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The NGR Logger consists of eight Sodium Iodide (Thallium) (NaI(Tl)) detectors surrounded by both passive and active shielding. The measurement of natural radioactivity from core samples faces the challenge of overcoming background noise, which consists of environmental radioactivity and cosmic radiation. In order to protect measurements from environmental noise the NGR system includes several layers of lead, which act as a passive shield. However, lead shielding is not enough to eliminate enough of the incoming cosmic radiation to measure low-count cores. To reduce the cosmic background further the NGR has a layer of active shielding consisting of plastic scintillator detectors and nuclear electronics. There are five plastic scintillators on the top of chamber and an additional plastic scintillator inside each NGR door. For rejection of counts in NaI(Tl) detectors associated with cosmic rays, fast–slow coincidence logic was implemented. In the event of coincidence within a 400-500 ns window between signals from the fast outputs of NaI(Tl) detectors and any of the seven plastic scintillators, a VETO signal is generated on the gate input of the multichannel analyzer modules (MCAs) and further readout of such an event is rejected.
A core section measurement consists of two positions, counted for at least 5 min each for a total of 16 measurements per section. A typical ~150 cm whole-round core section is wiped dry and placed in a boat on the loading end of the instrument, where a barcode scanner records the sample number and imports sample information from the encoded label. The length of the sample is record via the text-id. The boat stops at position #1, where the top of the boat is position positioned past (usually 10cm10cm is recommended, but no less than 8cm is acceptable) the center of Sensor #8 (starboard most detector). After measuring at position #1 for a user-defined time period (not less than 5 min), the boat moves 10 cm outward (port) and begins counting at position #2. When the run completes, the section returns to the starting position and can be unloaded.
NGR analysis results are expressed as spectra (counts vs keV energy) for each measurement and the raw spectra are saved in a zip folder in the database. The spectra are reduced by the NGRL software and produce total counts per second (cps), adjusted for energy threshold (>100 keV), edge corrections, and background radiation.
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