NEOS: Reactor neutrino experiment at short baseline 11 Sep @ TAUP - - PowerPoint PPT Presentation

NEOS:

Reactor neutrino experiment at short baseline

11 Sep @ TAUP 2019, Toyama Yoomin Oh

Yoomin Oh / NEOS TAUP 2019, Toyama

Outline

• Reactor neutrino
• Anomalies
• Short baseline (SBL)
• scillation
• Introduction/status of


the NEOS experiment

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2019.09.09

Yoomin Oh / NEOS TAUP 2019, Toyama

Reactor neutrinos and inverse beta decay

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In the detector

Neutrino Energy [MeV]

2 3 4 5 6 7 8

]

• 1

MeV

• 1

Flux [fission

0.0 0.2 0.4 0.6 0.8 1.0

]

2

cm

• 42

IBD cross section [10

1 2 3 4

ν̅ e p n e+ prompt delayed n-capture by (H, Gd, 6Li, …)

φ × σ

Huber, Mueller x Vogel

• Average of 6-7 β-decays and anti-v’s from a fission of an element.
• 2 x 1020 v / 4π / GWth

• O(10k) inverse beta decay events occur in 1000 kg of organic

scintillator with 10% mass of hydrogen atom at 10 m distance from a gigawatts-thermal-power reactor.

Yoomin Oh / NEOS TAUP 2019, Toyama

Anomalies and a new state?

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LSND (2001) MiniBooNE (2013) Gallex/SAGE (2009) white paper (2012) Gariazzo (2016) Mention (2011)

mixing with a ~1eV state?!

Yoomin Oh / NEOS TAUP 2019, Toyama

Miscalculated flux?

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• Disagreement in rate and shape, at hundreds
• f meters distances from LEU reactors.
• Not likely an energy scale problem.
• Should be checked with new SBL data.

RENO (2019) DChooz (v-fact ’19)

Yoomin Oh / NEOS TAUP 2019, Toyama

Reactor SBL oscillation

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P(¯ νe → ¯ νe) ∼ 1 − 22θ142

• 1.27Δm2L

Eν

• 2 ·
• Data / MC (3v)

sin2 2θ (1)

Antineutrino energy (MeV) 1.8

Δm2

10 Reconstructed prompt energy (MeV) ~1.0 10

*illustration

• Oscillation pattern smeared out by: distance / energy resolution,

uncertainties in reference spectrum, statistics, background, …

Yoomin Oh / NEOS TAUP 2019, Toyama

NEOS

• Neutrino Experiment for Oscillation at Short baseline,
• Test of RAA and search for active-to-sterile neutrino oscillation with

Δm2~ 1 eV2,

• Project launched in 2011,
• Using a commercial reactor core and 1000 L of homogeneous Gd-

loaded liquid scintillation detector,

• Phase-I data taking: Aug 2015 ~ May 2016,


Phase-II data taking: Sep 2018 ~ Now (~ Summer 2020),

• 20 collaboration members from 7 institutes.

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Yoomin Oh / NEOS TAUP 2019, Toyama

Reactor site

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• Hanbit NPP in Yeonggwang (靈光, ghost light), Korea,
• Distance between neighboring cores: 256 m (less than 1% contribution

from each of them),

• Same reactor complex used for RENO experiment

1

2

3

4

5

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Yoomin Oh / NEOS TAUP 2019, Toyama

Hanbit-5 reactor

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• OPR-1000 reactor, 2815 MWth,
• 177 fuel rods, low enriched (~4%) uranium-235 (LEU) fuel,
• Refueling by changing 1/3 of fuel rods for each burn-up cycle (~1.5 years),
• Active core size: 3.1 m (φ) x 3.8 m (h).

Burn-up [MTU/MWD]

5000 10000 15000

Fission Fraction

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

U-235 U-238 Pu-241 Pu-239

Yoomin Oh / NEOS TAUP 2019, Toyama

Tendon gallery

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• Not a radioactivity controlled area: no background related to reactor operation,
• Muon rate: about 1/5 of surface (~0.17 μ/cm2/min),
• Maintenance work every 5 years.

< L > = 2 3 . 7 m 8 m 3 m

3.3 +0.6 m

Yoomin Oh / NEOS TAUP 2019, Toyama

Environmental condition

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Yoomin Oh / NEOS TAUP 2019, Toyama

NEOS detector

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Homogeneous 1000 L (Φ103 x L121 cm) volume, 0.5% Gd-doped LS, 90% LAB+ 10% UG-F, seen by 2 x 19 8-inch PMTs, PTFE reflector on inner walls. 10 cm thick B-PE, 10 cm thick Pb for passive shieldings. Muon counter: 3(5)-cm thick plastic scintillator panels surround the most outside except for bottom. Data acquisition with

• 500 mega-sampling/sec FADC for target PMTs,
• 62.5 mega-sampling/sec ADC for muon counter PMTs.

Slow monitoring: temperatures, radon level, PMT HVs.

Yoomin Oh / NEOS TAUP 2019, Toyama

Phase-I result

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• LEU reactor, 24 m distance.
• The 5 MeV bump is there.
• Found no strong evidence of

active-to-sterile oscillation, compared to Daya Bay spectrum.

PRL 118 (2017) 121802

Yoomin Oh / NEOS TAUP 2019, Toyama

NEOS phase-II

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• Shape + rate analysis for sterile neutrino search, and/or,
• Precision measurement of the spectrum itself.
• Spectrum evolution with the fission fraction change.
• Measuring a full operation cycle (~500 calendar days) + two

background periods before/after the cycle (~100 days).


• Phase-I: 46 days OFF + 180 days ON as DAQ livetime.
• Same detector, same reactor and same baseline as in phase-I.

• Newly produced Gd-LS,

• Minor modifications: leak-proof maintenance, muon counter plastic

scintillator.

• Data taking started in September 2018.

Yoomin Oh / NEOS TAUP 2019, Toyama

NEOS phase-II

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• Not a dramatic improvement of eV

sterile neutrino search sensitivity,

• Decomposition of U/Pu spectra,

benefitted by large fission fraction changes in a single LEU reactor.

2 4 6 8

Prompt Energy [MeV]

20 40

Entries /day/MeV

Huber-235 Huber-239 σ 1 ± Decomposed 235 σ 1 ± Decomposed 239 Example 235 Example 239

Yoomin Oh / NEOS TAUP 2019, Toyama

NEOS phase-II

16 — NEOS-I — NEOS-II (Work in progress)

• On about half of planned

data taking:

• the second half of the fuel

cycle and another maintenance period (~100 days) to measure.

• measurement will be

completed in Summer 2020.

• Monte Carlo simulation is

being revised:

• light-energy non-linearity,
• n-Gd capture, etc.

Yoomin Oh / NEOS TAUP 2019, Toyama

Summary

• NEOS measures the inverse beta decay of the reactor electron

antineutrinos.

• Using a 2.8 GW commercial nuclear power reactor at 24 m distance.
• Homogeneous, 1000 L of Gd-loaded liquid scintillator target
• Precision measurement of the reactor v spectrum.
• Spectrum evolution to find the origin of the reactor anomaly.
• Testing of active-to-sterile neutrino oscillation for Δm2~1 eV2.
• We are in the middle of phase-II measurement.

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