ARE CALIBRATIONS NEEDED FOR THE FUTURE DUNE DETECTOR? BACKGROUND RADIOACTIVITY AND GEONEUTRINO FLUX ESTIMATION, SURROUNDING THE HOMESTAKE GOLD MINE, IN LEAD, SOUTH DAKOTA, USA

Gamma-ray spectrometer analyses of 736 samples collected within a 15-km radius of the Sanford Underground Research Facility (SURF) in the Northern Black Hills, SD provide an estimate of the local geoneutrino flux due to radioactive decay of U, Th, and K in the rocks. The samples include 240 igneous, 261 metamorphic, and 235 sedimentary rocks. Although igneous rocks typically have 2 to 3 times the radioelement contents of metamorphic and sedimentary rocks, the occurrence of roll front Uranium deposits in Tertiary sediments in the region and hydrothermal metamorphism during late Tertiary igneous intrusions suggest the possibility for outlier concentrations of radioelements. Igneous rocks, excluding Homestake samples, consisted of 215 samples, with the radioelement content averaging 5.27 ppm U, 15.80 ppm Th, and 2.19 pct K. Metamorphic rocks consisted of 143 samples, excluding Homestake samples, with the radioelement content averaging 2.95 ppm U, 8.81 ppm Th, and 2.16 pct K. Sedimentary rocks consisted of 235 samples, with the radioelement content averaging 1.60 ppm U, 1.95 ppm Th, and 0.63 pct K. U and Th contents in rocks within the Homestake Gold Mine have been enriched due to the hydrothermal and metamorphic activity. Igneous rocks present within the mine, consisted of 25 samples with an average radioelement content of 11.58 ppm U, 17.61 ppm Th, and 7.43 pct K. Metamorphic rocks present in the Homestake Gold Mine consisted of 118 samples, with the radioelement content averaging 6.40 ppm U, 13.80 ppm Th, and 3.49 pct K. SURF will be housing a geoneutrino detector at the -4850-ft level of the Homestake Gold Mine in the future, as part of the Deep Underground Neutrino Experiment. Natural radioactive decay also produces neutrinos, so the future neutrino detector will need to be calibrated regarding the localized radioelement content and contribution. The estimated neutrino flux within a 15-km radius of SURF is 2.30x10¹² antineutrinos per second.