Manual Information

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.

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(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 determined and manually entered. The boat stops at position #1, where the top of the boat is centered above 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 further inward 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.

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.

Apparatus, Reagents, & Materials

Hardware

The NGRL system consists of five major units (Fig. 1):

• Support Frame
• Main NGRL detector unit (NGR chamber)
• Electronics crate
• Core delivery system
• PC and APC uninterruptible power supply (UPS) battery system

Figure 1. NGR logger system components.

Support Frame

The support frame holds the NGRL components, including the heavy lead layers of the passive shield. It is constructed of steel that is welded to the support rails distributing the 5 tons of weight evenly on the deck, preventing it from shifting in heavy seas.

A steel I-beam frame above the chamber allows for transportation of heavy components during any assembly/disassembly activities or for opening the doors by using the chain hoist.

Note: Both NGRL doors contain plastic detectors inside and two PMT units beneath each detector. These are very fragile and care must be taken not to damage them while moving the doors!

Currently on the ship the NGRL operates with door #1 fixed in the open position. Door #2 (back door) stays closed. Open door #2 to place the standard over detector #8, as the standard will need to be positioned past the normal stop position.

Main NGR Detector Unit

The main NGR detector unit consists of the following (Fig. 2, Fig. 3):

• Passive lead shielding
• 8 NaI(Tl) scintillator detectors
• 7 plastic scintillator detectors
• 22 photomultiplier tubes (PMT)

Passive Lead Shielding

The NaI(Tl) detectors are covered by at least 8 cm of lead shielding. In addition, lead separators (~7 cm of low-background lead) are positioned between the NaI(Tl) detectors. The innermost 4 cm of the lead shielding is low-background lead, while the outer 4 cm is composed of virgin lead. The inherent radioactivity of the virgin lead is such low energy that the inner 4 cm of low-background lead shields nearly 100% of it. The internal radioactive rates of the lead shields are:

• Low-background lead = ~3 Bq/kg
• Virgin lead = typically 50–200 Bq/kg

NaI(Tl) Scintillators

The NaI(Tl) detectors are housed in stainless steel and hermetically sealed against atmospheric moisture. The sodium iodide crystals are extremely hygroscopic if moisture gets inside the housing they can lose their optical properties. For this reason, it is vitally important that the detector housings be protected from corrosion. Each detector is a half-ring of 10 cm thick x 10 cm wide NaI(Tl); the shape is to maximize the efficiency of capturing gamma rays emitted from whole-round core sections. Each detector has its own photomultiplier tube (PMT).

Plastic Scintillators

In addition to passive lead shielding, the NGR employs plastic scintillators to suppress the high-energy gamma and muon components of cosmic radiation by producing a VETO signal when charged particles from cosmic radiation pass through the plastic scintillators:

• 5 shell-shaped plastic detectors cover the upper hemisphere around the NaI(Tl) detectors
• 2 flat plastic shields placed inside the doors to cover the detectors from the ends

Each plastic detector has two PMTs to maximize light collection across a somewhat large detector surface.

Photomultipliers

Signal processing from all PMTs is organized through standard NIM electronics modules. The photomultipliers are located beneath the detectors, 1 for each NaI(Tl) detector and 2 on each door and shell-shaped plastic detector. 

Figure 2. NGR detector system schematic.

Figure 3. Internal view of NGR logger showing NaI(TI) and plastic scintillator detectors and lead shielding

NGR Electronics Crate

It should be noted that a professional nuclear electronics engineer has tuned the NGR electronics. As has been observed through years of operation, the NGR electronics show steady performance and there is usually no need to work with any of the electronic settings, except for voltage tuning in the calibration procedure. In all other cases, call an appropriate person with sufficient training in the NGR electronics before attempting to adjust any of the electronics settings.

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

Figure 4. Electronics crate.

