Investigating Optical Coupling Material Candidates in NaI(Tl) Detectors for the SABRE-North Dark Matter Experiment
Year: 2026
Type: preprint
Abstract: Direct dark matter detection with NaI(Tl) scintillators requires ultra-low-background materials. Silicone optical pads can couple NaI(Tl) crystals to photomultiplier tubes (PMTs) if they provide adequate optical transmission and radiopurity. We characterize four candidate materials for the SABRE-North experiment: commercial EJ-560, used as a reference, and three materials produced at the University of Salento (SYLGARD 184, EGel 3002 A/B, and a hybrid formulation) using different catalyst ratios and curing conditions, yielding ten samples in total. Optical transmittance was measured from 310 to 850 nm by UV/Vis/NIR spectrophotometry. Radiopurity was assessed at the STELLA facility at LNGS using high-purity germanium (HPGe) $γ$-ray spectrometry and, for EJ-560, high-resolution ICP-MS. At the 410 nm NaI(Tl) emission peak, EJ-560 shows an absorbance coefficient of $0.064\pm0.017$ mm$^{-1}$. The best locally produced samples reach $0.097\pm0.015$ and $0.093\pm0.014$ mm$^{-1}$ for SYLGARD 184 with a 10:2 base-to-catalyst ratio cured at room temperature and at $40^\circ$C, respectively, $0.125\pm0.015$ mm$^{-1}$ for EGel 3002 with a 2B:A ratio cured at $40^\circ$C, and $0.106\pm0.014$ mm$^{-1}$ for the hybrid 10:1 EGel A:CataSYL formulation cured at room temperature. EJ-560 exhibits detectable $^{232}$Th-chain activity at about 1.8 mBq/kg and the highest $^{40}$K activity, $(120\pm20)$ mBq/kg. The locally produced materials show no detectable Th-chain contamination but exhibit $^{226}$Ra activities of about 2.3 mBq/kg and lower $^{40}$K levels. No single material is optimal across all criteria. Additional ICP-MS screening, HPGe measurements of cured pads, and direct NaI(Tl)-PMT light-yield tests are required for a conclusive SABRE-North material selection.
Source: arXiv (Cornell University)
Authors: Ananna, Chemseddine, Cataldi, Gabriella, D'Angelo, Davide, D'Imperio, Giulia, De Giorgi, Maria Luisa, Di Carlo, Giuseppe, Di Giacinto, Attilio, Diemoz, Marcella, Ianni, Aldo, Khattak, Sana Gul, Martinenghi, Edoardo, Miccoli, Alessandro, Misiaszek, Marcin, Montanino, Daniele, Pettinacci, Valerio, Pietrofaccia, Lidio, Rahatlou, Shahram, Szczepaniec, Krzysztof, Tomei, Claudia, Toso, Valerio, Vignoli, Chiara, Zuhra, Samina, Ferella, Francesco, Nisi, Stefano
Institutions: Istituto Nazionale di Fisica Nucleare, Sezione di Lecce, University of Milan, Istituto Nazionale di Fisica Nucleare, Sezione di Milano, Istituto Nazionale di Fisica Nucleare, Istituto Nazionale di Fisica Nucleare, Sezione di Roma I, Sapienza University of Rome, Istituto Nazionale di Fisica Nucleare, Laboratori Nazionali del Gran Sasso, University of Salento, University of Padua
Language: English
Subfield: Radiation
Field: Physics and Astronomy
Domain: Physical Sciences
Open Access status: green
Awards: G231133531, G972786147
OpenAlex ID: W7220711193