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 uninterruptable 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).