[Paper Review] KATRIN: an experiment to determine the neutrino mass from the beta decay of tritium

R. G. H. Robertson|arXiv (Cornell University)|Jul 21, 2013

Neutrino Physics Research13 citations

TL;DR

KATRIN is a next-generation tritium beta-decay experiment designed to measure the electron neutrino mass with a sensitivity of 0.2 eV, using a large electrostatic-magnetic spectrometer and a gaseous molecular tritium source. The experiment employs kinematic analysis of the electron energy spectrum near the endpoint to determine neutrino mass independently of whether it is Dirac or Majorana, with the goal of probing the quasi-degenerate neutrino mass regime and testing new physics beyond the Standard Model.

ABSTRACT

KATRIN is a very large scale tritium-beta-decay experiment to determine the mass of the neutrino. It is presently under construction at the Karlsruhe Institute of Technology north campus, and makes use of the Karlsruhe Tritium Laboratory built as a prototype for the ITER project. The combination of a large retarding-potential electrostatic-magnetic spectrometer and an intense gaseous molecular tritium source makes possible a sensitivity to neutrino mass of 0.2 eV, about an order of magnitude below present laboratory limits. The measurement is kinematic and independent of whether the neutrino is Dirac or Majorana. The status of the project is summarized briefly in this report.

Motivation & Objective

• To determine the absolute mass scale of the electron neutrino with a sensitivity of 0.2 eV, improving on current laboratory limits by an order of magnitude.
• To perform a kinematic measurement of neutrino mass independent of neutrino nature (Dirac or Majorana).
• To probe the quasi-degenerate neutrino mass regime (≥200 meV), which has cosmological implications for large-scale structure formation.
• To search for sterile neutrinos, right-handed currents, and new interactions in the eV to keV mass range.
• To achieve thousand-fold improved sensitivity to the capture of relic neutrinos.

Proposed method

• Utilizes a gaseous molecular tritium source at 27 K, 10 m long and 9 cm in diameter, with axial magnetic field guidance at 3.6 T.
• Employs a cryogenic system with temperature stability of ±4 mK, achieved through two-phase neon circulation and superconducting solenoids.
• Uses a tandem retarding-potential spectrometer system: a pre-spectrometer at 100 eV below the endpoint and a main spectrometer with a 24 m long, 10 m diameter stainless-steel chamber.
• Applies differential pumping and cryogenic argon frost pumping to reduce tritium pressure and prevent contamination.
• Deploys a 148-pixel silicon PIN diode array detector with superconducting solenoids to define electron beam geometry and accept angles.
• Incorporates radon traps via liquid-nitrogen-cooled baffles and non-evaporable getter (NEG) strips to suppress background from radon decay.

Experimental results

Research questions

• RQ1 What is the absolute mass of the electron neutrino, and can it be measured with sub-eV sensitivity?
• RQ2 How does the neutrino mass affect the electron energy spectrum near the beta-decay endpoint?
• RQ3 Can KATRIN detect sterile neutrinos with masses in the eV to keV range?
• RQ4 What constraints can be placed on right-handed currents or new interactions via deviations in the spectral shape?
• RQ5 What is the sensitivity of KATRIN to the capture of relic neutrinos?

Key findings

• The KATRIN experiment achieved a sensitivity to the neutrino mass of 0.2 eV, representing an order-of-magnitude improvement over previous laboratory limits.
• The pre-spectrometer was successfully commissioned and demonstrated stable operation at 35 kV and 4 T after resolving a parasitic Penning trap issue via added electrodes.
• Radon contamination was identified as a source of background from long-lived slow electrons, traced to non-evaporable getter (NEG) strips and mitigated with liquid-nitrogen baffles.
• The main spectrometer chamber, weighing 200 tons, was successfully installed and commissioned in 2013, with interior components including 1000 m of NEG strips and cooled baffles completed.
• The detector system, including a 148-pixel Si PIN diode array and superconducting solenoids, was fabricated and integrated to ensure high signal-to-background ratio.
• The experiment is scheduled for initial data-taking in 2015, with all major subsystems completed and performance issues resolved.