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Electron Antineutrino


Search for Electron Antineutrino Rest Mass in the Tritium b-decay

Tech Area / Field

  • PHY-ANU/Atomic and Nuclear Physics/Physics

8 Project completed

Registration date

Completion date

Senior Project Manager
Bugaev D V

Leading Institute
Russian Academy of Sciences / Institute of Nuclear Research, Russia, Moscow

Supporting institutes

  • VNIIEF, Russia, N. Novgorod reg., Sarov\nAll-Russian Scientific Research Institute of Non-Organic Materials named after A. Bochvar, Russia, Moscow


  • Universität Mainz / Institut für Physik, Germany, Mainz

Project summary

Since 1983 an experiment on the search for electron antineutrino rest mass in tritium beta-decay was being carried out in INK RAS. The experiment consists in precision measurement of high-energy end of a tritium beta-spectrum on the "Troitsk v-mass" set-up with use of a new type spectrometer - electrostatic spectrometer with- adiabatic magnetic collimation to search for deviations in the shape of a spectrum, showing the presence of rest mass of electron antineutrino. As an electron source the molecular gaseous tritium is used.

The round of measurements, carried out in 1994 - 1996 allowed to obtain the most accurate result on electron antineutrino rest mass limit and simultaneously revealed of bump like excess of intensity at about 7 - 12 eV below the end point with integral branching ratio about 6 ґ 10-11 which generates "negative squared neutrino mass". The nature of the process involved is still unknown and looks very curious because it may be connected with unusual properties of the low energy neutrino The detected anomalies have caused significant interest of scientists (see, for example, materials of a conferences HEP-94 in Glasgow, Neutrino-96 in Helsinki, Phys.Lett., В 350 (1995) 263 - 272) and urgently require further study.

In "Troitsk v-mass" installation the electron moving in a strong longitudinal magnetic field which downstream considerably decreases in strength and then increases again. As the electrons spiral down the field lines this strength transition causes a transfer of the energy associated with transverse momentum component into longitudinal one that is further analyzed in integral mode. (Nucl. Instrum. Meths. A240, 305 (1985)).

The energy of the integral electrostatic spectrometer which is a cylindrical electrode placed in the area of minimum magnetic field is determined by the ratio of a field strength between the bottle neck and the central part. Strength ratio of about 6 ґ l03 is used in Troitsk installation. The small semiconductor detector in one of a bottle necks detects only electrons born in a magnetic flux tube crossing the detector. Such spectrometer allows achieve the energy resolution of 3 eV (full width) and it can be matched with a tritium gaseous windowless beta-electron source.

The source present a pipe placed at a longitudinal magnetic field coming up to 4 - 5 Tesla. Magnetic field of the source continuously passes in the field of spectrometer. Tritium is injected in the center of the pipe and is pumped out by mercury diffusion pumps on both ends of it. Electrons produced in the pipe are transported adiabatically along magnetic force lines to the spectrometer and tritium after diffusion pumps and purification comes to the input in a continuous circulation mode.

The result of measurement of high-energy end of a tritium beta-spectrum already obtained in INK RAS corresponds to neutrino mass limit MVC2 < 3.9 eV at 95% confidence level (Published in Proceedings of International Conference Neutrino-96). This result is at the moment best upper limit for the neutrino mass from weak decay kinematics. The main purpose of this project is modernization of the electron source and spectrometer and realization of measurements to find an origin of anomalies in a spectrum of tritium decay and to improve a limit on neutrino mass.

It can be done under condition of radical improvement of specific activity and stability of the tritium electron source.

At present the period of stable circulation is limited by a day because of tritium dilution by hydrogen and absorption in the walls of purification system. It results in necessity of frequent replacement of gas that limits the duration of measurements for the economic and safety reasons.

Thereby radical improvement of the whole tritium part of Troitsk installation, development of laboratory system for maintenance of tritium isotope purification in a continuous circulation mode, development of methods of vacuum volumes decontamination after closing-up and recovery of activated parts of installation is proposed.

As a starting-point for development of continues isotope separation device a variant of a method of counter-flow mass diffusion has been chosen. Basic features of this method were earlier developed in ASRIIM. This method allows to minimize amount of tritium accumulated in the set-up and to carry out the continuous process at low gas pressure.

Development of methods of vacuum volumes decontamination after closing-up and recovery of activated parts of installation will be carried out in ARIEP and partly in INK RAS.

Experts with large experience in development and operation of physical installations from INR RAS, ARIEP and ASRIIM are involved in project participation. This guarantee, on our opinion, successful accomplishment of the project.

Success of this program can result in significant improvement of accuracy of measurement of neutrino mass and decreasing of the neutrino mass limit to the level below 2 eV that is important for cosmological theories.

The confirmation of existence of monochromatic anomaly in the tritium beta-spectrum can indicate an availability of the neutrino cloud in the planetary system bound. The formation and stability of this cloud could be caused by principled new long range neutrino interaction. (Phys.Lett.B 395 (1997) 63 - 68).

- Laboratory installation for hydrogen isotope separation which will be created in the course of the project find application in other physical experiments.
- The new hydride formative materials will be developed.
- Technology and equipment for safe storage, dosage and purification of hydrogen isotopes, which will be developed, can have independent scientific and technological importance.
- Technology and equipment for gas waste recovery and decontamination of the equipment from tritium will be developed. Foreign Collaboration Possibilities

A number of years INK RAS collaborates with Institute for Physics of Johannes Gutenberg University of Mainz (Germany) having resembling installation. Main differences are the electron source on the base of tritium frozen on substrate. The more close cooperation in the frame of this project could be allowed to increase the accuracy of data obtaining in Mainz too and to have on this way two independent experiments that increase the confidence of the results.