Methods and devices are disclosed for determining if hydrocarbons or water is present in a subterranean reservoir are described. An anchor base having an aperture with at two continuously opening surfaces is used with a post-tension tendon. The locating pins are moveable from the first forward position and the second retracted position by a user via a mechanism accessible outside of the housing and without opening the housing. Since the amount of charge stored in the charge collecting electrode is clamped to the amount of charge corresponding to the clamp signal voltage by the MIS diode, the read gate is not destroyed even with excessive X-ray irradiation.