The effect of oxygen on anode wire swelling under high accumulated radiation dose

We present results from an aging investigation of straw drift tubes placed under sustained irradiation from a 90Sr (2 Ci) β-source. Aging tests were performed with two different gas mixtures 70% Xe+10% CO2+20% CF4 and 40% Ar+40% CO2+20% C2H2F4. A thorough study of the aging phenomenon of gold-coated tungsten wires with a diameter of 35 μm has been carried out with accumulated charges of 3.6 and 1.6 C/cm. As a result of the aging process, the gold coating on the wires was damaged, and the wire diameter increased by about 5–6%. Using nuclear reaction analysis in combination with an SEM/XEM analysis we determined that there is a direct correlation between the amount of oxygen inside the gold coating and the anode wire diameter. This anode swelling effect can be explained as a result of tungsten oxidation underneath the wire coating. This phenomenon of anode wire swelling by forces applied from within the anode wire is a possible new phenomenon in anode aging.