The ACIS instrument aboard the Chandra Observatory can be easily damaged by low-energy charged particles, principally protons that implant themselves in the X-ray sensitive CCDs, creating charge traps that degrade the energy resolution and detection efficiency. During periods of high background radiation, ACIS must be moved out of the focal plane of the Chandra telescope and, whenever possible, this action should be taken autonomously since the spacecraft only maintains ground contact for limited periods. The EPHIN detector has been monitoring the particle background since Chandra was launched in 1999, but it is no longer sufficiently sensitive, so the question arose whether ACIS could take over this task. Examining the ACIS data archive, a particular measured quantity—the rate of occurrence of CCD pixels found to contain electric charge that exceeded a predetermined threshold—was often correlated with particle background flux. An algorithm was developed to distinguish this behavior from random fluctuations in the above-threshold rate and the algorithm parameters were adjusted to find the maximum number of high radiation flux “triggers” from the data archive with the minimum number of false positives. The algorithm has been encoded as a patch to ACIS flight software and, after extensive ground testing, has been installed within the instrument.