The coincidence logic NIM bin (left side) consists of the plastic signal flow units (Fig. 5, A through E), coincidence determination units (F and G), NaI(Tl) signal flow units (H through J), a summary coincidence unit (K), and an ORTEC 480 pulser. For a detailed description of the electronics bin, please see the NGR logger academy MS Power Point presentation.

Figure 5. Coincidence Logic NIM Bin

The spectrometric logic NIM bin (right side) consists of NaI(Tl) signal processing unit (Fig. 6), which is eight paired sets of ORTEC 855 amplifiers and ORTEC 927 APSEC multichannel analyzers (MCA).

Figure 6. NaI(Tl) spectrometric processing unit

The signal summary monitoring panel is a CAEN Quad Scaler and Preset Counter Timer (model N.1145) (Fig. 7).

• The signal reading in the top display originates from the plastic detectors (normally the sum of all detectors). The normal reading is approximately 400-700 counts.
• The signal reading in the second display originates from the NaI(Tl) detectors (also usually summed). A normal reading is usually in the range of 400-600 counts if no sample or standard is inside the NGR chamber. Samples and standards will significantly increase this value.
• The signal reading in the third display represents the number of coincidences between the plastic and NaI(Tl) detector arrays; these are usually in the range of 40-100 counts.

Note: that while the Galil motor is running, the counts may be very high due to radio frequency (RF) interference from the motor. During analysis, the motors are turned off to prevent this noise from affecting the measurement.

Figure 7. CAEN signal counter depicting summed plastic, NaI(Tl), and coincidence values

The electronics crate also contains the ISEG power supply for the plastic detectors, the PC, and various communications electronics (e.g., USB hubs and cables), not pictured.

Core Delivery System

The core delivery system consists of the Galil control panel and Galil servo motor assembly, the NSK actuator, Delrin rails, the titanium core boat, and electronic limit switches.

The Track Utility on the main NGRL Core Analyzer window 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 over the center of detector #8 (starboard detector, furthest from the door)
• Position II: the edge of the boat moves 10 cm deeper (starboard) so that the edge of the boat is past detector #8
• Calibration position: used for placing the disk-type radioactive sources for energy calibration between detectors, this position is exactly midway between positions I and II
• Note that if the time calibration is being done (rarely), the source must be placed in the standard holder directly over each NaI(Tl) detector, not between them.
• 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 Track Utility display also provides a Home position (loading position) as well as manual fine controls.

PC and UPS System

The PC is used solely for running the NGRL and reviewing data. It must never be connected to the internet or any devices which may interfere with the proper functioning of the instrument and its software. Users should also avoid using the PC for any other purpose while a measurement is running.

The APC UPS units provide a short window of normal operation (a few hours at most) if ship’s power is down. If ship’s power is not going to be restored quickly, the technician should shut down the NGRL following the shutdown procedure.

Software

The track control and interface to data acquisition software (Maestro 32 by ORTEC) is a LabVIEW application. The Main Control Panel provides access to the main data acquisition functions and utilities as well as:

• Current measurement parameters
• Program state and system status
• Sample information
• Real-time data display during collection

In addition to LabVIEW (and Maestro if one is performing calibration), the uploader software “MUT” is installed on the NGRL PC. MUT will upload the NGR raw and reduced data into the LIMS.

Inexperienced operators should only the LabVIEW application and MUT. Maestro has many features and controls and can alter the detectors’ settings and care should be taken when using it.

Laboratory Apparatus

The titanium core boat has a 3.5 cm diameter with welded ends, attached to a Delrin rod that connects it to the NSK actuator. The inherent radioactivity of the boat and the rod are very low and do not affect core measurement.

Standards

• Assorted 45 cm long epoxy cores with varying amounts of K and Th (Isotope Products Labs, Burbank, CA).
  • NOTE! Although some of the epoxy IPL cores state a U concentration, the U salts were not included in these standards!
• Two 60 cm long plaster cores with U (produced by Dr. Grigor Chubaryan, Texas A&M University Cyclotron Institute)
• Disk-shaped ¹³⁷Cs, ⁶⁰Co and ¹⁵²Eu radioactive sources (Eckert & Ziegler Isotope Products, Valencia, CA).

