radiopure

Radiopure describes materials that possess extremely low levels of radioactive contaminants, a quality essential for low-background experiments and high-sensitivity measurements. Radiopurity is defined by the absence or very small presence of radionuclides such as uranium-238, thorium-232, potassium-40, and their progeny, typically quantified in units like becquerels per kilogram (Bq/kg), parts per trillion (ppt), or parts per billion (ppb), depending on the material and isotope.

Achieving radiopurity involves careful material selection, purification, and handling. Techniques include chemical purification of metals and compounds, refining processes to remove trace radionuclides, and electroforming copper underground to minimize cosmogenic activation. Some components are sourced or stored underground to reduce exposure to cosmic rays, while surface cleaning, cleanroom handling, and radon-suppressed environments minimize surface contamination and radon emanation. Cosmogenic activation remains a key challenge, necessitating minimized surface time and shielding during production and transport.

Measurement of radiopurity employs radiometric and spectrometric methods. High-purity germanium (HPGe) gamma-ray spectrometry is commonly used to quantify gamma-emitting contaminants, while mass spectrometry techniques such as inductively coupled plasma mass spectrometry (ICP-MS) assess trace concentrations of U, Th, and other elements. Alpha counting and neutron activation analysis may be used for complementary assessments of specific isotopes.

Applications of radiopure materials are prominent in fundamental physics and rare-event searches, including neutrino detectors, dark matter experiments, and searches for neutrinoless double-beta decay. Materials frequently cited for radiopurity include copper, lead, certain steels, scintillators, and specially purified plastics. Ongoing advances in purification, assay, and underground fabrication aim to further reduce background levels and expand the sensitivity of low-background experiments.