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First measurement of flux of the neutron background induced by accelerated neutrinos at the J-PARC facility

This paper reports the first measurement of the neutron background flux induced by the neutrino beam at the J-PARC facility, utilizing BGO and liquid scintillation detectors to determine a flux of approximately $1.45 \times 10^{-7}~\rm{cm}^{-2}\rm{s}^{-1}\rm{POT}^{-1}$ based on $2.972 \times 10^{20}$ protons on target.

Original authors: Hiroshi Ito, Ryo Shibayama, Chisako Ise, Akihiro Minamino, Yusuke Koshio, Masaki Ishitsuka

Published 2026-07-07

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CC BY 4.0

Original authors: Hiroshi Ito, Ryo Shibayama, Chisako Ise, Akihiro Minamino, Yusuke Koshio, Masaki Ishitsuka

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine you are trying to listen to a very quiet whisper (a neutrino) in a room that is already filled with the roar of a crowd (background noise). That is the challenge scientists face when studying how neutrinos interact with atomic nuclei.

This paper is about a team of scientists at the J-PARC facility in Japan who decided to measure just how loud that "crowd roar" actually is, specifically the part caused by neutrons.

Here is the story of their experiment, broken down into simple concepts:

1. The Problem: The "Ghost" in the Machine

Neutrinos are tiny, ghost-like particles that rarely interact with anything. When they do hit an atomic nucleus, they can bounce a proton or a neutron off it. Scientists want to study these bounces to understand the secrets of the nucleus.

However, there is a problem. The area where these experiments happen is surrounded by thick walls made of sand and concrete. When the powerful neutrino beam hits the beam dump (a wall designed to stop the beam), it accidentally creates a shower of neutrons in that sand and concrete.

These stray neutrons wander into the experimental area and hit the detectors, creating "fake" signals that look exactly like the neutrino interactions scientists are trying to study. It's like trying to hear a whisper while someone is constantly dropping marbles on the floor nearby.

2. The Mission: Counting the Marbles

The scientists wanted to answer a simple question: How many of these stray neutrons are actually getting into our detectors?

To find out, they set up a "listening post" right next to the neutrino beam path. They didn't try to catch the neutrinos this time; they just wanted to count the accidental neutrons.

3. The Tools: A Specialized Detective Kit

They used a three-part detective kit to catch these neutrons:

The Plastic Shield (The Veto): Imagine a security guard at the door. This plastic detector sits in front of everything. If a charged particle (like a muon) tries to walk in, the guard shouts "Stop!" and ignores it. This ensures they only count the quiet, neutral neutrons.
The Crystal Eyes (BGO Detectors): These are heavy crystals that glow when hit by particles. They act like a secondary check to see what kind of energy is hitting the system.
The Liquid Trap (Liquid Scintillator): This is the main star of the show. It's a tank filled with a special liquid that glows when a particle hits it.
- The Magic Trick (Pulse-Shape Discrimination): When a neutron hits this liquid, it makes a specific kind of "flicker" in the light. When a gamma ray (a different type of radiation) hits it, the flicker looks different. It's like telling the difference between a drumbeat and a cymbal crash just by listening to the sound. The scientists used this trick to separate the "neutron drumbeats" from the "gamma cymbals."

4. The Experiment: Three Rounds of Testing

They ran the experiment in three different phases (Periods 1, 2, and 3), slightly changing how they arranged their detectors (adding aluminum frames or extra crystals) to make sure their results weren't just a fluke of one specific setup.

They ran the machine for a massive amount of time, hitting the target with a huge number of protons (about 300 quintillion, or $2.972 \times 10^{20}$ ).

5. The Results: Finding the Signal

After filtering out all the noise and the "fake" signals:

They found 88 specific events where a neutron hit a proton in their liquid detector.
These hits happened in a specific energy range (between 0.98 and 11.60 MeV).

By comparing what they saw with computer simulations (which act like a digital twin of their experiment), they calculated the flux (the flow rate) of these neutrons.

The Final Number:
They determined that for every second of beam time, there is a flow of approximately $1.45 \times 10^{-7}$ neutrons per square centimeter for every proton they shot at the target.

6. Why This Matters

The paper concludes that this measurement is a crucial piece of the puzzle. Before this, scientists had to guess how much "neutron noise" was in their data. Now, they have a real, measured number.

