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We have observed a rapid loss of helium from the discharge volume when operating certain types of sealed-off helium-cadmium laser tubes. In severe cases, a loss over a hundred times larger than that possible by diffusion through the glass walls has been observed. Experiments are described indicating that the lost helium is trapped in cadmium, which condenses on the tube walls in the vicinity of the active discharge. Possible trapping mechanisms are discussed. An improved discharge tube design is presented that effectively eliminates the helium cleanup.