Removal of 210po from Groundwater for Nuclear Astrophysics Applications

At the frontiers of physics research, searches for solar neutrinos, dark matter, and double beta decay have become more prevalent and the need for low background experiments has increased. New technology and equipment necessitate higher levels of sensitivity and the ability to extract weaker signals amidst high levels of background. The Borexino detector at the Laboratori Nazionalli del Gran Sasso (LNGS) in Italy is one such experiment which requires extremely pure liquid scintillator with low levels of radioactive isotopes in order to study solar neutrinos. The detector uses a liquid-liquid extraction system to purify the scintillator by removing polar impurities, but the naturally occurring radioactive isotope ²¹⁰Po is abundant in the groundwater used and not fully removed by the extraction. This motivated the development of a fractional distillation apparatus at Princeton University to remove compounds of ²¹⁰Po from groundwater, which would then be put in place at LNGS to use with Borexino. The apparatus consists of two distillation columns working in conjunction to remove the more volatile polonium hydrides followed by the heavier and less volatile dimethyl polonides; this results in purified water which would have lower concentrations of ²¹⁰Po. In order to determine the amount of polonium in the purified water, a precipitation reaction was run to create measurable samples and deposit the polonium onto thin disks of silver foil. A silicon surface barrier detector was used to count the alpha-emittance of samples, first for the well water samples and polished silver foils, followed by the purified water samples. Upon analyzing the data generated by the alpha radiation hitting the detector, it was possible to determine the presence of polonium in the groundwater and the effectiveness of the distillation column. While all samples had high levels of low-energy background, it was possible to see a reduction in the high-energy ²¹⁰Po decay peak after the purification process. The successful reduction of noise from ²¹⁰Po would allow for more sensitive detection of signals in such a low-background experiment. Further studies will determine the optimal reflux ratio resulting in the most efficient purification process while scaling up this distillation apparatus to the volumes necessary for LNGS and Borexino.