This helps them design better experiments in the future. If they know exactly how loud the "marble dropping" is, they can build their detectors in a spot where the noise is quieter, or they can subtract the noise more accurately to hear the "whisper" of the neutrinos clearly.

In short: They built a specialized microphone to measure the background static in a neutrino lab, found out exactly how loud it is, and proved that this noise level stays consistent regardless of how hard they push the beam. This helps future scientists tune their instruments to hear the universe's quietest signals.

Technical Summary: First Measurement of Neutron Background Flux Induced by Accelerated Neutrinos at J-PARC

Problem Statement
Neutrino-nucleus neutral-current quasi-elastic (NCQE) scattering events ( $\nu + N \to \nu + N'$ ) are critical for probing nuclear structure and constraining the axial strange coupling constant ( $g_A^s$ ). However, these events are frequently obscured by a massive background of neutron-nucleus scattering events. These background neutrons are not produced by the neutrino beam directly but are generated within the materials (specifically sand and concrete) located between the beam dump and the experimental area. The lack of precise data on this "accidental" neutron flux complicates the background estimation for current and future neutrino experiments, such as those searching for the diffuse supernova neutrino background (DSNB) or measuring NCQE cross-sections.

Methodology
The authors conducted the J-PARC T101 test experiment to measure this neutron flux directly. The experiment utilized three distinct detector configurations over three periods (February and June 2024) with a total integrated exposure of $2.972 \times 10^{20}$ protons on target (POT).

Detector Setup: The core detection system consisted of:
- Plastic Scintillation Detector (PlasSci): Served as an active veto counter to reject charged particles (muons).
- BGO Detectors: Two Bismuth Germanate (BGO) crystals (cylindrical and cubic) used to monitor potential charged-current interactions and provide context, though the primary flux measurement relied on the liquid scintillator.
- Liquid Scintillation Detector (LiqSci): Filled with BC-501A, this detector was the primary instrument for neutron identification. It utilized Pulse-Shape Discrimination (PSD) to distinguish neutron-induced recoil protons from gamma-ray events.
Data Acquisition: Signals were recorded using a 4-channel, 12-bit digitizer. The analysis focused on "beam-on" periods synchronized with the J-PARC Main Ring spills and subtracted "beam-off" background events.
Event Selection:
- Events were selected within an electron-equivalent recoil energy range of 0.98–11.60 MeVee.
- Charged-particle backgrounds were rejected using the PlasSci veto.
- Neutron events were isolated using PSD cuts calibrated at the RCNP facility with a 65-MeV neutron beam.
Simulation and Fitting: A Geant4.11.1 Monte Carlo simulation was employed. Since the exact production points and energy deposition of neutrons in the surrounding sand were unknown, the authors assumed an exponential neutron energy spectrum ( $\Gamma(E_n) = A \exp(-E_n/\lambda)$ ). The simulation parameters ( $A$ and $\lambda$ ) were re-weighted to fit the experimental data across the three setups.

Key Results

Event Observation: From the $2.972 \times 10^{20}$ POT dataset, the experiment observed 88 neutron-induced recoil proton events after background subtraction and selection cuts.
Flux Determination: By comparing the observed energy spectra with the re-weighted simulations, the neutron flux was determined to be:
$\phi = [1.45^{+0.22}_{-0.24} \text{ (stat.)} \pm 0.55 \text{ (sys.)}] \times 10^{-7} \text{ cm}^{-2} \text{ s}^{-1} \text{ POT}^{-1}$
This calculation assumes an exponential energy spectrum.
Consistency: The measured flux was consistent across all three experimental setups and varying beam powers (650 kW to 800 kW), indicating that the neutron flux per POT is invariant with respect to beam power in the tested range.
Systematic Uncertainties: The dominant systematic uncertainty (36% relative) arose from the PSD distribution calibration. Other contributions included the quenching factor (9%) and energy resolution (8%).

Significance and Claims
The paper presents the first direct measurement of the neutron background flux induced by accelerated neutrinos at the J-PARC facility. The authors state that this result will contribute to the evaluation of neutron backgrounds for neutrino experiments conducted at J-PARC. Specifically, the data provides a baseline for understanding the accidental neutron background that masks NCQE signals. The authors note that while the current data was taken in "neutrino" mode, future work will aim to determine the flux in "anti-neutrino" mode and investigate spatial dependencies within the experimental hall to identify locations where the neutron background is minimized for future short-baseline NCQE experiments.