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.

Launching the IMS-SRM application

The IMS-NGR software can be launched from the Windows Start menu or from a desktop icon (Fig. 8).

Figure 8- NGR Desktop Icon

At launch, the program begins the following initialization process:

• Tests all instrument communications
• Reloads configuration values
• Homes the tray in the track
• Sets the degauss controller to a known state

Figure 9- NGR IMS Main Window

After successful initialization, the main IMS-SRM window will appear (Fig. 9).

A Quick Introduction to the IMS Program Structure

IMS is a modular program. Individual modules are as follows:

• INST plug-in: code for each of the instruments
• MOTION plug-in: code for the motion control system
• DAQ Engine: code that organizes INST and MOTION plug-ins into a track system

The SRM system, specifically, is built with one INST module (SRM), one MOTION module, and one DAQ Engine module. 
The IMS Main User Interface (IMS-UI) calls these modules, instructs them to initialize, and provides a user interface to their functionality. 
Each module manages a configuration file that opens the IMS program at the same state it was when previously closed and provides utilities for the user to edit or modify the configuration data and calibration routines.
The five buttons on the IMS-UI window provide access to utilities/editors via dropdown menus as shown in Error! Reference source not found..

Figure 10- IMS Control Panel Drop down menus

Initial Instrument Setup

SRM Configuration

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

SRM Instrument General Setup

To open the SRM instrument setup window (Error! Reference source not found.), select Instruments > SRM: Setup from the IMS panel menu (Error! Reference source not found.).

1. Ensure the values in the window are set as shown in Error! Reference source not found..

• The SQUID constant values are provided by 2G and should not be altered.
• Once the offset to the SQUIDS and degauss coils is determined, these values should not change.

1. Click OK to save the changes and write them to the configuration file. Click Cancel to revert to previous values.

Figure 4- SRM Parameters Window

Motion Control Setup:

Motion control should be set during initial setup and further changes should not be necessary. Motion control setup can be accessed by selecting Motion > Setup from the IMS panel menu (Error! Reference source not found.).

Figure 59- M-Drive Motion Setup

• The M-Drive Motion Setup control panel will open (Error! Reference source not found.).
• The user may select between four setup panels from this window.
• Motor and Track Options
• Fixed Positions
• Limit and Home Switches
• Motion Profiles

Motor and Track Options menu

Once these values have been properly set, they should not change. This panel is only for initial setup.

• Make sure to use the values shown for the SRM (Error! Reference source not found.).
• User should be familiar with the M-Drive motor system prior to adjusting these settings.
• The relationship between motor revolutions and linear motion of the track is defined in this window and is critical to both safe and accurate operation.
• Select Axis: In the case of the SRM it is always X.
• Encoder Pulses/rev: Defined by the manufacturer of the M-Drive as 2048.
• Screw Pitch: Set to 0.0627 (the ratio of 2048 steps per revolution and chain displacement (cm) as a function of the drive gears diameter).
• Gear Ratio: There is not a reduction gear so this value is set to 1.
• Direction: Clockwise rotation moves the tray in a positive direction (from home to end of track).
• Click the Open Utilities and Test button to open the Motion Utility window (Error! Reference source not found.) and test the settings. Click Close to exit this window.
• Click Done to save the values or Cancel to return to previous values.
   
  Figure 60- SRM Motor and Track Options Setup Window
  
  

Figure 61- Motion Utilities Window

Fixed Positions menu

Once these values have been properly set, they should not change. This panel is only for initial setup.
In this window the user may define fixed track locations used by IMS motion control. For the SRM make sure to use these value unless there has been a physical change to the system (Error! Reference source not found.).

• Select Axis: In the case of the SRM it is always X.
• Max Section Length: Maximum length of section that can be placed in the track. This value is set to 160 cm.
• Track Length: Distance in cm between the limit switches. Use the Motion Utility (Error! Reference source not found.) to determine this value by moving from limit switch to limit switch plus the length of the tray used.
• Load and Unload: For the SRM set to 0. This will always bring the tray back to the sample load end after a measurement sequence.
• Top-of-Section Switch?: Not used.
• Top-of-Section Switch Offset: Not used.
• Fast Offset: Not used.
• Run Out Switch?: Not used.
• Click the Open Utilities and Test button to open the Motion Utility window (Error! Reference source not found.) and test the settings. Click Close to exit this window.
• Click Done to save the settings. Click Cancel to return to previous values.
   
  Figure 62- SRM Fixed Positions Window

Note, these values along with the positions set in the Degauss-Drift Location window and the SQUID offset set in the SRM setup window are necessary to fully define the track geometry. Take a lot of care in setting these values!

Limit and Home Switches Window

Once these values have been properly set, they should not change. This panel is only for initial setup.

• This panel is used to define the orientation of the track system, limit switches, and the home switch (Error! Reference source not found.). The MDrive can be used with either a dedicated Home switch or a limit switch as a home switch. In the case of the SRM track we use only a limit switch. There is no home switch installed.
• Select Axis: In the case of the SRM it is always X.
• Select Track and Home Geometry: For the SRM, select CW limit.
• Click the Open Utilities and Test button to the Motion Utility to test the settings.
• Click Done to save the settings. Click Cancel to return to previous values.
   
  Figure 63- SRM Limit and Home Switches Window

Motion Profiles

The motion profiles window can be used to adjust the speed and acceleration profiles used by the track (Error! Reference source not found.) for various types of movements.
Setting the correct values for the motion profile takes a little experimentation to make the track run efficiently and safely. It is not unusual to modify these values as the lithology changes to balance the need for speed without inducing flux jumps.

 Figure 64- Motion Profiles Window

• DAQ Move: This profile controls moves between measurement positions (leader and trailer measurements included) and the move to the drift 2 position. Set this to a reasonable speed with gradual acceleration and watch out for flux jumps. In addition, when you use the speed reduction feature to control flux jumps, this value is the base value for the reduction.
• Limit Seek: This profile is used for the following moves:
• This profile finds the limit switch location. Do not exceed 3 cm/sec. Do not use a large acceleration value, but always use a large deceleration value.
• Home Final: This profile finds the final location of the home switch. Do not exceed 3 cm/sec. Do not use a large acceleration value, but always use a large deceleration value.
• Load/Unload: This profile is used for moving the tray in and out of the SRM for general movements and moving out of the SRM to the final position prior to ending a measurement or beginning the next degaussing step.
• Drift 1: This profile is used to move from the drift 1 position to the leader position and to move from the last trailer position to the drift 2 position
• Drift 2: Unused
• Degauss Stage: This profile is used to move to the degauss start position
• Degaussing: This profile is used to move the section/discrete samples from degauss stage position through the in line AF degauss coils for X, Y, and Z. The profile is also used to move the tray to the drift 1 position.
• User Define: This profile is used for testing only in the Motion Utilities (Error! Reference source not found.) program.
  • Click the Open Utilities and Test button to open the Motion Utility (Error! Reference source not found.) window and test the settings.
  • Click Done to save the settings. Click Cancel to return to previous values.