...

1. Ensure the fans are plugged in and working.
2. Turn on the master power button above the middle NIM rack (blue circle in Fig. 13).
3. Turn on the computer.
4. Turn on the NaI(Tl) detector electronics (right-hand NIM rack next to iSEG crate, the right red circle in Fig. 13), only after ensuring that the fan under it is working.
5. Turn on the fast signal processing electronics (left-hand NIM rack, the left red circle in Fig. 13).
6. Turn on the iSEG voltage crate power found behind the unit near the power cord.
7. Launch the iSEG control software at the NGR computer (Fig. 14).
   
   
   
   Figure 14. iSEG Hard ware Setup and Main screen. The controller cards may be the EHS or EHQ model and are labeled accordingly.
   
   
8. Make sure the voltages (Vset) for ESQ00/EHS00 and EHQ01/EHS01 are set (Fig. 15). Modify the Vset fields as follows:
   
   • Channel 0 = 0 (unused and available for spare channel)
   • Channels 1 and 7 = 1100 V (plastic scintillators in the doors)
   • All other channels = 1300 V (shell-shaped plastic scintillators)
   • If one of the channels on the EHS/EHQ modules has failed, Channel 0 may be in use—be sure you understand which scintillators are connected to which channels, because the door and hoop PMTs require different operating voltages! 
   
   
   Figure 15. EHS/EHQ 00 and 01 iSEG Multi-Channel HIgh-Voltage Modules Screens; VRamp and IRamp fields are circled in red.
   
   
9. Make sure voltage ramp (VRamp) is 5% or lower and the current ramp (IRamp) is 50% or lower. The iSEG software does not remember these values between sessions!
10. Click on the Module access menu and click Instructions for all channels > On (ctrl+o) to start ramping up the voltage.
11. Wait until ramp-up completes (1–2 minutes if proper values are used).
12. Exit the iSEG program.
13. Answer No to the prompt when asked to ramp voltages back down. If you answered Yes inadvertently, start again at Step 8.

...

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.

   
   

Calibration Procedures

Even uncalibrated, the NGRL will still produce and record signals, but significant error will arise.

The multichannel analyzer (MCA) collects the analog signal from the PMT and divides it into channels, but without energy calibration, it is impossible to characterize the energy into scientific units (i.e., MeV). Radioactive materials of known energy are placed within the NGRL at specific locations and the ORTEC Maestro program is used to ensure that the signals from the standards lie in their proper channels.

In addition, it is necessary to calibrate the instrument in the time domain. If this is not done, the active shielding will not function properly without proper timing of the anti-coincidence logic. This will decrease the effectiveness of the active shielding.

Make sure to update the NGR’s NGR_configuration/Folders_and_Files dialog window with the correct location of the most recent calibration files.  Update the configuration files after you have completed the calibration procedure.

Equipment Needed

• Calibration core made of aluminium (Fig. 17)
• Plastic (PFTE) holder to hold calibration sources.
• ⁶⁰Co radioactive source (nominal activity 1 µCi; half-life 5.27 years)
• ¹³⁷Cs radioactive source (nominal activity 1 µCi; half-life 30.2 years)
• 77 IV Multimeter (cabinet NGR 1)
• NGRL Bias Voltage Calibration Worksheet (NGRL Bias Voltage Calibration Worksheet.pdf in the “NGR Manual/Log” binder. Attached at the end of this Chapter.)
• New folder created in Windows C/data/ngr/config/calibration/current expedition
  
  

Warning! The radioactive sources (kept in a black lockbox located in cabinet PPTRKF 13) generate a relatively small amount of radiation, but the user should take care to minimize interaction with them. The sources should be returned to the radioactive standards lockbox as soon as the procedure is finished.

Note: The ⁶⁰Co source has a much shorter half-life than the ¹³⁷Cs source as stated above. Getting low ⁶⁰Co peaks probably does not mean a problem with the instrument: check the date of the standard disk first and determine if the remaining activity seems reasonable compared to past experiments. When new, the ⁶⁰Co double peak is roughly the same size as the ¹³⁷Cs single peak; as the sources age, the ⁶⁰Co double peak will shrink relative to the ¹³⁷Cs peak.

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Figure 17. Aluminum Calibration Core.

Energy Calibration Procedure

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.
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   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.
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   Figure 19. Maestro Aquire menu showing the “MCB Properties” selection.
   
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   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.
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   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.)
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   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).
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    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.

Warning! This procedure may be necessary at the start of any expedition as a response to drift, but should not be undertaken without clearly understanding the process.

