one above the other, the installing of a second hollow cylinder having a top edge and an internal diameter which is larger by not less than 20 mm and not more than 60 mm than the target diameter of the single crystal, so that the second hollow cylinder is installed in such a way that it surrounds the single crystal to be pulled; pulling a cylindrical part of the single crystal which has a target diameter of not less than 290 mm and not more than 310 mm, where the feed rod at the melting front forms an outer melting edge and the monocrystalline ingot on the growth side forms a crystallizing edge, where the pulling speed is not less than 1.3 mm/min and not more than 1.5 mm/min, preferably not less than 1.35 mm/min and not more than 1.45 mm/min, where the vertical distance of the bottom edge of the first hollow cylinder from the outer melting edge is smaller than 2 mm, and the top edge of the second cylinder protrudes not less than 1 mm and not more than 10 mm over the crystallizing edge, removing a length of the single crystal to form an ingot piece having a length of not less than 15 cm and not more than 50 cm. the installing of a feed rod of silicon in a float-zone apparatus, the feed rod having a diameter of not less than 230 mm and not more than 270 mm, the installing of a first hollow cylinder having a bottom edge and an internal diameter which is larger by not less than 30 mm and not more than 50 mm than the diameter of the feed rod, so that during pulling, the hollow cylinder surrounds the feed rod and the longitudinal axis of the first hollow cylinder and the longitudinal axis of the feed rod lie one above the other, the installing of a second hollow cylinder having a top edge and an internal diameter which is larger by not less than 20 mm and not more than 60 mm than the target diameter of the single crystal, so that the second hollow cylinder is installed in such a way that it surrounds the single crystal to be pulled; pulling a cylindrical part of the single crystal which has a target diameter of not less than 290 mm and not more than 310 mm, where the feed rod at the melting front forms an outer melting edge and the monocrystalline ingot on the growth side forms a crystallizing edge, where the pulling speed is not less than 1.3 mm/min and not more than 1.5 mm/min, preferably not less than 1.35 mm/min and not more than 1.45 mm/min, where the vertical distance of the bottom edge of the first hollow cylinder from the outer melting edge is smaller than 2 mm, and the top edge of the second cylinder protrudes not less than 1 mm and not more than 10 mm over the crystallizing edge, removing a length of the single crystal to form an ingot piece having a length of not less than 15 cm and not more than 50 cm. <EOS>