Background studies for NaI(Tl) detectors in the ANAIS dark matter project

• Main Authors: Amaré, J., Borjabad, S., Cebrián, S., Cuesta, C., Fortuño, D., García, E., Ginestra, C., Gómez, H., Martínez, M., Oliván, M. A., Ortigoza, Y., Solórzano, A. Ortiz de, Pobes, C., Puimedón, J., Sarsa, M. L., Villar, J. A., Villar, P.
• Format: Conference Proceeding
• Language: English
• Subjects: ALPHA PARTICLES; BACKGROUND RADIATION; COMPUTERIZED SIMULATION; CONTAMINATION; CRYSTALS; HIGH-PURITY GE DETECTORS; INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND TECHNOLOGY; MODULATION; MONTE CARLO METHOD; MULTIPARTICLE SPECTROMETERS; NAI DETECTORS; NONLUMINOUS MATTER; PARTICLE IDENTIFICATION; POTASSIUM 40; PULSE SHAPERS; SHIELDING; THORIUM 232; UNDERGROUND; URANIUM 238
• ISSN: 0094-243X; 1551-7616
• DOI: 10.1063/1.4818100

Several large NaI(Tl) detectors, produced by different companies, have been operated in the Canfranc Underground Laboratory (LSC) in the frame of the ANAIS (Annual modulation with NaI Scintillators) project devoted to the direct detection of dark matter. A complete background model has been developed for a 9.6 kg detector (referred as ANAIS-0 prototype) after a long data taking at LSC. Activities from the natural chains of {sup 238}U and {sup 232}Th, and {sup 40}K in the NaI(Tl) crystal were evaluated applying different methods: discrimination of alpha particles vs beta/gamma background by Pulse Shape Analysis for quantifying the content of the natural chains and coincidence techniques for {sup 40}K. Radioactive contaminations in the detector and shielding components were also determined by HPGe spectrometry. Monte Carlo simulations using Geant4 package were carried out to evaluate their contribution. At high energies, most of the measured background is nicely reproduced; at low energy some non-explained components are still present, although some plausible background sources have been analyzed. The {sup 40}K content of the NaI(Tl) crystal has been confirmed to be the dominant contributor to the measured background with this detector. In addition, preliminary results of the background characterization, presently underway at the LSC, of two recently produced NaI(Tl) detectors, with 12.5 kg mass each, will be presented: cosmogenic induced activity has been clearly observed and is being quantified, and {sup 40}K activity at a level ten times lower than in ANAIS-0 has been determined.