[en] An irradiation rig with simple design has been constructed and installed in the IEA-R1 research reactor for neutron transmutation doping (NTD) of silicon with phosphorus. Crystal ingots with 3 and 4 in. diameter can be irradiated in this device. By adopting a procedure in which two ingots of 20 cm long each are irradiated simultaneously and their positions interchanged at a point when precisely half the total necessary neutron dose has been received, it has been possible to achieve the desired axial uniformity of the neutron dose. This method avoids the use of neutron absorbing shields around the crystals which necessarily compromise the overall irradiation capacity of the reactor. Test irradiations were performed with 50 float zone silicon crystals, and the results of radial and axial uniformities in the final resistivity values as well as the doping accuracy obtained in the test irradiations show an excellent doping quality achieved. (orig.)