ACIS PHA Randomization
For some observations, the way PHA channels are converted to PI channels will cause an aliasing problem in the final binned spectra. Since PHA is in discrete integer values, a single value of PHA corresponds to a single energy and therefore a single PI value for a given 32x32 pixel region in the gainmap file. Although it varies, a difference of one PHA bin corresponds to ~4 eV in energy space. Therefore, in the case of a sufficiently strong source in a small spatial region, the spectrum in energy will show a series of spikes and zeros. Since
PI = 1 + (energy / 14.6)
the PI spectrum will also show a strange series of jumps. This problem primarily affects sources with a large number of counts; randomizing the PHA distribution alleviates this effect. PHA randomization has been applied in standard data processing since software version DS 6.1 (02 July 2001).
Removing the randomization
Removing the PHA randomization should only be done by advanced users who know exactly what they are doing. Since half an ADU is much smaller than the energy resolution at all energies, disabling the PHA randomization should not significantly affect CCD spectra.
To remove the PHA randomization, run acis_process_events with rand_pha=no.