The procedure for the STAR Barrel Electromagnetic Calorimeter (BEMC) absolute calibrations, using penetrating charged particle hits (MIP-hits) from physics events at RHIC, is presented. Its systematic and statistical errors are evaluated. It is shown that, using this technique, the equalization and transfer of the absolute scale from the test beam can be done to a percent level accuracy in a reasonable amount of time for the entire STAR BEMC. MIP-hits would also be an effective tool for continuously monitoring the variations of the BEMC towers gains, virtually without interference to STARs main physics program. The method does not rely on simulations for anything other than geometric and some other small corrections, and also for estimations of the systematic errors. It directly transfers measured test beam responses to operations at RHIC.
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