The Anti-Coincidence Detector (ACD) of the Fermi Large Area Telescope (LAT) serves to identify charged particles, which cross the LAT at a rate orders of magnitude higher than that of the gamma-ray signal. We have developed a method that uses heavy cosmic rays, Z > 3, as a calibration source to improve signal uniformity, gain linearity, and charge resolution of light deposit measurement in the ACD at high light levels. Improving the charge resolution gives the LAT an additional heavy-ion charge detector and therefore enhances the LAT's capability for analysis of heavy cosmic rays. Using the calorimeter and tracker, we select populations of heavy cosmic rays with the same charge and roughly minimum ionization.

In this sample, we are able to distinguish eight heavy cosmic-ray species: boron, carbon, nitrogen, oxygen, neon, magnesium, silicon and iron from the calorimeter and tracker data. We use the tracker and calorimeter information to predict the position of the ion in the ACD and its energy deposit, which allows us to better calibrate the gain across the ACD and improve the overall uniformity. We present the results of our method, and demonstrate improved signal uniformity and charge resolution for heavy cosmic rays in the ACD.