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Contents

Introduction

This guide describes standard operating procedures for the Natural Gamma Radiation Logger (NGRL), designed and built at the Texas A&M University IODP-JRSO facility in 2006-2008. The NGRL measures gamma ray emissions emitted from whole-round core sections, which arise primarily due to the decay of U, Th, and K isotopes. Minerals that fix K, U, and Th, such as clay minerals, are the principal source of natural gamma radiation.

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The NGR electronics crates (Fig. 4) include:

• 2 NIM bins populated with 21 NIM standard electronic modules
• ISEG high-voltage supply crate for the plastic detectors’ PMTs
• PC computer
• Power supply
• Amplifier for core delivery system motor

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Figure 8. Power switch placement 

Partial Shutdown Procedure

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Figure 13. Master Power Switch.

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Startup Procedure

Before proceeding, the air conditioning system should be functioning and atmospheric conditions stable. If stable, ensure the equipment surfaces are dry. Wait at least 4 hours after the air conditioning turns on.

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                                    6,400 counts x 300 seconds / 21000 seconds = 91.4 counts

Background procedure

1. Use the empty 150 cm whole-round core liner with the background label; this is normally stored on the top of the core rack next to the NGRL. Load it onto the Ti boat.
2. In the NGRL configuration/System Setup dialog window (Fig. 16) change the settings to: no data reduction, live time, 21,000 seconds acquisition time.
3. In the NGRL configuration/File and Folders window verify the data folder (so you can find the background files later).
4. Click on “Core Analyzer”> “Summary Display” tab, Use the barcode scanner to scan the background label on the core liner.
5. Run the experiment (same run button as for sample analysis); this will take ca. 12 hours.
6. After the run is complete, copy the 16 background files from the c:\data\ngr\ archive to c:\data\ngr\.config\background\[EXP#]\data folder, where EXP# is the current expedition (you will have to create this folder).
7. In the NGRL configuration/File and Folders window indicate (select current folder) this Directory as the background; the files should appear in the window.
8. Important! Return the acquire time to 300 seconds and turn “reduce data” back on.
   

   Figure 16. Typical background file acquisition parameters in NGR configuration dialog window; note background acquire time should be 21,000 seconds, not 20,000.

   
   

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1. Place the calibration core on the core boat so the round holes face upward. The highest-numbered end (#8) should be closest to the NGR chamber opening (starboard) and the #1 end should be closest to the catwalk hatch.
2. Insert calibration source holder containing both the ⁶⁰Co and ¹³⁷Cs sources into the hole marked 2-1 (Fig. 18). Match up the red marks on the calibration source holder with the marks on the calibration core so that the plastic holder lies flush into its position and will not strike the edge of the NGR chamber opening.
   
   

   Figure 18. Calibration Source Holder.

   
   
   
3. Make sure the track pathway is free from obstacles.  Prepare Bias Voltage Calibration Worksheet (see NGR Log black binder – a blank sheet is attached at the end of this Chapter) to record initial readings.
4. Advance the core boat into the calibration position by using NGR Core Analyzer software, “Track Utility” tab, “Calibration Position”, “Move In button.”
5. Start Maestro. From the Maestro tool bar (Fig. 19) choose the detectors of interest (starting with #1 and #2) and close any other detector windows; your Maestro window should show two channels (Fig. 20). Record initial readings on Worksheet.
   
   
   
   Figure 19. Maestro Aquire menu showing the “MCB Properties” selection.
   
   
   Figure 20. Maestro window showing NaI #1 and NaI #2 detector responses
   
   
6. Clear any results for both windows (right-click in the dark blue area and select “Clear” from the drop-down mouse menu.
7. From the Maestro tool bar open the “Acquire” drop-down menu (Fig. 19) and open “MCB Properties;” this should be the last item on the menu as shown.
   

   Figure 21. Maestro MCB Properties dialog window.

8. For each detector window, In the MCB Properties dialog window (Fig. 21), make sure the Gate setting in the “ADC” tab is set to Anticoincidence. In the “Presets” tab, enter the Live time; exact Live time is not important so long as the Cs and Co peaks are sharp. For a new 1 µCi standard, 60 seconds is sufficient; as the standard ages (esp. the ⁶⁰Co source with its short half-life), it will be necessary to use a longer Live time.
9. For each detector window, right-click in the dark blue area and select “Start” from the mouse window. The progress for the spectra can be observed in the “Pulse Ht Analysis” box on the right side of the Maestro window. Clicking the left mouse button on the spectrum will activate the detector window of interest (Fig. 22.)
   

   Figure 22. Detectors #7 and #8 after acquiring signal from the ¹³⁷Cs and ⁶⁰Co sources; the lower window, detector #8.

10. Click with the left mouse button in the middle of the left (first) peak; this is the ¹³⁷Cs line. Use the zoom functions if it will help see the peak. Go to the tool bar menu, choose the “Calculate,” then “Calibration” commands. A small dialog window will show up (Fig. 23) by peak. Fill the “Calibration (Energy)” field with 662.0, then click OK. Confirm that the calibration units are in keV (not MeV) in the subsequent pop-up window. Peak in channel should be at 226 (+/- 2).
    

    Figure 23. Calibrate dialog window to set a peak’s energy

11. Repeat this operation for the right (third) peak; this is the higher-energy ⁶⁰Co line (448 +/- 2). When the Calibrate dialog window appears, enter 1330.0 in the “Calibration (Energy)” field.
12. Check the calibration by clicking on the top of the middle (second) peak; this is the lower-energy ⁶⁰Co line (394 +/- 2). Go to the Calculate/Calibration dialog window to see if the value in the “Calibration (Energy)” field is close to 1172. If the observed value for the second peak is within +/- 3 keV, you may click OK and proceed to the save step. If the value is outside of this range, click the “Destroy Calibration” button and return to step 11 until in-range values are obtained. It may be necessary to adjust voltage, see following section on Tuning the NGRL Voltage Settings.
13. Save the energy calibration file by clicking the in the detector window and choosing File: Save. Save the calibration file in C:\data\ngr\.config\calibration\[expedition]\date folder (you may have to create this folder, where [expedition] is the current expedition number). Ensure the detector number in the file name matches the actual detector number in the title bar above the spectrum.
14. Retract the core boat to the loading position (Click “Find Home” in the NGR Core Analyzer Software)
15. Remove the source holder from the current position and place it in the next position.
16. Repeat steps 2–8 until all four positions (2-1, 4-3, 6-5, and 8-7) and all eight detectors have been calibrated.
17. After all eight detectors are calibrated and each calibration file is properly saved, close the Maestro window. Make sure to update the NGR’s NGR_configuration/Folders_and_Files dialog window with the correct location of the most recent calibration files. When done, close the configuration window.
    

Tuning the NGRL Voltage Settings

If the normal energy calibration procedure does not bring the ¹³⁷Cs peak (662 keV) close to channel #226, then it may be necessary to tune the NGRL’s detector voltage and then repeat the calibration procedure. This is done detector-by-detector as noted below.

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