Photons detection using a cloud chamber I know that a cloud chamber can be used to detect charged particles. In fact, when they pass through it, they ionize the fluid in the chamber and so their trajectory is visible. But why can't I detect photons in the same way? I mean through the ionization of the medium.
 A: The problem with photons is that they are not charged and so do not produce enough ionisation to be detected in a cloud chamber directly.
So you never see the track of a photon as a line in a cloud chamber.
If the photon energy is above about $1 \, \rm MeV$ then pair production is possible as is shown in the photograph below.
There is no evidence of the incoming gamma photons but once the charged electron/positron pair are produced the effect of their ionisation of the air can be seen.
This photograph was taken in 1933 by Iréne and Frédéric Joliot-Curie.  

Such an event is very rare and most of the pair production photographs were produced when high energy gamma photons passed through a thin sheet of lead as in the photograph below.  
 
There is a very good one taken in 1937 with a Getty Images copyright here.
The other way in which gamma and X-ray photons were detected in a cloud chamber was by the photons knocking out electrons from atoms either in the chamber itself or in the walls of the chamber and these secondary electrons produced tracks.