The activity of the epoxy and plaster cores is extremely low—significantly lower than background levels—and can be handled safely even for extended periods of time. The disk-shaped sources are 1 µCi nominal activity and should be handled only by properly trained operators. They do not represent a short-term hazard, but long-term exposure would be harmful.

NGRL Startup & Shutdown Procedures

Do not switch off the power supply when it is not necessary!

Utilizing standard NIM modules and electronics crates, the NGRL detectors/electronics are designed to run uninterrupted for years at a time. Switching the power supply off/on is one of the worst operations for the instrument! Do it only if it is necessary!

Shutdown Criteria

Partial (see Partial Shutdown) or complete (see Full Shutdown) shutdown may become necessary in the following conditions:

• If air conditioning (AC) is not functioning properly
  • If atmospheric condensation forms on the NGRL and other instrument surfaces, shut down the high-voltage supplies to all the detectors to avoid an electrical short.
  • Shut down other NGRL equipment (fans, cooling crates, iSEG crates, delivery system, and computer) only when very long AC interruptions with heavy condensation formation are expected or occur. Note that condensation typically occurs when the AC is restored; it can also happen if humid fresh air continues to be pumped in but the AC is not working.
  • After partial shutdown, do not power back up for at least 4 hours after AC is functional without additional condensation.
• If an extended ship’s power supply shutdown is scheduled
  • Full shutdown may be necessary.
  • During short power supply interruptions, the power supply UPS, through which all NGRL electronics connect to the power line should provide enough electricity until power returns; however, it is important to monitor the AC situation and if humidity becomes an issue, a shutdown is needed.
  • After full shutdown, wait at least 4 hours after normal AC function returns and condensation disappears.

Figure 8, below, shows the location of the main power switch and the two NIM 4001B crate power switches.

Figure 8. Power switch placement 

Partial Shutdown Procedure

• Make sure the core boat is at home position outside of the NGR.
• Start the iSEG High-Voltage Control software using the desktop icon (Fig. 9a and 9b) or select Start > All Programs > iSEGHVwithCan > iSEGCANHVControl to open the iSEG Main window. This program controls the high-voltage power supply for the plastic scintillator MPTs through the iSEG power supply.
  

  Figure 9a. iSEG Hard ware Setup and Main screen

  

  Figure 9b. The controller cards may be the EHS or EHQ model and are labeled accordingly.
  
  
  

• Exit the iSEG program. When the program asks if you wish to ramp the power down on all channels, select Yes (Fig. 10).

Figure 10. Program termination query.

• Click the iSEG High-Voltage Control Icon to open the Main window.
• Click on the EHQ00 and EHQ01 (or EHS00 and EHS01) modules. A window opens for each module (Fig. 11a and 11b). Check the following items:
  • Vmeas values: verify the actual channel voltages have ramped to near zero (1–3 V is acceptable)
  • Current indicators (i.e., Imeas values) have decreased.
  • Status of all channels is OFF (i.e., no green color in the menu status bars).
• Close all iSEG windows after checking the channel voltages and current indicators.

Figure 11a. iSEG Multi-Channel High-Voltage Modules; Ch00.

Figure 11b. iSEG Multi-Channel High-Voltage Modules; Ch01.

• Turn off the NaI(Tl) detector electronics (right-hand NIM rack next to iSEG crate; Fig. 12). This will turn off the high-voltage supply to all 8 NaI(Tl) detectors. Do not turn off the left-hand NIM rack (marked with a circled X in Figure 12 below) or the main power (blue circle) at this time.
• Leave all other electronics and fans running to prevent additional condensation from forming.

Figure 12. Electronics Crate: Shutdown.

