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Poster excerpt 9: Reconstruction of Spallation Neutron Kinematics in Antineutrino Detectors

March 3, 2015

(The following text has been written for the conference blog by Sheng-Chao Li)

Fast spallation neutrons from cosmic-ray muons are essential candidates of background signals in underground neutrino experiments. We study their properties in the Daya Bay antineutrino detectors (ADs) with a method that is also applicable to other scintillator based neutrino detectors.  The energetic cosmic-ray muons can penetrate tons of rocks and reach at our antineutrino detectors, causing a line of scintillation through various physical processes; it may also cause the spallation of a nucleus, producing isotopes and fast neutrons [1]. Fast neutrons can scatter off protons, and the excited protons recoil and cause scintillation in liquid scintillator, all happen in a transient from the passage of muon. Both muons entering ADs and passing near ADs are vetoed in Daya Bay Experiment, the latter of which produce fast neutrons that can spill into ADs and be captured, mimicking the prompt-delay pairs of the desired inverse beta decay (IBD). Yet it might be just as much interesting to study the spallation neutron and proton recoil with AD muons, whose contributions of scintillation sum up in the PMT wave pulses and cannot be separated easily.

spallationOur algorithm considers muon entering AD with average stopping power of 0.22MeV/mm as a line of scintillation, utilizing the vertex of RPC hit to settle its angle; then we fit the residue PMT response to reconstruct the energy and vertex of proton recoils (as in figure), which hint on the initial state of the spallation neutron. The very preliminary result of this generic algorithm shows a correlation in features of proton recoil with delayed neutron capture events, and the index obtained in the fitting may place a handle on selecting spallation neutron events.

In order to evaluate the sensitivity of our method to proton recoils, toy Monte Carlo (MC) simulations are performed generating muon events according to the Daya Bay muon flux profile [2], and neutron events with diverse energy levels at the center of ADs. It is also verified by the simulations that the average time interval from muon passing through ADs to the spallation neutron causing protons to recoil is around tens of nanoseconds, which is possible to be recorded in a single AD muon readout.

References:

[1]. F. P. An et al. (Daya Bay collaboration), Phys. Rev. Lett. 108, 171803 (2012).

[2]. F. P. An et al. (Daya Bay collaboration), NIM-A 773 (2015) 8-20 (2015).

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