The Hamamatsu UVtron was installed on the top deck facing to the left. This location was chosen for two reasons. At one time, I was attempting to determine whether a candle was in a room just by slowing down while passing the doorway (staying in the center of the hall). The route I had planned would generally result in passing doorways on the left side. I later decided this scheme was not very reliable as a candle hidden well around the corner from a doorway did not trip the UVtron very often, particularly if furniture was in the way. On the other hand, other people seem to have pretty good luck with this method.
The other reason was for for my final plan which was to enter each room fully, then pivot to the right 180 degrees to aim out of the room for exit if no candle was sensed. The UVtron is most sensitive in about a 90 degree arc around the front of the sensing tube, hence pivoting allowed the tube to point in all directions. For most rooms, having the UVtron on the left resulted in a good scan of the whole room.
I build a semi-enclosure for the board and the tube out of plastic. Prior to doing this, I often bent the tube leads while working on the robot. The plastic prevents most damage while not keeping any UV light from reaching the tube.