Muon and cosmogenic neutron detection in Borexino

Abstract

Borexino, a liquid scintillator detector at LNGS, is designed for the detection of neutrinos and antineutrinos from the Sun, supernovae, nuclear reactors, and the Earth. The feeble nature of these signals requires a strong suppression of backgrounds below a few MeV. Very low intrinsic radiogenic contamination of all detector components needs to be accompanied by the efficient identification of muons and of muon-induced backgrounds. Muons produce unstable nuclei by spallation processes along their trajectory through the detector whose decays can mimic the expected signals; for isotopes with half-lives longer than a few seconds, the dead time induced by a muon-related veto becomes unacceptably long, unless its application can be restricted to a sub-volume along the muon track. Consequently, not only the identification of muons with very high efficiency but also a precise reconstruction of their tracks is of primary importance for the physics program of the experiment. The Borexino inner detector is surrounded by an outer water-Cherenkov detector that plays a fundamental role in accomplishing this task. The detector design principles and their implementation are described. The strategies adopted to identify muons are reviewed and their efficiency is evaluated. The overall muon veto efficiency is found to be 99.992 % or better. Ad-hoc track reconstruction algorithms developed are presented. Their performance is tested against muon events of known direction such as those from the CNGS neutrino beam, test tracks available from a dedicated External Muon Tracker and cosmic muons whose angular distribution reflects the local overburden profile. The achieved angular resolution is ∼3 • -5 • and the lateral resolution is ∼35-50 cm, depending on the impact parameter of the crossing muon. The methods implemented to efficiently tag cosmogenic neutrons are also presented.

A Search for Low-energy Neutrinos Correlated with Gravitational Wave Events GW 150914, GW 151226, and GW 170104 with the Borexino Detector

“…A detailed description of the detector could be found elsewhere [17][18][19][20][21][22][23][24][25][26][27][28][29].…”

“…Cosmogenic backgrounds can be reduced by applying the detector temporal veto after each muon that could be discriminated through coincidence with outer veto as well as by pulse-shape discrimination [25]. A veto length of Visible energy spectrum of Borexino detector data passing the selection procedure, obtained using weekly runs containing events GW150914, GW151226 and GW170104.…”

Cosmic-muon flux and annual modulation in Borexino at 3800 m water-equivalent depth

“…The detection efficiencies are 0.9925(2), 0.9928(2) and 0.9890(1) respectively. Details on the muon tagging methods and on how the efficiencies have been evaluated can be found in [15].…”

“…These curves are illustrative only as the definition of effective temperature used to calculate the experimental values also depends on the K/π ratio. The data from other experiments are shown for comparison only, and are from Barrett 1, 2 [2], AMANDA [4], MACRO [11], Torino [12], Sherman [15], Hobart [16] and Baksan [17].…”

Fast neutron detection with a segmented spectrometer

“…It is clear to the community of researchers in underground physics that reliable measurements of both the fluxes and energy spectra are essential to understanding their backgrounds [19]. A num-ber of collaborations address this concern by measuring fast neutron parameters directly at their experimental facility [20,21,22,23,24,25,26,27] and by benchmarking the codes that they use for simulations [17,18].…”