CPP magnetic read head designs have been improved by increasing the length of the AFM layer relative to that of both the free and spacer layers. The metal gate electrode is formed by forming a metal gate electrode pattern which is comprised of a polysilicon layer and a metal layer, and performing a selective oxidation process. At least one of the detector elements is connected to a time to digital converter to allow counting of the ions and at least one of the detector elements is connected in parallel to both a time to digital converter and an analogue to digital converter. The structure includes a first base unit having a first wall mounting portion and a first low profile cover, the cover being mountable over at least a portion of the duct. The contacts and electrical connections of the structural element are electrically insulated from the metal of the shielding structure.