  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.
