[Paper Review] Future Reactor Experiments

Miao He|arXiv (Cornell University)|Oct 28, 2013

Neutrino Physics Research3 references4 citations

TL;DR

This paper proposes the JUNO experiment, a 20-kiloton liquid scintillator detector located 53 km from multiple nuclear reactors in China, designed to determine the neutrino mass hierarchy and measure mixing parameters with sub-1% precision. By exploiting the interference of two oscillation modes in the reactor antineutrino energy spectrum, JUNO achieves a sensitivity of Δχ² > 16 for mass hierarchy determination using a 3% energy resolution and self-calibration techniques to mitigate nonlinearity in detector response.

ABSTRACT

The measurement of the neutrino mixing angle $θ_{13}$ opens a gateway for the next generation experiments to measure the neutrino mass hierarchy and the leptonic CP-violating phase. Future reactor experiments will focus on mass hierarchy determination and the precision measurement of mixing parameters. Mass hierarchy can be determined from the disappearance of reactor electron antineutrinos based on the interference effect of two separated oscillation modes. Relative and absolute measurement techniques have been explored. A proposed experiment JUNO, with a 20 kton liquid scintillator detector of $3%/$$\sqrt{E(MeV)}$ energy resolution, $\sim$ 53 km far from reactors of $\sim$ 36 GW total thermal power, can reach to a sensitivity of $Δχ^{2}>16$ considering the spread of reactor cores and uncertainties of the detector response. Three of mixing parameters are expected to be measured to better than 1% precision. There are multiple detector options for JUNO under investigation. The technical challenges are new type of PMTs with high efficiency and highly transparent liquid scintillator. Funding has been approved from Chinese Academy of Sciences. A similar proposal was from Korea known as RENO-50. Both of them are going to start data taking around 2020.

Motivation & Objective

• To determine the neutrino mass hierarchy using reactor antineutrinos via precision energy spectrum measurements.
• To achieve sub-1% precision in measuring three neutrino mixing parameters.
• To develop and validate self-calibration techniques to correct for energy nonlinearity in liquid scintillator detectors.
• To support the global effort in measuring leptonic CP violation by providing a clean, CP-phase-independent measurement of mass hierarchy.
• To advance detector technology through high-efficiency PMTs and highly transparent liquid scintillator.

Proposed method

• Utilize a 20-kiloton liquid scintillator detector with 3% energy resolution at 1 MeV to measure the energy spectrum of reactor electron antineutrinos.
• Apply Fourier transform to the L/E spectrum (baseline over energy) to enhance oscillation frequency information and identify interference peaks from Δm²₃₁ and Δm²₃₂.
• Implement self-calibration using redundant oscillation cycles to correct for energy nonlinearity caused by quenching and Cherenkov effects.
• Employ a dual-trigger system using prompt positron and delayed neutron capture signals to identify inverse beta decay events.
• Use microchannel plate photomultiplier tubes (MCP-PMTs) with 35% quantum efficiency and near-4π acceptance to maximize light collection.
• Apply advanced purification techniques to achieve an attenuation length of 24 m at 430 nm in the liquid scintillator.

Experimental results

Research questions

• RQ1 Can the neutrino mass hierarchy be determined with high significance using medium-baseline reactor antineutrino experiments?
• RQ2 What level of energy resolution and detector coverage is required to achieve Δχ² > 16 for mass hierarchy sensitivity?
• RQ3 How can energy nonlinearity in liquid scintillator detectors be corrected without external calibration?
• RQ4 To what extent can the precision of mixing parameters be improved beyond current reactor experiments?
• RQ5 What technical advancements are necessary to achieve 3% energy resolution and high light collection efficiency in large-scale liquid scintillator detectors?

Key findings

• JUNO achieves a sensitivity of Δχ² > 16 for neutrino mass hierarchy determination in the ideal case of a single reactor and detector, based on interference in the oscillation spectrum.
• The experiment is expected to measure three mixing parameters with better than 1% precision, significantly improving current measurements.
• Self-calibration using multiple oscillation cycles reduces the impact of energy nonlinearity, preserving sensitivity even with imperfect energy resolution.
• The use of MCP-PMTs with 35% quantum efficiency and reflection photocathodes increases light collection efficiency, enabling 80% PMT coverage with 20" tubes.
• The liquid scintillator achieves an attenuation length of 24 m at 430 nm after purification, supporting high light yield and energy resolution.
• Accidental background is estimated at less than 10% of IBD signals and can be precisely measured, while cosmic muon backgrounds are suppressed below 1% with veto systems.