Full Shutdown Procedure

Notes

• Take your time. Let the electronics settle. This is not a race.
• Voltages across any given NaI(Tl) detector may be monitored with a volt-meter at the bias box with 8 red touch points and a white single ground point.
• Do not change the voltage at any NaI(Tl) detector at the junction box!!! Remember if you need to switch off the High Voltage you must switch off the whole right-hand NIM crate (right-hand NIM crate button in Fig. 12), which will turn the high voltage off from all NaI(Tl) detectors simultaneously.
• Follow all steps in the Partial Shutdown procedure first.
• Turn off the power switch to the iSEG voltage control crate (behind iSEG unit).
• Turn off the fast signal processing electronics (Fig. 13; leftmost NIM rack).
• Shut down the computer.
• Turn off the master power switch above the middle NIM rack (blue circle in Fig. 13).
• The fans are connected to the uninterruptible power supply (UPS) and must be unplugged to turn off.

Figure 13. Master Power Switch.

 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.

Note: Allow the exterior surfaces to dry by evaporation so there is some assurance the internal surfaces are dry as well.

If the A/C is stable and the instrument surfaces are dry for 4 hours after partial or full shutdown, start the NGRL as follows:

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.

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(Tl) detector and measurement position, utilizing the titanium boat with an empty core liner to create conditions as close as possible to core measurement. The background measurement is taken for a much longer period of time for a good statistical spectrum. Typical measurement time is 300 seconds; the background is normally done for 21,000 seconds (almost 6 hours per measurement position; 12 hours total).

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

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.

   
   

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.

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.
   
   

   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.

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.

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. 

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

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.

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.

Figure 28. Thorium Decay Chain. By http://commons.wikimedia.org/wiki/User:BatesIsBack  - http://commons.wikimedia.org/wiki/File:Decay_Chain_of_Thorium.svg, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=16983885

Figure 29. Uranium Decay Chain. By User: Tosaka - File:Decay chain(4n+2, Uranium series).PNG, CC BY 3.0, https://commons.wikimedia.org/w/index.php?curid=33293646

Time Calibration

Time calibration refers to the process of ensuring that the fast signal from the NaI(Tl) detectors and the fast signal from the plastic detectors occur close enough in time for the coincidence logic modules to work properly. In general terms, a radioactive source (the disk sources) is placed over the detector and the acrylic tube is rotated off its normal position to move the lead spacers away from the plastic scintillators. Gamma and charged particle signals produced by the source are now at such a frequency that the logical coincidence signal is clear and the two detectors’ fast outputs can be aligned to allow the proper anticoincidence VETO signal to be produced.

This procedure is beyond the scope of this manual.

Sample Preparation and Analysis

This section gives instructions for running whole-core sections through the NGR system.

Preparing Samples

It is very important that no water or mud enters the NGR, as the NaI(Tl) 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:

• Equilibrate cores to room temperature
• Degas cores as much as possible to avoid “mud worms” in the bore of the NGR
• Use electrical tape to cover holes

Running Samples

1. Place the core section in the tray and ensure the top of the section is against the top of the boat.
   Note: the PMTs for the door plastic scintillators are visible (and exposed to potential damage) so significant care must be taken during core section movement to avoid the possibility of damaging the detector or PMTs.
2. Place the core section in the titanium boat on the NGR delivery system loading track. Ensure the blue end of the core section is against the leading edge of the boat (closest to the NGR door
3. Open NGR Core Analyzer (Fig. 30), which is usually found on the NGR’s workstation desktop. If the configuration files are corrupted or missing, load them from the Main control menu by choosing Configuration.
4. Press the Scan button (Fig. 30, #1) on the NGR Core Analyzer screen to initiate a run. The Sample Information dialog window will appear (Fig. 30, #2).
   Important! Ensure that the cursor bar is blinking in the Text ID box before scanning the section’s barcode. The NGRL can be set to run without a sample identifier if no Text ID is present and this will waste time.
5. Ensure the barcode reader has read the sample identification and the Text ID and Sample Name, fields have been completed on the Sample Information screen. Sample length is very important for proper edge correction and must be entered manually. (Text ID and Sample Name fields can also be entered manually if the barcode won’t scan.)
   Note: the “Run the Experiment” button will not activate unless a length >0.0 cm is entered in the Sample Length field. The user must click elsewhere, tab out of the field, or click press enter (user’s choice) in order for the software to register the entered value and enable the “Run the Experiment” button.
   

