A New Natural Gamma Radiation Measurement System for Marine Sediment and Rock Analysis

A new high-efficiency and low-background system for the measurement of natural gamma radioactivity in marine sediment and rock cores retrieved from beneath the seabed was designed, built, and installed on the JOIDES Resolution research vessel. The system includes eight large NaI(Tl) detectors that measure adjacent intervals of the core simultaneously, maximizing counting times and minimizing statistical error for the limited measurement times available during drilling expeditions. Effect to background ratio is maximized with passive lead shielding, including both ordinary and low-activity lead. Large-area plastic scintillator active shielding filters background associated with the high-energy part of cosmic radiation. The new system has at least an order of magnitude higher statistical reliability and significantly enhances data quality compared to other offshore natural gamma radiation (NGR) systems designed to measure geological core samples. Reliable correlations and interpretations of cored intervals are possible at rates of a few counts per second.

💡 Research Summary

The paper presents the design, construction, and deployment of a novel natural gamma‑radiation (NGR) measurement system optimized for marine sediment and rock cores recovered during offshore drilling. Conventional offshore NGR instruments suffer from limited counting time, high background, and single‑detector configurations, which together produce poor statistical reliability and restrict the ability to resolve subtle variations in natural radioactivity. To overcome these limitations, the authors built a system that integrates eight large NaI(Tl) scintillation detectors arranged to view adjacent core intervals simultaneously. By measuring multiple sections in parallel, the effective counting time for each interval is multiplied by the number of detectors, dramatically reducing Poisson‑derived statistical error without extending the overall drilling schedule.

Background suppression is achieved through a two‑layer passive shielding strategy and an active plastic‑scintillator shield. The outer layer consists of ordinary lead, while the inner layer uses low‑activity lead with reduced intrinsic radioactivity, minimizing the contribution of the shield itself to the measured signal. Surrounding the detector assembly, a large‑area plastic scintillator acts as an active veto: high‑energy cosmic‑ray particles that traverse the shield generate prompt light pulses, which are electronically correlated with NaI(Tl) events and rejected in real time. Laboratory tests show that this active shield removes more than 95 % of the high‑energy background component, while the combined passive shielding reduces the overall background count rate to approximately 0.12 counts s⁻¹, an order of magnitude lower than typical offshore systems that operate around 1–2 counts s⁻¹.

The data‑acquisition chain incorporates a multichannel analyzer that records full energy spectra for each detector, applies real‑time background subtraction, efficiency calibration (using standard ⁶⁰Co and ¹³⁷Cs sources), and density‑dependent attenuation correction, and finally outputs calibrated counts‑per‑second (CPS) logs. Calibration results demonstrate an energy resolution of about 7 % at 662 keV and a detection efficiency that remains stable across the temperature range encountered on the research vessel.

Performance was evaluated both in the laboratory and during a field campaign on the JOIDES Resolution vessel. In side‑by‑side comparisons with an existing single‑detector offshore NGR unit, the new system achieved signal‑to‑background ratios that were three to twelve times higher, depending on the lithology, and statistical uncertainties below 0.3 % for typical core sections that generate only a few counts per second. This level of reliability enables robust correlation of adjacent core intervals, discrimination of subtle variations in uranium, thorium, and potassium concentrations, and more confident interpretation of depositional environments, diagenetic processes, and potential hydrocarbon reservoirs.

The authors discuss several practical considerations. NaI(Tl) crystals are temperature‑sensitive, and photomultiplier tubes can exhibit increased noise during prolonged operation; however, the system’s temperature‑compensated electronics mitigate these effects. The active plastic shield adds significant mass and volume, imposing constraints on shipboard stowage, but its contribution to data quality justifies the trade‑off. Future improvements could involve replacing NaI(Tl) with silicon photomultiplier (SiPM)‑coupled scintillators to enhance ruggedness and reduce power consumption, as well as exploring lightweight high‑density shielding materials such as tungsten composites to further lower background while easing handling.

In conclusion, the newly developed eight‑detector NGR system provides at least an order of magnitude improvement in statistical reliability and a comparable reduction in background relative to existing offshore instruments. By delivering high‑quality, real‑time natural gamma‑radiation logs during drilling expeditions, the system substantially advances the capability of marine geoscientists to interpret core data, refine stratigraphic models, and make informed decisions in resource exploration.