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Radiative electron capture (REC) cross sections for 40-80 MeV Cl ions incident on thin C foils are reported and compared with previous results using Cu targets. We find that the measured REC cross section per K vacancy scales according to the number of "free" electrons on the target atom, i. e., those bound target electrons with a velocity much less than the incident projectile velocity. Comparison is made with the free-electron theory of Bethe and Salpeter and good agreement is obtained. Experimental and theoretical results are compared with those of Lindskog et al., and consideration is given to different ways of computing the K-shell binding energy of the projectile at the time of REC which enters into the calculation of the theoretical REC cross section in an important way.