The advanced design uses highly enriched selenium-74 in conjunction with high flux irradiations, but it replaces elemental Selenium with a thermally stable metal-selenide compound in a quasi-spherical focal geometry. This has been introduced to mitigate safety concerns over the use of elemental Selenium and to provide better control and definition of the focal spot geometry. In this paper, the design of an advanced second-generation source is described. As a result, not all countries permit the use of elemental selenium sources for gamma radiography applications. The use of elemental selenium in radiography sources has given rise to some safety concerns because of its high volatility and chemical reactivity. This can melt during irradiation and move around inside the capsule it can even corrode the capsule inside walls at high temperature, potentially with adverse consequences on the focal geometry and the source integrity. This is because sources contained Selenium-75 in elemental form.
Early Selenium-75 source designs were too large to fit many source holders and projectors commonly used by the industry and they failed to optimize the focal geometry. Gamma radiography requires the use of compact, robust, high activity sources with small focal dimensions. The general features and benefits of these sources and their performance relative to other techniques has been thoroughly researched and published elsewhere. For these reasons Selenium-75 provides real performance benefits and working life advantages. Selenium-75 has a softer gamma ray spectrum than Iridium-192 and it has a significantly longer half-life.
Gamma radiography using Selenium-75 is now generally acknowledged throughout the world to provide performance benefits relative to Iridium-192 in the working range of 5-30mm steel.
Methods and Instrumentation Advanced, Second Generation Selenium-75 Gamma Radiography SourcesĪEA Technology QSA, B329, Harwell, Didcot Oxon, OX11 0RA, UK.Advanced, Second Generation Selenium-75 Gamma Radiography Sources
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