In the Homestake experiment, why did they alternate runs with $^{36}\rm Ar$ and $^{38}\rm Ar$? I understand why they use a carrier isotope like $^{36}\rm Ar$ or $^{38}\rm Ar$ to mesure the efficiency of the extraction process. But I can't understand how it is useful to alternate the isotope to mesure also the purification efficiency separately.
For the details of this, see B.T. Cleveland et al., Measurement of the Solar Electron Neutrino Flux with the Homestake Chlorine Detector, Astrophys. J. 496, 505 (1998), with the description of alternate runs in §4.3.3. A summary of the extraction process is in p48, §4, of Neutrino 94: Proceedings of the 16th International Conference on Neutrino (Nucl. Phys. B (Proc. Suppl) 38, 47-53 (1995); Google Books link).