[en] The efficiency of radiation sources used for radiation treatment is important in the overall economics of irradiators. The report examines design optimization for a ¹³⁷Cs radiation source. The initial source design used is currently being fabricated at the Waste Encapsulation and Storage Facility (WESF). Although the current source was intended for long term storage and isolation of the cesium, it has potential for use as an irradiator. Modifications to the source which were examined include: optimum cylinder dia is 1 in.; optimum stainless steel wall thickness is about 0.1 in., thicker walls will reduce source efficiency, while thinner walls increase the likelihood of wall failure; and a cesium-titanate ceramic combines the advantages of good source efficiency with low solubility and low leachability. The cost to modify the present WESF facility does not appear to justify these design changes. However, new facilities could incorporate this design to obtain optimum source efficiency