I. 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.

Concentrations of uranium, thorium and potassium in geological materials provide insight into many important lithological characteristics and geologic processes. In marine sediment, they can aid in identifying clay compositions, depositional environments, and diagenetic processes. In hard rock, they can yield information about the alteration and heat production of rocks (Dunlea et al., 2013). A high-efficiency, low-background system for the measurement of natural gamma radioactivity in marine sediment and rock cores designed and built by the JRSO at Texas A&M University is used aboard the JOIDES Resolution.

Electromagnetic gamma rays are emitted spontaneously from an atomic nucleus during radioactive decay. Each nuclear isotope emits gamma rays of one or more specific energies. NGR data are reported in total counts per second, a quantity dependent on instrument and core volume, derived from the integration of all counts over the photon energy range between 0 and ~3.0 MeV. Total counts represents the combined contributions by K, U, and Th, matrix density resulting from Compton scattering, and matrix lithology resulting from photoelectric absorption. Data generated from this instrument are used to augment geologic interpretations.

Theory of Operation

The NGR Logger consists of eight Sodium Iodide (Thallium) (NaI) detectors surrounded by both passive and active shielding (Appendix B.3). 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 titanium boat on the loading end of the instrument, where a barcode scanner reads the section label and imports sample information from the database. The length of the section is input by the user.  The boat stops at position #1, where the top of the boat is positioned 10cm past the edge of NaI detector #8 (starboard-most detector). After measuring at position #1 for a user-defined time period (not less than 5 minutes), the boat moves 10 cm outboard (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 offset 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.

Energies below 100 keV (and into the X-ray portion of the spectrum) are not recorded, as the NGRL has not been designed to characterize the natural radioactivity below this level.

For more details, please see Vasilyev et al. (2011)

II. Procedures

A. Preparing the Instrument

1. Double-click the MUT icon on the desktop (Figure 1a) and login using ship credentials. For more information on data uploading see the "Uploading Data to LIMS" section below.

2. Double-click the IMS icon (Figure 1b). IMS initializes the instrument. Once initialized, the logger is ready to measure the first section.

At launch, IMS begins an initialization process:

• Testing instrument communications

• Reloading configuration values

• Homing the pusher arm of the motion control system.

After successful initialization, the main NGRL window will appear (Figure 2).

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B. Instrument Calibration 

a) Background Measurement

Measure the background periodically, whenever the ship changes latitude by more than 1-2 degrees and at least twice an expedition. A data file is generated for each NaI detector and measurement position. The titanium boat with an empty core liner are no longer used in the background measurements as of Exp. 368X.  The background measurement is taken for a much longer period of time for a good statistical spectrum. Typical core measurement time is 300 seconds per position; the background is normally done for 21,000 seconds (almost 6 hours for a single measurement position).  On the main (left-hand) IMS panel, go to Instruments > NGR: Background (Figure 3) to start a measurement.

During data reduction for core analysis, the background spectrum is normalized to its equivalent at 300 seconds’ duration. For example, if the background at channel 200 of the spectrum were 6,400 counts in the 21,000-second background experiment, that specific channel’s background would be normalized as follows:

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

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b) Energy 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 IMS 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. This was performed during the initial instrument validation.

1. Collimated Sources (CURRENTLY NOT BEEN USED)

This calibration is done using IMS, placing the collimated sources on the top of each detector.

Equipment Needed

• Fan-collimated Lead holder for containing the Co and Cs 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 section.)
  
  

Warning: The radioactive sources (kept in a black lockbox; ask a technician if you need them) 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.

Energy Calibration Procedure

1. Place the collimated Lead holder into the boat, ensuring that it clips over the end closest to the logger.

2. On the main IMS (left-hand) panel, go to Instruments > NGR: Energy Calib, the Energy Calibration Setup window will be open (Figure 4), check the values and click START to allow the calibration to commence.  The calibration process takes approximately 40 minutes to complete.

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• In Tray Offset: Distance from the top of the boat to the opening in the collimator. This used to be 5.9 cm but should be checked if a new collimator is built. Using this offset will place the sources on top of the detectors.

• Live Time: Sample time for each detector. Should not be less that 120 sec.

• Hold and Repeat: If you select ON, the measurement will continue on the same detector until it is switched off.

• Peak Search Width: Definition??? A usual value is 10.

• Peak Search Threshold: Definition??? A usual value is 20%.

• Fit Width: Definition??? A usual value is 10.

    3. If during the calibration the positions of ¹³⁷Cs or ⁶⁰Co peaks are not between the reference channels (table 1), the bias of that detector should be changed following the procedure explained below. When the calibration finishes, review the values for each detector and click Accept if everything is correct, if not, you can do it again by selecting Cancel.

Table 1. Calibration channel position for ¹³⁷Cs and ⁶⁰Co peaks.

2. Non-Collimated Sources

This calibration is done using IMS, placing the non-collimated sources between detectors. The calibration of each detector will be performed with the sources 10 cm away from the detector.

Equipment Needed

• NGR Metal core

• ⁶⁰Co radioactive source (nominal activity 1 µCi; half-life 5.27 years)

• ¹³⁷Cs radioactive source (nominal activity 1 µCi; half-life 30.2 years)

• Plastic container for holding the sources

• 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 section.)
  
  

Warning: The radioactive sources (kept in a black lockbox; ask a technician if you need them) 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.

Energy Calibration Procedure

1. Place the metal core into the boat, ensuring that hole 8 is closest to the logger (Figure 5).

2. Place the sources inside the plastic holder and insert it into the hole between holes 8 and 7. Note that the red marks on the plastic holder should be aligned with the red marks on the metal core.

