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Gulf Coast Repository

Natural Gamma Radiation Logger (NGRL)

Overview

Thumbnail image of Natural Gamma Radiation Logger Gamma ray emissions emitted from sediments and rocks are 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, these data can yield information about the alteration and heat production of rocks.

Maximum core diameter: 90 cm (o.d.)
Maximum core length: 160 cm

Instrument Make & Model

Custom-built system.

Estimated Measurement Time

  • Sample Preparation: none
  • Measurement Time: 10 mins/1.5 m section

Measurement Resolution

The measurement resolution of the NGRL is 10 cm, as each sodium iodide detector integrates the radioactive signal over this distance. This resolution is fixed by the NGRL’s geometry.

Principles & Analytical Capabilities

The Natural Gamma Radiation Logger (NGRL) is a high-efficiency, low-background system for the measurement of natural gamma radioactivity in marine sediment and rock cores. 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. The total count value 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 system can be used to measure whole-round sections, section halves, and discrete samples (although these may require significantly longer acquisition time because less mass is over a given detector).

The NGRL is a custom-built system1. The system is a lead-shielded chamber with eight sodium iodide-thallium, NaI(Tl), scintillation detectors custom-designed for the greatest possible efficiency to capture gamma rays from a cylindrical object (i.e., a core section). In addition, the system includes active shielding, a combination of plastic scintillators and electronics, which significantly reduces the noise from cosmic radiation events. The combination of passive (lead) shielding and active shielding makes the effective background inside the NGR between 135 and 150 times lower than the environment outside of the system. This makes it possible to “see” gamma spectra from very low-activity samples.

Data Generated

Data components:

  • NGR total counts (cps): rate of natural gamma radiation across the entire spectrum of ~0.1-3 MeV.
  • Error (cps): absolute error of measurement; 1 divided by the square root of the total counts (s) divided by the number of seconds.
  • Relative Error: relative error of measurement; Error (cps) / NGR total counts (cps).

Data files:

  • NGR Results (CSV): tabular file of NGR results.

Raw files:

  • NGR Compilation (ZIP): contains raw files for each detector/position, backgrounds, energy calibration, edge corrections, and a tabular file of compiled raw and reduced data.
  • NGR (instrument file): Data file containing NGR reduced data and metadata.
  • Sodium-iodide detector measurement (SPE): contains raw spectral data.
  • Sodium-iodide detector calibration (SPE): contains detector spectrum generated during detector calibration. There is 1 file for each of 8 detectors.
  • Sodium-iodide detector background calibration (SPE): contains detector spectrum generated during ambient background signal collection. There is 1 file for each of 8 detectors.
  • NGR edge correction (TXT): contains correction coefficients for detector positions close to annulus openings.
  • IMS configuration (INI): IMS configuration file.

Data Example

Graph of natural gamma radiation from Hole U1624B.
Natural gamma radiation (cps = counts per second) from Hole U1624B of IODP Expedition 403 in the Eastern Fram Strait.2

Procedures & Manuals

References

1  PM.A. Vasiliev, P. Blum, G. Chubarian, R. Olsen, C. Bennight, T. Cobine, D. Fackler, M. Hastedt, D. Houpt, Z. Mateo, Y.B. Vasilieva, A new natural gamma radiation measurement system for marine sediment and rock analysis, Journal of Applied Geophysics, Volume 75, Issue 3, 2011, Pages 455-463, https://doi.org/10.1016/j.jappgeo.2011.08.008

2  Lucchi, R. G., St. John, K., Ronge, T. A., Barcena, M. A., De Schepper, S., Duxbury, L. C., Gebhardt, C., González-Lanchas, A., Goss, G., Greco, N. M., Gronmyr, V., Gruetzner, J., Haygood, L., Husum, K., Iizuka, M., Kapuge, A. K. I. U., Lam, A. R., Libman-Roshal, O., Liu, Y., … Zhong, Y. (2026). IODP Expedition 403 Natural gamma radiation [Data set]. International Ocean Discovery Program. https://doi.org/10.5281/zenodo.18437051