Use of ceramic capacitors in HV system
Very often. the high DC voltage from the power supply is boosted from a much lower DC voltage (perhaps a battery). A DC-to-AC-to-DC converter circuit is used.
Some pulses inside this converter may appear superimposed on its high-voltage DC output. Since a Geiger tube is sensitive to variations of its high-voltage input, C1 helps to smooth any variations caused by the AC portion of the DC-to-AC-to-DC supply. C2 helps as well.