This is the fifth article in the Detector Deep Dives series. Where previous parts covered detectors that count neutrino events, this part covers a detector that doesn’t directly detect neutrinos at all. It measures the electrons that come out of a particular beta decay, with such extreme precision that the neutrino mass can be inferred from the spectrum shape. The detector: KATRIN, the Karlsruhe Tritium Neutrino experiment.
The physics
Tritium (³H) beta-decays to ³He, an electron, and an electron antineutrino: $$^3\text{H} \to {}^3\text{He}^+ + e^- + \bar\nu_e$$
The 18.6 keV of energy released is shared between the electron and the antineutrino. The exact partitioning is random event-by-event, but the shape of the electron energy spectrum at its high-energy endpoint depends on the neutrino mass.
If the neutrino were exactly massless, the electron spectrum would extend smoothly to exactly 18.6 keV. If the neutrino has mass m, the spectrum must turn over and end at $E_0 - mc^2$, with a characteristic curvature in the last few electron-volts before the endpoint.
KATRIN’s job: measure the shape of the electron spectrum in the last ~30 eV near the endpoint precisely enough to extract that curvature.
The detector — beamline overview
KATRIN is not a “detector” in the usual sense; it is a 70-meter-long beamline with five main stages:
1. Windowless gaseous tritium source (WGTS): A 16-meter-long tube held at 30 K with a small steady flow of gaseous T₂ at ~10 mbar. About 10¹¹ beta decays per second occur inside. The source is “windowless” because any solid window would distort the electron spectrum. The temperature stability of the WGTS is held to better than 30 mK across the 16-meter tube.
2. Differential and cryogenic pumping: Tritium gas must be removed from the beam before reaching the spectrometer. A multi-stage pumping system removes gas by factors of ~10¹⁴ while letting beta electrons through.
3. Pre-spectrometer: A smaller spectrometer that rejects most low-energy electrons.
4. Main spectrometer (MAC-E filter): The signature element of KATRIN. A vessel 10 meters in diameter and 23 meters long maintained at vacuum better than 10⁻¹¹ mbar. Inside, a calibrated electric retarding potential combined with strong magnetic fields filters electrons by energy with sub-eV resolution.
5. Focal-plane detector: A 148-pixel silicon detector that counts the electrons that make it through.
The MAC-E (Magnetic Adiabatic Collimation with Electrostatic) filter is the key technology. Magnetic-field guidance turns the electron’s transverse motion into longitudinal motion, allowing a single retarding voltage to filter by total energy with resolution of about 0.93 eV at 18.6 keV.
How it works in practice
KATRIN scans the retarding voltage in fine steps near 18.6 keV. At each step, electrons with energy above the threshold pass; electrons below are rejected. The count rate vs voltage traces out the integrated spectrum’s last few electron-volts.
A non-zero neutrino mass would produce a characteristic dropoff before the kinematic endpoint. The fit returns the effective electron neutrino mass squared $m_\beta^2 = \sum_i |U_{ei}|^2 m_i^2$ — the incoherent sum over the mass states weighted by the PMNS matrix.
The systematic challenge
Subtleties that have to be controlled at sub-eV scale:
- Source dynamics: Vibrational and rotational excitations of the daughter ³He molecule.
- Energy losses: Inelastic scattering of electrons inside the source.
- Source-volume systematics: Density, temperature, and tritium isotopic purity.
- Field stability: Voltage stability of the retarding potential to better than 60 mV.
- Background characterization: Cosmic-ray-induced and material-induced backgrounds at the focal-plane detector.
Each of these is a major experimental program in itself. The collaboration has spent over two decades on systematic preparation.
The results so far
2019 (first physics run, 4 weeks of data): m_β < 1.1 eV. Already a factor-of-2 improvement over the previous Mainz/Troitsk record from 2003.
2022 (combined data): m_β < 0.8 eV.
2024: m_β < 0.45 eV (90% CL). World-leading direct limit.
Target by 2026-2027: m_β < 0.2 eV at design sensitivity, after the full statistics campaign.
If the neutrino is at or near the cosmological mass scale (~50-100 meV), KATRIN won’t see it — direct kinematic measurement isn’t sensitive there. But if the absolute mass is closer to the quasi-degenerate ~0.2 eV scale, KATRIN could detect it.
