spaced sodium iodide scintillometer detectors 16 is arrayed on tool sonde 17 suspended within tubing 10 by conventional logging cable 18 scintillometer detectors 16 are located both above and below perforations 14 the spacing between vertically adjacent sensors 16 should be selected to provide adequate depth resolution over the thickness of formation to be measured enough detectors 16 are used to allow measurement across the entire selected depth interval the total formation to be measured during a hydraulic fracturing treatment may have a thickness ranging from less than twenty five meters to more than 250 meters although the sensor string is shown in fig1 as suspended within tubing string 10 it can also be attached to or incorporated in the tubing itself bowspring caliper 19 provides tool centralization and several such units can be spaced on the tool sonde 17 as needed the fracturing fluid or gel injected into the well contains initially non radioactive tracer nuclei 20 a neutron emitting chemical source or electromagnetic generating neutron minitron 22 is supported by cable 18 amidst scintillometer detectors 16 and adjacent to perforations 14 when neutron emitting source 22 is active nuclei 20 in the fracturing fluid are bombarded by neutrons and activated converting them into radioactive tracer isotopes 23 activated isotopes 23 then pass through perforations 14 and into the fracture that the fluid is forcing open in reservoir zone 12 another logging tool available from schlumberger called the gamma ray spectrometry tool gst contains a minitron of a sort.