The clean, annealed W(110) and Mo(110) surfaces were bombarded by 1013-1015 noble-gas ions having kinetic energies between 100 and 1000 eV. The degradation of intense peaks in the major beams was observed with a rapid-scan, Faraday collector system. The dependence of the rate of degradation on ion mass and energy and target mass was determined to depend on the energy transferred in two-body, linear, hard-sphere collision. Comparison with sputtering yields and thermal desorption of embedded ions indicated when knock-on lattice distortion rather than direct sputtering or ion embedding probably caused the intensity degradation.