  In order to cope with increasing lifetime radiation damage expected at
collider experiments, silicon sensors are becoming increasingly thin. To
achieve adequate detection efficiency, the next generation of detectors may
have to operate with thresholds below 1000 electron-hole pairs. The readout
chips attached to these sensors should be calibrated to some known external
charge, but there is a lack of traditional sources in this charge regime. We
present a new method for absolute charge calibration based on Compton
scattering. In the past, this method has been used for calibration of
scintillators, but to our knowledge never for silicon detectors. Here it has
been studied using a 150 micron thick planar silicon sensor on an RD53A readout
integrated circuit.
