Eur. Phys. J. C (2021) 81: 1075
https://doi.org/10.1140/epjc/s10052-021-09799-x

Regular Article - Experimental Physics

Identification of the cosmogenic ${}^{11}$C background in large volumes of liquid scintillators with Borexino

¹ Department of Physics and Astronomy, University College London, London, UK
² Physik-Department and Excellence Cluster Universe, Technische Universität München, 85748, Garching, Germany
³ National Research Centre Kurchatov Institute, 123182, Moscow, Russia
⁴ Institute for Nuclear Physics IKP-2, Forschungszentrum Jülich, 52428, Jülich, Germany
⁵ Dipartimento di Fisica, Università degli Studi e INFN, 20133, Milan, Italy
⁶ Chemical Engineering Department, Princeton University, 08544, Princeton, NJ, USA
⁷ INFN Laboratori Nazionali del Gran Sasso, 67010, Assergi, AQ, Italy
⁸ Physics Department, Princeton University, 08544, Princeton, NJ, USA
⁹ Dipartimento di Fisica, Università degli Studi e INFN, 16146, Genoa, Italy
¹⁰ Physics Department, Virginia Polytechnic Institute and State University, 24061, Blacksburg, VA, USA
¹¹ Lomonosov Moscow State University Skobeltsyn Institute of Nuclear Physics, 119234, Moscow, Russia
¹² St. Petersburg Nuclear Physics Institute NRC Kurchatov Institute, 188350, Gatchina, Russia
¹³ Joint Institute for Nuclear Research, 141980, Dubna, Russia
¹⁴ AstroParticule et Cosmologie, Université Paris Diderot, CNRS/IN2P3, CEA/IRFU, Observatoire de Paris, Sorbonne Paris Cité, 75205, Paris Cedex 13, France
¹⁵ Gran Sasso Science Institute (INFN), 67100, L’Aquila, Italy
¹⁶ Physics Institute IIIB, RWTH Aachen University, 52062, Aachen, Germany
¹⁷ Institute of Physics and Excellence Cluster PRISMA+, Johannes Gutenberg-Universität Mainz, 55099, Mainz, Germany
¹⁸ M. Smoluchowski Institute of Physics, Jagiellonian University, 30059, Kraków, Poland
¹⁹ Kiev Institute for Nuclear Research, 03680, Kyiv, Ukraine
²⁰ Department of Physics, Royal Holloway University of London, TW20 0EX, Egham, Surrey, UK
²¹ Institute of Nuclear Research (Atomki), Debrecen, Hungary
²² National Research Nuclear University MEPhI (Moscow Engineering Physics Institute), 115409, Moscow, Russia
²³ Department of Physics, Technische Universität Dresden, 01062, Dresden, Germany
²⁴ Dipartimento di Chimica, Biologia e Biotecnologie, Universitàa degli Studi e INFN, 06123, Perugia, Italy
²⁵ Amherst Center for Fundamental Interactions and Physics Department, University of Massachusetts, 01003, Amherst, MA, USA
²⁶ Department of Physics, University of California, Berkeley, 94720, Berkeley, CA, USA
²⁷ Universidad Autónoma de Madrid, Ciudad Universitaria de Cantoblanco, 28049, Madrid, Spain
²⁸ Departement Fysica en Sterrenkunde, Universiteit Gent, Gent, Belgium
²⁹ Dipartimento di Fisica e Astronomia dell’Università di Padova and INFN Sezione di Padova, Padua, Italy
³⁰ Dipartimento di Fisica, Università degli Studi Federico II e INFN, 80126, Naples, Italy

^s davide.dangelo@mi.infn.it
^cb spokesperson-borex@lngs.infn.it

Received: 19 June 2021
Accepted: 2 November 2021
Published online: 7 December 2021

Abstract

Cosmogenic radio-nuclei are an important source of background for low-energy neutrino experiments. In Borexino, cosmogenic ${}^{11}$C decays outnumber solar pep and CNO neutrino events by about ten to one. In order to extract the flux of these two neutrino species, a highly efficient identification of this background is mandatory. We present here the details of the most consolidated strategy, used throughout Borexino solar neutrino measurements. It hinges upon finding the space-time correlations between ${}^{11}$C decays, the preceding parent muons and the accompanying neutrons. This article describes the working principles and evaluates the performance of this Three-Fold Coincidence (TFC) technique in its two current implementations: a hard-cut and a likelihood-based approach. Both show stable performances throughout Borexino Phases II (2012–2016) and III (2016–2020) data sets, with a ${}^{11}$C tagging efficiency of $\sim 90$ % and $\sim$ 63–66 % of the exposure surviving the tagging. We present also a novel technique that targets specifically ${}^{11}$C produced in high-multiplicity during major spallation events. Such ${}^{11}$C appear as a burst of events, whose space-time correlation can be exploited. Burst identification can be combined with the TFC to obtain about the same tagging efficiency of $\sim 90\%$ but with a higher fraction of the exposure surviving, in the range of $\sim$ 66–68 %.