Heidelberg-Moscow experiment

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The Heidelberg-Moscow experiment was an experiment to search for the neutrino-free double beta decay in the germanium isotope 76 Ge. Unlike the normal double beta decay , in which two neutrinos are emitted, this process can not be explained with the standard model of particle physics , so experimental evidence would be a sensation. The experiment was led by a collaboration between the Max Planck Institute for Nuclear Physics in Heidelberg and the Kurchatov Institute in Moscow . The results of the experiment are controversial.

Look for neutrino-free beta decays

The nuclide 76 Ge is stable against single, but not against double beta decay. The half-life is about 1.5 · 10 21 years. The energy of 2039 k eV released during the decay is distributed over two electrons and two neutrinos. The detectable total energy of the two electrons therefore has a broad distribution.

A neutrino-free double beta decay would be much rarer. It would be characterized by the fact that the sum of the energies of the emitted electrons is always equal to the decay energy. With a good energy resolution of the detector, both types of decay could be distinguished from one another, because a single "line" would have to be superimposed on the continuous distribution.

construction

In the experiment, five individual semiconductor detectors made from a total of 11.5 kg of high-purity germanium were used, which were also the radiation source. The examined isotope 76 Ge is only 7% contained in natural germanium. In order to increase the sensitivity of the experiment, it was enriched to a level of 86% .

To reduce the background effect , the experiment was carried out in the Laboratori Nazionali del Gran Sasso 1400 m underground. In order to reduce disturbing radiation from the outside, the detectors were also shielded by lead . However, since cosmic muons cannot be completely shielded, scintillators were installed above the structure to detect them so that they could be removed from the data analysis.

Result of the experiment

Data were collected from August 1990 to May 2003. The evaluation under the direction of Hans Klapdor-Kleingrothaus showed evidence of neutrino-free double beta decays with a significance of 6.4 standard deviations. The partial lifetime of this decay channel was calculated to be 2.2 · 10 25 years. It would be ten thousand times rarer in the Ge-76 than the normal double beta decay.

The result is controversial. Other analyzes could not find any clear evidence of neutrino-free double beta decay in the data. In the GERDA experiment with its improved sensitivity, no neutrino-free double beta decay was observed either.

Individual evidence

  1. ^ Karlsruhe nuclide map , 1998
  2. ↑ Alexander Dietz: Evaluation of the neutrino-free double beta decay of 76Ge in the Heidelberg-Moscow experiment with improved statistical methods . Dissertation, 2003 ( urn : nbn: de: bsz: 16-opus-34162 ).
  3. ^ HV Klapdor-Kleingrothaus, IV Krivosheina: The Evidence for the Observation of 0 νββ Decay: The Identification of 0 νββ Events from the full Spectra . In: Modern Physics Letters A . tape 21 , no. 20 , June 28, 2006, pp. 1547–1566 , doi : 10.1142 / S0217732306020937 ( PDF online (PDF)).
  4. ^ HV Klapdor-Kleingrothaus, A. Dietz, HL Harney, IV Krivosheina: Evidence for Neutrinoless Double Beta Decay . In: Modern Physics Letters A . tape 16 , no. 37 , December 7, 2001, p. 2409-2420 , doi : 10.1142 / S0217732301005825 .
  5. ↑ CE Aalseth u. a .: Comment on “Evidence for Neutrinoless Double Beta Decay” . In: Modern Physics Letters A . tape 17 , no. 22 , July 20, 2002, pp. 1475-1478 , doi : 10.1142 / S0217732302007715 .
  6. ↑ GERDA collaboration, M. Agostini et al .: Background-free search for neutrinoless double-β decay of 76Ge with GERDA . In: Nature . tape 544 , April 5, 2017, p. 47 , doi : 10.1038 / nature21717 , arxiv : 1703.00570 .