[en] Particles or 'seeds' are manufactured for implantation into tumors within a human body for emitting X-rays to destroy or reduce the tumors. The seeds contain palladium which is substantially enriched in palladium-102 and which is activated by exposure to neutron flux so as to contain a minor, but significant, fraction of X-ray-emitting palladium-103. The palladium is distributed on or throughout a base material so as to reduce selfshielding by the palladium. The seeds include an X-ray-opague marker to faciliate external visualization of the seeds after their implantation, the marker preferable being formed of a material, such as lead or rhodium, which does not activate to contain undesirable isotopes under the radiation conditions in which palladium-102 is activated to palladium-103. The base material-distributed palladium and the marker are encased in an elongated shell which is formed by welding a pair of end caps to a tubular member, the construction of the shell reducing the mass of material at the end of the seed, thereby providing a more isotropic angular distribution of X-rays