Analysis of natural gamma-ray spectra obtained from sediment cores with the shipboard scintillation detector of the Ocean Drilling Program: example from Leg 156

Abstract

Natural gamma-ray (NGR) spectrometry allows estimation of the elemental concentrations of K, U, and Th, which can be used to help interpret sediment composition, provenance, and diagenesis. Spectral data obtained with the NGR multichannel device installed on the Ocean Drilling Program’s multisensor track in 1993 are presented here for the first time. The spectra were divided into 16 energy intervals using a minima search algorithm that defined all peaks observed in 79 sample NGR spectra. The intervals were further subdivided into peak area and background area segments using a peak baseline algorithm, which allows optimal assessment of the usefulness of spectral segments to estimate elemental abundance. Linear regression with laboratory (X-ray diffraction, inductively coupled plasma-mass spectrometry, and instrumental neutron activation analyses) data was used to estimate elemental concentrations of K, U, and Th for each spectral segment. Conservative estimation errors for the best estimator spectral segments are 16%, 30%, and 20% for K, U, and Th, respectively. These errors also reflect analytical errors of the reference data, and the true estimation error may be significantly smaller. Our method suggests that the best K e stimates (± 16%) are obtained using the peak area segment between 1335 and 1580 KeV. In our study, which uses 4-hr counting times, the best U and Th estimates are obtained using peak areas between 1695 and 1885 KeV and 550 and 700 KeV, respectively. If low counting times are used for routine core logging, however, regressions using the total counts of the entire spec trum yield more reliable U and Th estimates because of Poisson’s law, with maximum total errors of about 35% and 23%, respectively. Spectral analysis using 2048-channel data has no advantage over 256-channel analyses, even with the extremely high counting times used for our study. The full character of natural gamma-ray spectra, as revealed by scintillation detectors , can be defined and measured in full detail with 16 energy intervals.