Experiments

If an experiment works, something has gone wrong.
Finagle's First Law

All limits below correspond to 90% Confidence Level, if not stated explicitly otherwise. For references, see publications of the LPD group. The abbreviation «UL» below is for Underground Laboratory.

1. First observation of the two-neutrino 2β decay of ¹¹⁶Cd. The LPD group took part in all three experiments, based on different experimental technique, where 2β2ν decay of ¹¹⁶Cd was observed at the first time and the T_1/2 value and energy and angular distributions of the electrons were measured:

(1) experiment with isotopically enriched scintillator ¹¹⁶CdWO₄ (83% of ¹¹⁶Cd, Solotvina UL, Ukraine, 13316 h of measurements in the last modification of the set-up; T_1/2 = 2.9^+0.4_-0.3×10¹⁹ yr [42, 103, 140]);

(2) measurements with the ELEGANTS-V set-up (Kamioka UL, Japan, 1875 h; T_1/2 = 2.6^+0.9_-0.5×10¹⁹ yr [40, 50]);

(3) measurements with the tracking detector NEMO-2 (Frejus UL, France, 6588 h; T_1/2 = 3.8±0.4×10¹⁹ yr [47, 67]).

2. Lower limits on T_1/2 for neutrinoless modes of ¹¹⁶Cd 2β decay. The limits on 2β0ν decay of ¹¹⁶Cd and decays with emission of one or two Majorons, determined with the ¹¹⁶CdWO₄, are the most stringent to-date limits for this nucleus [140]:

T_1/2(0ν, g.s.) ≥ 1.7×10²³ yr,
T_1/2(0ν, 2⁺₁) ≥ 2.9×10²² yr,
T_1/2(0ν, 0⁺₁) ≥ 1.4×10²² yr,
T_1/2(0νM1) ≥ 8.0×10²¹ yr,
T_1/2(0νM2) ≥ 8.0×10²⁰ yr,
T_1/2(0νMbulk) ≥ 1.7×10²¹ yr.

The value of 1.7×10²³ yr is fifth world result (after ⁷⁶Ge, ¹³⁶Xe, ¹³⁰Te, ¹⁰⁰Mo) for T_1/2 limits, determined for all known 2β candidates. The upper limits on the neutrino mass (m_ν ≤ 1.7 eV), right-handed admixtures in the weak currents, neutrino-Majoron coupling constant and other theoretical parameters were calculated. Full time of measurements with the ¹¹⁶CdWO₄ detector(s) in the Solotvina UL is near 60 000 h.

3. Participation in the experiments with the NEMO collaboration to observe 2β2ν decays of ⁸²Se, ⁹⁴Zr, ⁹⁶Zr and ¹⁰⁰Mo. In the experiments with the tracking detector NEMO-2 (Frejus UL, France) the existence of 2β2ν decays of ⁸²Se, ⁹⁶Zr and ¹⁰⁰Mo was confirmed, and half-lives and energy and angular distributions of emitted electrons were measured:

¹⁰⁰Mo - 6140 h, T_1/2 = 9.5±1.0×10¹⁸ yr [48];
⁸²Se - 10357 h, T_1/2 = 8.3±1.1×10¹⁹ yr [84];
⁹⁶Zr - 10357 h, T_1/2 = 2.1^+0.8_-0.4×10¹⁹ yr [92] (this is the first observation of ⁹⁶Zr 2β2ν decay in direct experiment).

The lower limits on 2β decays of ⁹⁴Zr were established [92] (T_1/2 ≥ 1×10¹⁷ - 2×10¹⁹ yr for different modes, which are the best known limits for this nucleus).

4. Search for 2β decay of ¹⁰⁶Cd. Two experiments were performed (both in the Gran Sasso UL, Italy):

(1) with big (1.046 kg) CdWO₄ crystal (6701 h, T_1/2(2β⁺0ν) ≥ 2.2×10¹⁹ yr and T_1/2(εβ⁺0ν) ≥ 5.5×10¹⁹ yr) [66];

(2) with two NaI detectors and enriched (68%) ¹⁰⁶Cd sample (4321 h, for different modes of 2β⁺, εβ⁺ and 2ε decays T_1/2 ≥ (0.3-4)×10²⁰ yr, which are 6 to 60 times better than determined in other experiments) [87].

5. Search for 2β decays of ¹⁰⁸Cd, ¹¹⁴Cd, ¹⁸⁰W, ¹⁸⁶W. The experimental limits on 2β decays of these nuclei (Solotvina UL, ¹¹⁶CdWO₄ and CdWO₄ scintillators) were determined at the first time and are in the range of T_1/2 ≥ 6.0×10¹⁶ yr to T_1/2 ≥ 1.1×10²¹ yr (the best up-to-date limits) [45, 140, 141].

6. Limits on 2β decays of ⁴⁰Ca, ⁴⁶Ca. In the experiment with two CaF₂ detectors with mass of 370 g each (Gran Sasso UL, Italy, 1906 h), the limits on 2ε decays of ⁴⁰Ca (T_1/2 ≥ (3.0-5.9)×10²¹ yr, these are the record values for the limits on probability of 2ε decay for all nuclei-candidates) and 2β^- decay of ⁴⁶Ca (T_1/2 ≥ 1.0×10¹⁷ yr, the best for this nucleus) were determined [91].

7. Investigation of 2β decays of ¹⁶⁰Gd, ¹³⁶Ce, ¹³⁸Ce, ¹⁴²Ce. In the experiment with Gd₂SiO₅(Ce) detector with mass of 635 g (Solotvina UL, 13949 h), the limit T_1/2(2β0ν, ¹⁶⁰Gd) ≥ 1.3×10²¹ yr was established (near one order of magnitude higher than that reached by other groups). The limits on probabilities of 2β processes were established, most of them at the first time, for Ce isotopes:

T_1/2(2β⁺0ν, ¹³⁶Ce) ≥ 1.9×10¹⁶ yr,
T_1/2(Kβ⁺0ν, ¹³⁶Ce) ≥ 3.8×10¹⁶ yr,
T_1/2(Kβ⁺2ν, ¹³⁶Ce) ≥ 1.8×10¹⁵ yr,
T_1/2(2K0ν, ¹³⁶Ce) ≥ 6.0×10¹⁵ yr,
T_1/2(2K0ν, ¹³⁸Ce) ≥ 1.8×10¹⁵ yr,
T_1/2(2β^-0ν, ¹⁴²Ce) ≥ 2.0×10¹⁸ yr,
T_1/2(2β^-2ν, ¹⁴²Ce) ≥ 1.6×10¹⁷ yr [115].

8. Search for β and 2β decays of ⁴⁸Ca was performed with CaF₂(Eu) scintillator (1.1 kg, 3236 h, Gran Sasso UL, Italy); obtained limits were from T_1/2 ≥ 1.2×10¹⁸ yr to T_1/2 ≥ 5.5×10¹⁹ yr [128].

9. Limits on two-neutrino double electron capture in ¹³⁶Ce, ¹³⁸Ce were obtained in measurements with CeF₃ scintillator (49 g, 2142 h, Gran Sasso UL, Italy) as

T_1/2(¹³⁶Ce) ≥ 2.7×10¹⁶ yr,
T_1/2(¹³⁸Ce) ≥ 3.7×10¹⁶ yr [145] (the best limits).

10. Limits on double electron capture in ