Abstract: An ionization detector, such as an electron capture detector for use in pesticide residue analysis, utilizes a scandium tritide beta particle source. The scandium tritide forms a surface portion of a metallic foil. If the foil initially exhibits an unacceptably high tritium emanation rate at desired high operating temperatures, e.g., at temperatures above 250.degree. C, the foil can be treated so as to exhibit an acceptable tritium emanation rate at such temperatures. The treatment comprises heating the foil at a predetermined treatment temperature, e.g., at a selected temperature in the 300.degree. to 400.degree. C range, until the tritium emanation rate for the foil at the selected treatment temperature reaches a value which correlates with an acceptable tritium emanation rate for the foil at a particular desired operating temperature.