Abstract: X-rays of a specific wavelength are required for some analytical applications. Such soft X-rays can be obtained by irradiating a secondary target with X-rays of a suitable wavelength. According to the invention there is formed an X-ray tube 1 with an anode surface layer 7 containing a rare earth metal, preferably Gd or Dy. When a secondary target containing La is irradiated with K radiation from such a tube, La K radiation is generated which can be used for analysis of elements of the fifth row of the periodic table. In a preferred embodiment the rare earth metal is bonded to the anode body 4 by way of an intermediate layer 8 of Ti or Mo.