Author(s): | M. Hastedt, M. Vasilyev, Y. Vasilyeva |
Revised by: | Exp376 Techs |
Reviewer(s): | D. Houpt |
Supervisor Approval (Name, Title, Date): | draft Exp. 362T (July 2016) |
Audience: | Marine Laboratory Specialists |
Origination date: | 4/28/08 |
Current version: | 376 |
Revised: | V1.3 | 7/5/2017 (IODP-II); 372 | 03/02/2018 ; 374; 375:376 |
Domain: | Physics |
System: | Natural Gamma Radiation Logger |
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.
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.
The NGRL system consists of five major units (Fig. 1):
Figure 1. NGR logger system components.
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.
The main NGR detector unit consists of the following (Fig. 2, Fig. 3):
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:
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).
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:
Each plastic detector has two PMTs to maximize light collection across a somewhat large detector surface.
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
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
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).
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.
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:
The Track Utility display also provides a Home position (loading position) as well as manual fine controls.
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.
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:
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.
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.
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.
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!
Partial (see Partial Shutdown) or complete (see Full Shutdown) shutdown may become necessary in the following conditions:
Figure 8, below, shows the location of the main power switch and the two NIM 4001B crate power switches.
Figure 8. Power switch placement
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.
Figure 10. Program termination query.
Figure 11a. iSEG Multi-Channel High-Voltage Modules; Ch00.
Figure 11b. iSEG Multi-Channel High-Voltage Modules; Ch01.
Figure 12. Electronics Crate: Shutdown.
Figure 13. Master Power Switch.
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:
Ensure the fans are plugged in and working.
Figure 14. iSEG Hard ware Setup and Main screen. The controller cards may be the EHS or EHQ model and are labeled accordingly.
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.
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!
Click on the Module access menu and click Instructions for all channels > On (ctrl+o) to start ramping up the voltage.
Wait until ramp-up completes (1–2 minutes if proper values are used).
Exit the iSEG program.
Answer No to the prompt when asked to ramp voltages back down. If you answered Yes inadvertently, start again at Step 8.
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