The image above show how a shielding against a electrical field works. Please have a look on the page LearnEMC which explains quite well the background to shielding.
In the image above I show how the shielding issue looks like. in the right corner, I connected the high voltage supple with the yellow and green cable. At the end of the cables I connected a screw drive with at voltage indicator. This voltage indicator is actually a glow lamp with a resistor. I placed the tip and the ground of scope close close to cables. In the right bottom, is the image of the scope shown. The image shows voltage peaks with approx. 12V. In the left corner I covers almost everything with aluminum foil. I left only a small hole to see the glow lamp. I was also important to cover the low voltage cables to the high voltage supply to get the best result. After all the effort in the left bottom corner is the result shown. The voltage peaks decreased to approx. 2V.
Maybe the shielding issues are the reason that photo multiplier are so popular.