Normally, every event in the RHESSI germanium detectors is recorded and sent to the ground.  Decimation is a mechanism for eliminating some events to keep the SSR at a reasonable fill level. Decimation is characterized by two quantities: an energy threshold below which counts will be removed, and the fraction of counts retained.   The front and rear segments of the detectors can be set to independent decimation levels.  For example, the normal mode for the rear segments is to decimate counts below ~200 keV and retain 1 out of every 4 counts.

Decimation usually occurs automatically in the front segments whenever the SSR is more than 50% full. The energy level and depth of decimation is a function of the exact SSR state and the attenuator state.  Decimation in the rear segments is commanded for particular fixed times rather than being dependent on spacecraft conditions.

More details are available in the Tohban Decimation Guide and the RHESSI Decimation/Event Download Status Record (covers 2002-2005).

The decimation state is part of the data stream*, and can be retrieved in the RHESSI object software via an info parameter called DECIM_TABLE. 

* Except the rear decimation state was not in the data stream until December 2002.  See Rear Decimation in 2002 for more details.