To attain the high sensitivity needed to detect a dark matter particle with a xenon time-projection chamber, all other sources of particle interactions need to be eliminated or minimized. These interactions are classified as background events. Radiogenic backgrounds, in particular, come from radioactive isotopes within the detector materials that decay and lead to alpha, beta, or gamma emissions. Neutrons from spontaneous fission of heavy isotopes or from secondary reactions within the detector materials also contribute to the radiogenic background and can mimic a dark matter signal.
To minimize the radiogenic background, the goal of the XENON1T radioassay program is to measure the radioactivity of all materials that are needed to build the detector and to select only the most radiopure materials for the final construction. To do this, we use mass spectrometry techniques and high-purity germanium spectrometers that are capable of measuring radioactivity at the level of 10-6 decays per second in a kilogram of material (Bq/kg). As comparison, a typical banana has an activity of ~102 Bq/kg!
Because natural radioactivity is present in the soil, the water, and in the air, it is also present in the XENON detector materials. The Figure shows a measurement obtained with a germanium spectrometer of the gamma rays emitted from a sample of photomultiplier tubes. The background (purple spectrum) is subtracted from the sample (pink spectrum) in order to quantify the expected activity from a XENON1T component or material sample.
The most common radioactive isotopes present in the Earth are primordial uranium and thorium, each of which decays into a series of other radioactive isotopes (marked in green in the Figure). Potassium (red) is also a common, primordial isotope that is found in soil, and subsequently in food and in your body. Other isotopes that are found in detector materials come from interactions with cosmic rays (yellow) or from man-made activities (blue), i.e. industrial or medical use, nuclear power plant emissions, nuclear accidents, and military testing.
The measured activities of each material selected for detector construction are used in simulations of XENON1T to determine the expected background. This allows for a prediction of the attainable sensitivity of the detector to dark matter interactions. The radioassay measurement results from over 100 material samples are presented in our new paper “Material radioassay and selection for the XENON1T dark matter experiment”.