The total number of channels in the high voltage divider of the MCA is 1024. The ¹³⁷Cs decay has a peak of 662 keV and the two ⁶⁰Co peaks are 1173.2 and 1332.5 keV, respectively. Normally the 662 keV ¹³⁷Cs peak appears close to channel #226, however this will drift over time with a different drift rate for each detector. By changing the voltage in the bias adjustment box (Fig. 24), the operator can control the position of a given peak and bring it to (or close to) the appropriate channel. If the ¹³⁷Cs peak is not within a few channels of #226, it may be necessary to use the adjustment box to adjust the peak. Using the Maestro program, the operator can see the position of the peak from the sources and make adjustments. The potentiometers in the bias adjustment box are very sensitive and nonlinear, so only a lite touch to avoid moving the peak too far.

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Figure 24. NaI(Tl) bias adjustment box

The gross voltage of the PMT should be in the range of 650 to 750 volts; the leads for the bias adjustment box have been stepped down to the equivalent in millivolts, so a voltmeter can be used without the presence of dangerous voltages. Thus, the voltmeter should display a value somewhere between 650 and 750 mV. Each detector has a positive (red) lead; they share a common ground (white) lead. The row of silver screws above the numbers are the potentiometers. 

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Figure 25. Maestro window showing the ¹³⁷Cs and two ⁶⁰Co peaks.

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Figure 26. Zoomed-in Maestro window of the ¹³⁷Cs peak, showing that the peak is at channel 226.

Step-by-step procedure for setting the NaI(Tl) detector bias voltage:

1. Place the aluminum calibration core (source holder) on the titanium boat, making sure that hole #8 is on the NGR chamber end and hole #1 is closer to the catwalk hatch. Place both the ¹³⁷Cs and ⁶⁰Co calibration sources in the white PFTE holder and insert it into the hole between #1 and #2 (i.e., hole 2-1) as for the energy calibration procedure above.
2. Ensure no obstructions are on the track or inside the chamber.
3. From the main NGRL screen, select the Track Utility dialog box and click “Calibration position” to send the core into the chamber.
4. Open the ORTEC Maestro program. From the top menu choose Display/Detector/00001 PC to open the signal spectrum for NaI(Tl) detector #1. Clear the previous spectrum, if any.
5. On the Acquire menu, select MCB Properties and on the properties window, ensure that the gate is set to “anticoincidence” on the ADC tab and that the Live time on the Presets tab is approximately 60 seconds. Close the properties box.
6. On the Acquire menu, click on Start and allow the spectrum to be collected. It should look like one above (Fig. 25). Bring the Maestro cursor to the middle of the ¹³⁷Cs peak and check the corresponding channel number on the bottom of the screen. If this number is 224 to 228, a bias voltage correction is not necessary, but could still be performed if desired. (Expected behavior is for the channel number for ¹³⁷Cs to be 226±2 channels (Fig. 26).)
7. If the drift is sufficiently large to require an adjustment, or if the spectrum appears compressed or stretched compared to the other detectors, perform a bias voltage tuning.
8. Use a multimeter. Set it to DC current in the millivolt range. Read the voltage in the bias adjustment box. The multimeter’s black probe goes into the white fitting and the red probe into the appropriate red fitting for the detector being examined (see Fig. 27).
9. Note the current voltage setting and the position of the pulser channel (if the pulser is used), the ¹³⁷Cs 662 keV peak position, and the ⁶⁰Co 1170 and 1330 keV peak positions in the table below (Table 1).
10. Using the potentiometer screw just above (aft of) the red fitting, gently turn the screw to increase voltage (clockwise rotation) if the ¹³⁷Cs peak is less than channel 226, or to decrease voltage (counterclockwise rotation) if the ¹³⁷Cs peak is greater than channel 226. Note that you must rerun the 60-second acquisition (step 6) each time to see the new channel.

11. Once you have set the ¹³⁷Cs peak close enough to channel 226, record the new voltage setting and the new positions of the pulser channel, the ¹³⁷Cs 662 keV peak, and the ⁶⁰Co 1170 and 1330 keV peaks on the table.

12. You must now set the software calibration as noted in the “Energy Calibration Procedure” section above. Once you have done this for detectors #1 and #2, repeat for the other detector pairs.

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Figure 27. Insert the multimeter probes into the bias detector box to measure the voltage. Black to white. Red to red

Table 1. NGRL NaI(Tl) Detector Bias Voltage Table

ORTEC 480 Pulser

It should be noted the ORTEC 480 pulser as well as the pocket pulsers can be used to generate a signal for the NaI(Tl) detectors. At a setting of 30 mV for 50 ¿ input impedance, the signal from the ORTEC 480 will fall onto approximately channel 236. It will be necessary to set the voltage of the pulser with an oscilloscope, and detailed procedures can be found in the NGRL electronics manual.