   Figure 30. NGRL Core Analyzer Screen, Ready to Start Scan.

   
   
   
6. After all necessary lines are filled click “Run the Experiment” in the Sample Information window and the boat will begin moving. The boat will proceed automatically into the chamber without further intervention, so be sure the track is clear of obstructions!
7. As soon as the sample reaches measurement position I, the measurement will begin. Eight channels will collect gamma ray counts from measurement points along the core section in Position #1 for a user-defined time period.
8. The core will move automatically forward 10 cm into Position #2 and again collect data for another eight positions (for a total of 16 measurement positions every 10 cm).
9. As the sensors take measurements, counts are plotted vs. sensor number on the mainscreen. Hit the Detail/Summary button at the upper right of the screen to switch between the histogram vs. binned mode displays. The button name changes from Summary to Detail, depending on which view is selected (Fig. 31 and 32).
   

   Figure 31. Core Analyzer Screen Showing Histogram.

   
   

   Figure 32. Core Analyzer Screen in Binned Mode Display.

10. Elapsed time (on the left side of window) during normal run (e.g., with 10 min acquisition time per position should go up to 21–22 min).
11. After data acquisition finishes, the boat will retract from the NGR chamber automatically. It is therefore important not to have anything blocking the actuator’s path!
12. The analysis is now complete.

Running the NGR during High Recovery Cruises

During high-recovery cruises, the normal ~11-12 min. NGRL run may be too long to keep up with core flow. The 300 seconds-per-position measurement time is the minimum time needed for good statistical measurements, so reducing it is not recommended. Instead, the software can be instructed to only measure at position #1 or position #2, cutting overall measurement time in half. This will create an interleaved measured/not-measured 10-cm pattern in the data, but the actual data points will be of the normal NGRL data quality.

Again, reducing the NGRL runtime below this single-position measurement is not recommended!

LIMS Integration

Data Handling

Once the run is accepted and the data saved, the results and associated calibration files are saved into the LIMS database. The uploader program (“MUT”) is used to put the files in LIMS; this normally runs in the background, set to automatically upload new files, but can be set to manual upload only. There are two main files to be uploaded:

• ngr_sample_{label id}_{text id}_{timestamp}_{detector/position indicator}.spe
  • This file lists all the raw spectra files generated by a sample measurement experiment at a single measurement position.
  • {label id} corresponds to a name for the sample in LIMS, e.g. 300-1054-A-123H-1 or 419-1200-A-35X-CC.
  • {text id} corresponds to a unique LIMS sample catalog identifier, e.g. SECT103597, SECT10325.
  • {timestamp} disambiguates repeated measurements against the same sample. This timestamp is generated from the NGR host PC’s clock, which should be set to UTC (Zulu time). It is in the format of 4-digit year, 24-hour clock, seconds precision.
  • {detector/position indicator} is in the format of d#p# where d represents detectors 1 to 8 and p represents either position 1 or 2.
• ngr_sample_{label_id}_{text_id}_{timestamp}.xls
  • This file is an Excel file containing all of the summary information and constants used in the calculation of background-corrected, energy threshold-limited data. All eight detector/position combinations are in the single file.
• ngr_sample_{label_id}_{text_id}_{timestamp}.zip
  • The zip file contains all of the calibration spectra, the edge correction table, the background spectra, and another copy of the individual ngr_sample files mentioned above.

