Crosstalk issues with Photomultiplier Circuit
Assuming your crosstalk is Efield, you have MANY methods to reduce crosstalk.

Any bit of grounded metal, sticking out above the PCB, will capture some flux lines and reduce crosstalk.

Any nearby component (especially if Grounded, like BypassCaps) reduce crosstalk.

If the transmitter of energy has direct line of sight  to the receiver of energy, then block that line of sight.

Any small grid of wires, or large grid of wires, GROUNDED, is a fine gatherer of flux lines and will improve your quietness.