The pulsers are not necessary unless the user wishes to see a sharply-defined channel marker to facilitate adjustments. The user can complete the energy calibration without using either type of pulser.

Energy Calibration Evaluation

After all eight detectors are calibrated; Maestro shows energy-corrected results. Core spectra from these detectors display both channel and energy information. The ASCII files with the calibration coefficients are saved and available for the data reduction software. The operator should evaluate the position of the K peak (1.460 MeV) in core samples to ensure the calibration has been recorded with reasonable results. (Known U and Th peaks can be used for this purpose as well.)

When the ¹³⁷Cs peak is calibrated to channel 226±2, the ⁴⁰K peak should fall roughly at channel 498±4, in the same direction of error as the ⁴⁰K peak because energy vs. channel is quite linear.

(If the ¹³⁷Cs peak is found at channel 224, the ⁴⁰K peak is likely to lie at or about channel 494; if the ¹³⁷Cs peak is found at channel 228, the ⁴⁰K peak is likely to lie at or about channel 502.)

The system is now calibrated sufficiently to perform analysis on a total counts basis. Further calibration with known values of K, U, and Th (KUT) must be performed before KUT abundance can be determined. The scientist must do this reduction for KUT from the spectral data and no automated process exists for this.

Exact Source Placement

The above procedure presupposes the calibration sources are positioned exactly in the center of the lead separator between each NaI(Tl) detector and on the top of the aluminum standard holder. Any significant error in this positioning (especially if the source is too close vertically to the detector) will introduce systematic errors in the calibration, as the lead shielding will interact with the gamma rays differentially between the two detectors. Systematic errors can be controlled by making measurements placed from both the right and left of detectors #2 through #7. (It is physically impossible to make this determination for detectors 1 and 8 but we can use the systematic error determined from the other six detectors to estimate the error for these detectors.) Calibrations done with manual positioning demonstrate that peak position can shift up to 5–6 channels (~15–18 keV) with a typical value of 2–3 channels (~6–10 keV). It is therefore important that the sources be placed precisely (the normal procedure does this).

If you are performing time calibration, different holders and positioning are used; refer to that section for details.

Calibration of NGRL Spectra for Analysis of ⁴⁰K, ²³⁸U, and ²³²Th (“KUT”) Concentrations

Note: that the NGRL software does not produce “KUT” data. The spectral data (found in the ZIP file produced by the NGRL software) is there, but significant post-acquisition work (beyond the scope of the IODP marine technician’s duties) is needed to derive it.

The raw spectrum recorded in the file contains the spectral information to identify the ⁴⁰K line and the several ²³²Th and ²³⁸U lines. In order to use this information to produce calibrated % K and ppm Th and U, it is necessary to measure the intensity of the related peaks from standards of known activity. (It is also necessary to have a good enough quality spectrum, which for geologic cores with low activity (<10-15 cps) may require longer read times as well as additional calibration.)

Aboard the JOIDES Resolution, a number epoxy core sections containing K and Th salts are available with different levels of activity.

Note! Even if the epoxy core label indicates that it contains U, it does not; the manufacturer inadvertently omitted the radioisotope!

Two gypsum plaster core sections are available with known concentrations of U salt as well. As stated elsewhere, the epoxy and plaster cores’ activities are so low that they can be considered effectively non-radioactive for handling purposes.

In order to perform a KUT study, each detector must be calibrated against the standards, which in turn requires that the core section be centered over each detector. Do not try to use one epoxy or plaster core to cover more than one detector, although it is possible to position multiple standards over different detectors simultaneously.

Collect spectra for the standards, from each detector, for at least 30 minutes (preferably 2-4 hours) to acquire a high-quality spectrum. For good quantitation, it is necessary to have individual radioisotope peaks on the order of 2000 counts (each).

Note that in order to position the standard core sections over detector #8, the rear door must be opened with the chain hoist and the plug at the end of the acrylic tube removed.

⁴⁰K decay produces only a single peak within the NGRL’s sensitivity range, and presuming ²³⁸U concentration is not high, evaluation of the ⁴⁰K peak is relatively straightforward. High ²³⁸U concentration creates a “shoulder” peak on the ⁴⁰K peak and must be deconvoluted from the potassium signal to produce the correct ⁴⁰K concentration.

²³²Th and ²³⁸U decay produces a large number of gamma-ray peaks, making quantification much more challenging. For reference, a decay isotope diagram for the Th and U chains are provided here (Fig. 28 and 29).

It is beyond the scope of this manual to describe the full analytical procedure. Dunlea et al. (2013) provides some guidance on this matter.