TRISTAN: the upgrade
After the main neutrino-mass run finishes, KATRIN’s beamline will be re-purposed. The TRISTAN upgrade instruments the focal-plane detector with new silicon drift detectors capable of full spectral measurement (not just integrated rate). TRISTAN’s primary target: a sterile-neutrino search at the keV mass scale. Sterile neutrinos at keV mass would leave a kink in the full tritium beta spectrum, observable as a discontinuity in the differential rate.
Beyond TRISTAN: ideas for atomic tritium experiments using cooled and trapped tritium atoms instead of T₂ molecules, eliminating molecular final-state systematics and potentially reaching meV-scale sensitivity.
Complementary approaches
Project 8: Cyclotron radiation emission spectroscopy. Tritium gas in a magnetic trap; each electron’s cyclotron radiation reveals its energy. Demonstrated proof-of-principle. Long-term goal: meV-scale sensitivity.
HOLMES and ECHo: Cryogenic bolometers measuring the electron-capture spectrum of holmium-163. Independent systematics from tritium-based experiments.
Cosmological constraints: CMB-S4 should reach 30-meV-scale precision on Σm_ν by the early 2030s.
All four approaches probe different aspects of the absolute mass scale. KATRIN remains the dominant direct laboratory measurement.
The bottom line
KATRIN is the rare detector built around a single very precise question: what is the absolute mass scale of the lightest neutrino? Decades of careful systematic preparation, a 70-meter beamline, a 10-meter spectrometer at parts-per-billion vacuum, and the patience to scan retarding voltage steps thousands of times — that’s what it takes to weigh a particle that’s at most a few hundred milli-electron-volts.
The next part of this series turns to a very different kind of detector, designed not to weigh neutrinos but to study how they oscillate over thousand-kilometer distances: DUNE.
Frequently asked
What does KATRIN actually measure?
The shape of the tritium beta decay electron energy spectrum near its endpoint at 18.6 keV. The neutrino mass distorts this shape — heavier neutrinos pull the endpoint downward and bend the spectrum in a calculable way. KATRIN measures the spectrum to sub-eV precision and extracts the effective electron neutrino mass m_β.
What is the current KATRIN limit?
As of 2025, KATRIN reports an effective electron neutrino mass below 0.45 eV at 90% confidence — the world's tightest direct laboratory limit. The collaboration aims to reach the project goal of about 0.2 eV sensitivity by 2026-2027 after full statistics, then transition to its successor program TRISTAN.
How is KATRIN different from cosmological mass constraints?
KATRIN measures only kinematics — the recoil of the electron in beta decay. It makes no cosmological or particle-physics assumptions about how neutrinos couple to anything else. Cosmological measurements give roughly Σm_ν < 70-100 meV but depend on the assumed cosmological model. KATRIN is the cleanest direct, model-independent measurement available.
Where is KATRIN located?
At the Karlsruhe Institute of Technology (KIT) in southwestern Germany. The KATRIN beamline runs across the KIT campus connecting the windowless gaseous tritium source, a series of differential pumping and cryotrapping stages, and the giant main spectrometer at the far end.
Cite this article 5 formats
APA
Neutrino Times Editorial Team. (2026, February 16). Detector Deep Dives — Part 5: KATRIN, the world's most precise neutrino mass scale. Neutrino Times. https://neutrino-times.com/articles/detector-deep-dives-part-5-katrin/
Chicago
Neutrino Times Editorial Team. "Detector Deep Dives — Part 5: KATRIN, the world's most precise neutrino mass scale." Neutrino Times, February 16, 2026. https://neutrino-times.com/articles/detector-deep-dives-part-5-katrin/.
MLA
Neutrino Times Editorial Team. "Detector Deep Dives — Part 5: KATRIN, the world's most precise neutrino mass scale." Neutrino Times, 16 Feb. 2026, https://neutrino-times.com/articles/detector-deep-dives-part-5-katrin/.
BibTeX
@misc{neutrino-times-detector-deep-dives-part-5-katrin,
author = {Neutrino Times Editorial Team},
title = {Detector Deep Dives — Part 5: KATRIN, the world's most precise neutrino mass scale},
howpublished = {Neutrino Times},
year = {2026},
month = {feb},
url = {https://neutrino-times.com/articles/detector-deep-dives-part-5-katrin/},
note = {Accessed: 2026-02-16}
} RIS
TY - GEN TI - Detector Deep Dives — Part 5: KATRIN, the world's most precise neutrino mass scale AU - Neutrino Times Editorial Team PY - 2026 DA - 2026-02-16 PB - Neutrino Times UR - https://neutrino-times.com/articles/detector-deep-dives-part-5-katrin/ ER -