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Neutrino mass measurement

Neutrinos are the most numerous massive particles in the Universe. The oscillation experiments proved that the neutrinos created in weak interaction processes are a quantum superposition of three states with the masses m1, m2 and m3. The experiments also proved that the mass of at least one of these states is larger than 0.05 eV. Precision measurements of the uppermost part of the tritium beta spectrum yielded the upper bound of the electron antineutrino mass mν < 2 eV. This is a model independent result relying on the laws of energy and momentum conservation only. Cosmological observations provide the sum m1 + m2 + m3 < 0.7 eV but substantially larger or smaller values can be obtained in dependence on applied models and data sets. Assuming that the neutrinoless double beta decay exists in Nature, the measurement of its half-life enables to estimate the neutrino mass, too. The result, however, strongly depends on the nuclear models predictions.

Aiming to bring the measurement of the tritium beta spectrum to current technological limits, the international project KATRIN (the Karlsruhe Tritium Neutrino experiment, http://www.katrin.kit.edu) was founded in 2001. The founding members are physicists from Germany, Russia, USA and the NPI of the Czech Academy of Sciences. The experiment strives for a tenfold improvement in the neutrino mass sensitivity up to 0.2 eV. In comparison with the best previous experiments this requires a hundred times increase of the beta particle intensity and five times better energy resolution while keeping the lowest possible background.

The KATRIN measuring complex is installed at the Karlsruhe Institute of Technology (Germany) that enables to utilize the unique tritium laboratory of that institute. The laboratory supplies the gaseous molecular tritium into a 16 m long windowless tritium source that with its five hundred sensors ‒ that together with temperature of -243°C stabilized to ±0.03°C ‒ is apparently the most complex cryostat built up to now. The tritium source provides 1011 beta particles per second the movement of which is governed by almost thirty superconducting magnets.

The energy analysis of beta particles is carried out by an electrostatic retardation spectrometer with adiabatic magnetic collimation (the MAC-E-filter). This principle allows analyzing 18% of beta particles emitted into a full solid angle with the outstanding resolution of 0.93 eV at 18.6 keV. In order to achieve these parameters the size of the main spectrometer had to be enlarged to 23 m in length and 10 m in diameter. The spectrum is taken point-by-point by stepwise changes of the retarding voltage in the region of 18.6 kV that has to be stabilized with a relative precision of ±3x10-6.

The low-energy part of the beta spectrum that carries no information about the neutrino mass is removed by a smaller pre-spectrometer of the same type. An integral part of the KATRIN complex is a monitoring spectrometer that independently checks the energy scale stability of the main spectrometer by continuous measurement of an appropriate conversion electron line (Fig. 1.). At the end of the 70 m long KATRIN complex, there is a semiconductor detector of electrons composed of 148 independent pixels.

Fig. 1. The spectrum of monoenergetic electrons emitted from the K atomic shell of gaseous krypton in internal conversion of the 32 keV nuclear transition in 83mKr and measured with the KATRIN main spectrometer. A function, the integral of which describes the expected shape of the spectrum, is shown by dashed line. The statistically reliable fit is depicted by the full line.

In the frame of the KATRIN project, we are responsible for checking the stability of the main spectrometer energy scale as well as for the verification of operation the whole system, in particular that of the tritium source. To achieve this goal we have developed radioactive sources of monoenergetic conversion electrons that are based on the decay chain 83Rb/83mKr/83Kr. The sources are of two types: a solid source with outstanding energy stability of 0.5 ppm/month for the monitoring spectrometer and a gaseous source with activity of 1 GBq for other purposes. Required 83Rb is produced with the cyclotron of our Institute. An interview about the Czech road to KATRIN can be found here.

The KATRIN inauguration took place in June, 2018. Shortly prior to this event the first beta spectrum was measured with a small amount of tritium (Fig. 2.). The circulation of our gaseous 83mKr in tritium loops was successfully tested in October, 2018. During the spring of 2019, 83mKr will circulate together with tritium in order to explore a possible space charge in the beta particle source. Then, a systematic measurement of tritium beta spectrum for the neutrino mass determination will start. The full sensitivity of 0.2 eV should be achieved after 1000 measuring days, i.e. within five calendar years.

Fig. 2. The upper part of the beta spectrum of a trial amount of gaseous tritium measured with the KATRIN main spectrometer. Full line denotes fit of the expected spectrum shape assuming zero neutrino mass.

 

Czech members of the KATRIN team

Scientists

  • Mgr. Drahoslav Vénos, CSc. (Head of the group)
  • Ing. Otokar Dragoun, DrSc.
  • Ing. Alojz Kovalík, DrSc.
  • doc. Ing. Ondřej Lebeda, Ph.D.
  • RNDr. Miloš Ryšavý, CSc.

Postdoc

  • Ing. Martin Suchopár, Ph.D.

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