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       3. On the main IMS (left-hand) panel, go to Instruments > NGR: Energy Calib, the Energy Calibration Setup window will be open (Figure 4), check the values and click START to allow the calibration to commence. The calibration process takes approximately 40 minutes to complete.

• NOTE: For placing the sources 10 cm away from the detectors, the Setup offset should be set to 5.0 cm (Figure 6).

• When using weaker sources for a calibration, the sources can be placed on top of the detectors. The source holder stays in the hole between holes 8 and 7, but the offset has to be set to 15.0 cm. Never place the plastic holder in hole 1 or 8, because the metal core end cap prevents it from going all the way in, and it will not have enough space to go into the NGRL (Figure 7).

       4. If during the calibration the positions of ¹³⁷Cs or ⁶⁰Co peaks are not between the reference channels (table 1), the bias of that detector should be changed following the procedure explained below. When the calibration finishes, review the values for each detector and click Accept if everything is correct, if not, you can do it again by selecting Cancel.

Tuning the NGRL Voltage Settings

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 ¹³⁷Cs and ⁶⁰Co peaks appear close to the appropriate channels (Table 1), however this will drift over time with a different drift rate for each detector. By changing the voltage in the bias adjustment box (Figure 8), the operator can control the position of a given peak and bring it to (or close to) the appropriate channel. If ¹³⁷Cs or ⁶⁰Co peaks are not within the channels range, it may be necessary to use the adjustment box to adjust the peak.

Using the IMS 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 use a very light touch to avoid moving the peak too far.

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Figure 8. NaI 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. 

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

1. Place the lead source holder (collimated sources) or the metal core (non-collimated sources) on the titanium boat, making sure that holder is slotted on the Ti boat correctly.

2. Ensure no obstructions are on the track or inside the chamber.

3. On the main IMS (left-hand) panel, go to Instruments > NGR: Energy Calib, the Energy Calibration Setup window will be open, check the values (These will vary depending on whether the sources are collimated or not) and click START to allow the calibration to commence.

4. Check if ¹³⁷Cs and ⁶⁰Co peaks are in the right channels (Table 2). If the peaks are between the range wait until the calibration time finish (usually 120 s), the program will automatically move to next detector.

5. If any of the peaks is not between the range, the bias voltage should be corrected:

   1. 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 (Figure 9).

   2. 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 3).

   3. Using the potentiometer screw just above (aft of) the red fitting, e.g. 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 acquisition each time to see the new channel.

   4. Once you have set the peaks, record the new voltage setting and the new positions of the pulser channel on the table.

   5. Wait until the calibration time finish (usually 120 s), the program will automatically move to next detector.

6. After complete the procedure for the eight detectors, follow the steps on Verifying Threshold Values section.

Table 3. NGRL NaI Detector Bias Voltage Table

Download a Copy

NGRL Bias Voltage Calibration Worksheet.pdf

Verifying Threshold Values

After completing an energy calibration in the IMS software, the user must verify the threshold values selected by IMS in the .ini file.

1. Close the IMS software.

2. Open the NGR Threshold Check spreadsheet.  IMS-NGR threshold Verification.xlsx.

   1. Each channel covers a range of keV, approximately 3 keV.  The value shown in the spreadsheet is the initial voltage.

3. Open the .ini file for the NGR.

4. Compare the "hand picked" channel number to the IMS selected channel number in the .ini file.  The channel number will be listed with a text string similar to:  value 0.NGR927.CALB.>100 Kev.   

5. Update the >100 keV threshold values to the hand picked threshold values.

6. Save the .ini file

7. Reopen IMS

8. Reopen the .ini file and verify that the threshold values are set properly. If IMS was open when the .ini file was edited, the changes made manually may have been overwritten.  It is important to verify that the manual edits stayed in the .ini file after IMS is launched.

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 detectors. At a setting of 30 mV for 50 Ohm 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; IMS 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. 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.

C. Set Measurement Parameters

Configuration values should be set during initial setup and configuration by the Physical Properties technician or scientist(s). There should be no need to change these values unless the configuration file is corrupted.

NGRL Instrument General Setup

To open the NGRL instrument setup window (Figure 10), select Instruments > NGR: Setup from the IMS panel menu (Figure 3).

1. Ensure the values in the window are set as shown in the detector values given are constants and measured from the front door to each detector.  Once the offset to the detectors are determined, these values should not change. Note: During Expedition 401, a detector profiler was run and new positions for the detectors were determined from 42.12 cm (Detector #1) to 182.12 cm (Detector #8). The NGR Parameters of Figure 10 below were updated.

2. Click Accept to save the changes and write them to the configuration file. Click Cancel to revert to previous values.

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The Utility on the main NGRL Motion menu is used to control of the boat position. There are three basic positions of the boat inside NGR chamber:

• Position I: the edge of the boat (and top of section) is positioned past the edge of detector #8 (starboard detector, furthest from the door)

  • Note that this analyzes "odd" intervals, i.e., 10-20, 30-40, 50-60, and so on.

• Position II: the edge of the boat moves 10 cm outboard (port) so that the edge of the boat is directly over detector #8

  • Note that this analyzes "even" intervals, i.e., 0-10, 20-30, 40-50, and so on.

• Single Detector Scan:  For collimator experiments, done only rarely to test each detector’s spatial characteristics.  It is important to open the rear door with the chain hoist and to remove the rubber stopper before attempting to calibrate detector #8.

The Utility display also provides a Home position (loading position) as well as manual fine controls.

D. Preparing Sections

It is very important that no water or mud enters the NGRL, as the NaI detectors are hygroscopic and there is high voltage (~1 kV) applied to the scintillators and photomultipliers. Any mud or dust inside the system will stay there and become an additional source of radioactive background, which will negatively affect all following measurements. All core sections and boat surfaces must be cleaned and wiped dry. Additional preparations include: