Swelling phenomena in anode wires aging under a high accumulated dose

We present results from an aging investigation of straw drift-tubes placed under sustained irradiation from a 2 Ci 90Sr β-source. The aging phenomenon of gold-plated tungsten wires with diameters 25, 30 and 35 μm with accumulated charges up to 9 C/cm was studied thoroughly. Aging tests were carried out with different gas mixtures: 70% Xe+10% CO2+20% CF4, 60% Ar+30% CO2+10% CF4, 40% Ar+40% CO2+20% C2H2F4, and 60% Ar+30% CO2+10% C2H2F4. As a result of the aging process, the gold coating on the wires was damaged, and the wire diameter increased. For the first two gas mixtures, the wire swelling effect was concentrated only in the center of the irradiated zone. In comparison, the performance deterioration in the mixtures containing C2H2F4 proved to be quite different. The gold coating on the wires was damaged, but outside the irradiating area. Large deposits were found on the wire surface, both in the vicinity of the irradiated zone and downstream of the gas flow. All deposits contained tungsten and oxygen, resulting in the local increase of the wire diameter by 36% (from 30 to 41 μm). Our data show that both aforementioned aging effects have a similar reason: the swelling of the wire material that creates forces within the anode wire and cracks the gold coating.