The LIMS components for the NGR analysis are as follows:

Data Upload

• Once analysis is complete, open the uploader “MUT.” The icon is a dog face on the desktop but may also be found in START > ALL PROGRAMS > IODP.
• Log in to MUT (the PP technician can help with this).
• Usually, the uploader is set to automatically upload data (check the “automatic” button) but if troubleshooting, it may be desirable to leave this unchecked and trigger upload manually.
• Please remember to put the uploader back on automatic in normal operation so that NGR results are available to other scientists and technicians as soon as sections are completed!

Data Management

Once all sections for the Expedition have been sent through the track, all data needs to be placed in the appropriate folders on data1 (S:\data1).

1. Copy all files from archive and place them in the 4.1 Petrophysics NGR natural gamma folder. Confirm relocation. Delete all files off the local drive.

Health, Safety, and Environment

Safety

NGR operational safety guidelines were developed with regard to automatic equipment operation, laser and radiation sources, high voltage electronics, and general laboratory procedures.

• Keep extraneous items and body parts away from the moving actuators, titanium boat, and motor.
• The doors are extremely heavy (ca. 400 kg) and care should be taken when using the chain hoist to open or close the rear (starboard) door for calibration.
• The track system has an emergency stop button to halt the titanium boat motion in case of a jam or other problem. Once this button is pressed it is necessary to restart the Galil motor (see Galil instruction manuals).
• Do not attempt to work on the system while a measurement is in progress.
• Do not lean over or onto the track. During disassembly (for example, to replace a NaI(Tl) detector), it may be necessary to stand on parts of the track; do not do this unless you know where weight can be placed!
• Do not place anything but samples or standards on the boat and don’t place anything on the track as the boat will move automatically.
• Do not splash water at the base of the NGR (e.g., during mopping). Although shrouded, it is not waterproof and there are high voltage leads underneath.
• When not in use, keep radioactive standards and calibration sources in the specially designated location only. Do not use the calibration sources unless you are properly trained in radioisotope safety.

         Important! Do not keep the calibration source disks on your person! They are small and could be placed in a pocket; do not do this!

• This analytical system does not require personal protective equipment.
• Visually inspect the instrument before operation for exposed electronic cable wires or unusual parts alignment.
• Keep radiation sources in the designated place.
• Do not use any other radiation sources other than those described in this document, especially open sources (e.g., dissolved europium standard or powdered U ore).

Environmental Hazards

• Keep radiation sources in the designated place.
• Do not use any other radiation sources other than those described in this document, especially open sources (e.g., dissolved europium standard or powdered U ore).
  
  

Maintenance/Troubleshooting

Common Issues

Scheduled Maintenance

• Daily: keep barcode scanner and laser windows clean by wiping with a Kimwipe. If necessary, use isopropyl alcohol to remove soil.
• As needed: ensure that acrylic tube inside NGR stays clean and dry at all times. Keep the titanium boat free of dirt and moisture as well.
• Monthly or as needed: clean the air filters on the NGR electronics crates.
• Semiannually open the NSK face plate and lubricate ballscrew.
• Annually or as needed:
  • Examine the cable management system for abraded cables or other indications of wear.
  • Remove electronics shroud at the base of the NGR and carefully clean underneath.

Related Documentation and Links

• Blum, P., 1997. Physical properties handbook. ODP Tech. Note, 26. doi:10.2973/odp.tn.26.1997
• Servo motor data sheet: blm_n23.pdf
• “A New Natural Gamma Radiation Measurement System for Marine Sediment and Rock Analysis”, Vasiliev, M.A., et al. Journal of Applied Geophysics, 75, 2011, 455-463.
• “Assessment and Use of NGR Instrumentation on the JOIDES Resolution to Quantify U, Th, and K Concentrations in Marine sediment”, Dunlea, A., Murray,R.W., Harris, R.N., Vasiliev, M., Evans, H., Spivack, A.J., and D’Hondt S., Scientific Drilling No.15, March 2013, 57-63.