[Paper Review] Double Calorimetry System in JUNO

Miao He|arXiv (Cornell University)|Jun 27, 2017

Neutrino Physics Research2 references3 citations

TL;DR

JUNO proposes a double calorimetry system using 25,000 3-inch photomultiplier tubes (PMTs) alongside 18,000 20-inch PMTs to achieve sub-percent energy resolution. The small PMTs operate in photon-counting mode, enabling precise calibration of the large PMT system’s nonlinear response, thereby controlling systematic uncertainties and improving energy resolution for neutrino mass hierarchy measurement and high-energy physics.

ABSTRACT

The Jiangmen Underground Neutrino Observatory (JUNO) is a multipurpose neutrino-oscillation experiment, with a 20 kiloton liquid scintillator detector of unprecedented 3\% energy resolution (at 1 MeV) at 700-meter deep underground. There are ~18,000 20-inch photomultiplier tubes (PMTs) in the central detector with an optical coverage greater than 75%. Control of the systematics of the energy response is crucial to archive the designed energy resolution as well as to reach 1% precision of the absolute energy scale. The detected number of photoelectrons in each PMT differs by two orders of magnitude in the reactor antineutrino energy range in such a large detector, which is a challenge to the single channel charge measurement. JUNO has approved a new small-PMT system, including 25,000 3-inch PMTs, installed alternately with 20-inch PMTs. The individual 3-inch PMT receives mostly single photoelectrons, which provides a unique way to calibrate the energy response of the 20-inch PMT system by a photon-counting technology. Besides, the small-PMT system naturally extends the dynamic range of the energy measurement to help the high-energy physics, such as cosmic muons and atmospheric neutrinos. We will present the physics concept of this double calorimetry, the design and implementation of the 3-inch PMT and its readout electronics system.

Motivation & Objective

• To achieve 3% energy resolution at 1 MeV in the JUNO liquid scintillator detector by controlling systematic uncertainties in the energy response.
• To address the challenge of nonlinear charge response in large 20-inch PMTs, which vary by two orders of magnitude across the reactor antineutrino energy range.
• To extend the dynamic range of energy measurement for high-energy physics applications such as cosmic muons and atmospheric neutrinos.
• To implement a dual-PMT system that combines stochastic (20-inch PMTs) and non-stochastic (3-inch PMTs) response control for improved energy resolution.
• To calibrate the 20-inch PMT energy scale with sub-percent precision using photon-counting from 3-inch PMTs

Proposed method

• Install 25,000 3-inch PMTs (XP72B22) in the gaps between 20-inch PMTs to form a dual-layer PMT system.
• Use the 3-inch PMTs in photon-counting mode, where >98% detect only single photoelectrons in the reactor antineutrino energy range.
• Employ a multichannel front-end ASIC (CATIROC) for high-bandwidth, dead-time-free readout of charge and time information from the 3-inch PMTs.
• Apply the total charge measured by the 3-inch PMTs as a reference to calibrate the nonlinear charge response of the 20-inch PMT system.
• Use the 3-inch PMT system to correct for non-uniformity in energy response due to variable event vertex positions in the detector.
• Integrate the 3-inch PMT system with a custom underwater box and power/data transmission system for long-term stability and low background

Experimental results

Research questions

• RQ1 How can the nonlinear charge response of large 20-inch PMTs be calibrated to sub-percent precision in a large-scale liquid scintillator detector?
• RQ2 Can a secondary small-PMT system operating in photon-counting mode provide a reliable reference for calibrating the energy response of the primary large PMT system?
• RQ3 To what extent does the double calorimetry system reduce systematic uncertainties in energy resolution for neutrino oscillation measurements?
• RQ4 How does the inclusion of 3-inch PMTs extend the dynamic range of energy measurement for high-energy physics beyond reactor antineutrinos?
• RQ5 What are the performance characteristics of the HZC XP72B22 3-inch PMTs under JUNO’s stringent requirements for timing, resolution, and radioactivity?

Key findings

• The 3-inch PMTs (XP72B22) achieve a single photoelectron resolution of 35% ± 2%, demonstrating excellent uniformity across samples.
• The time walk of single photoelectrons is less than 5 ns FWHM, meeting JUNO’s timing requirements for high-precision energy reconstruction.
• The pre- and after-pulse charge ratios are below 5% and 15%, respectively, minimizing spurious signal contributions.
• The nonlinearity of the 3-inch PMTs is less than 10% across the 1–100 photoelectron range, ensuring accurate photon counting.
• The 3-inch PMTs exhibit low dark rates (1,000 Hz at 0.25 PE) and ultra-low radioactivity, with 238U, 232Th, and 40K below 400 ppb, 400 ppb, and 200 ppb, respectively.
• The double calorimetry system is expected to enable 3% energy resolution at 1 MeV with systematic uncertainty below 1%, critical for determining the neutrino mass hierarchy