[en] The increasing application and use of radioisotope x and γ-ray sources based on transuranic nuclides has been accompanied by significant improvements in source designs and production methods. At the same time, considerable progress has been made in both prototype and long term testing. This critical approach to design and testing has emphasized that due to inevitable ageing effects such as radiation damage, sources cannot be expected to retain their initial high performance standards indefinitely but should be regarded as having a limited working life not related to the long half-life of the nuclide