NeutrinoResearch Hub
Taking data

KATRIN

Karlsruhe Tritium Neutrino Experiment

A 200-tonne electrostatic spectrometer designed to measure the absolute neutrino mass through the shape of the tritium-β endpoint, model-independently and free of cosmological assumptions.

Specifications

Full name Karlsruhe Tritium Neutrino Experiment
Host institution Karlsruhe Institute of Technology (KIT)
Spokesperson C. Weinheimer
Status Taking data
First data 2019
Location Karlsruhe, Germany
Technique MAC-E filter electron spectroscopy of β decay
Channel(s) Tritium β-decay endpoint spectrum
Depth Surface
Fiducial mass / volume Tritium gas in 70-m main spectrometer
Energy range 18.6 keV (T endpoint region)

Sensitivity Final design sensitivity m_β < 0.2 eV / c² (90% C.L.).

Recent result

Combined analysis of KNM-1 to KNM-5 yields m_β < 0.45 eV / c² (90% C.L.) — the tightest direct limit to date (Nature Physics 20, 1112 (2024)).

Detector architecture

KATRIN schematic 70 m beamline. Tritium source on the left, differential and cryogenic pumping, then the dominant 23 m main spectrometer, with the focal-plane detector on the right. The β-electron path is animated. WGTS Tritium source 10 m · ~30 K DPS + CPS ×10¹⁰ pumping pre-spec Main spectrometer 23 m · MAC-E filter retarding-potential adiabatic bundle FPD Detector 148 px Si β-electron path ~70 m total · 0.93 eV resolution at 18.6 keV endpoint
Schematic of the KATRIN detector.

KATRIN is a high-resolution MAC-E (Magnetic Adiabatic Collimation with Electrostatic) filter at the Karlsruhe Institute of Technology (KIT). Tritium gas circulates through a 10 m windowless source (WGTS) at ~30 K. β-electrons are guided along magnetic field lines through differential and cryogenic pumping sections that suppress tritium contamination by ten orders of magnitude before they enter the spectrometer.

The 23 m-long main spectrometer combines a magnetic field that drops by 20,000× along its central axis with an adjustable retarding potential. Only electrons with kinetic energy above the retarding potential reach the focal-plane detector — a 148-pixel Si PIN array. The energy resolution is 0.93 eV at the tritium endpoint of 18.6 keV.

Scientific case

The shape of the tritium β spectrum near its endpoint is sensitive to the effective electron-neutrino mass mβ2 = Σ |Uei|2mi2. KATRIN was designed to push the model-independent kinematic limit from 2 eV (Mainz/Troitsk) to 0.2 eV — the lower edge of the quasi-degenerate region. Unlike cosmology, the result is free of assumptions about Λ-CDM and dark-energy parameters.

Milestones

  1. 2001 KATRIN Letter of Intent.
  2. 2013 Spectrometer arrived at KIT (the famous "60-tonne tour" through Europe).
  3. 2018 First tritium operation.
  4. 2019 KNM1 result: m_β < 1.1 eV (90% CL).
  5. 2022 KNM1+2 result: m_β < 0.8 eV — first sub-eV kinematic limit.
  6. 2025 Combined KNM1–6 expected to reach the design goal m_β < 0.2 eV.

Recent highlights

The 2024 combined KNM1–5 result tightened the upper limit to mβ < 0.45 eV (90% CL, Nature Physics), already inside the region disfavoured by Planck+BAO under standard cosmology. KATRIN-3rd-Gen analyses include sterile-neutrino searches in the keV mass range as a probe of warm-dark-matter scenarios.

Future plans

TRISTAN: a 21-pixel Si-drift detector array replacing the focal-plane detector to enable a high-rate keV-scale sterile-neutrino search using the full β spectrum.

Project 8 / PTOLEMY are next-generation programmes that aim to follow KATRIN with cyclotron-radiation emission spectroscopy or atomic tritium, targeting mβ ≈ 40 meV — the inverted-ordering threshold.

Collaboration

Collaboration size ≈ 200 members, 20 institutions

Coverage in this issue

References

  1. KATRIN Collab., Nature Physics 20, 1112 (2024)  doi:10.1038/s41567-024-02497-x

Collaboration website ↗

Read next — across the hub

Glossary

Kinematic neutrino mass

The model-independent effective mass measured by tritium β-decay endpoint experiments such as KATRIN.

Formula

Beta-decay endpoint

How KATRIN reads the absolute neutrino mass off the spectrum.

Other experiments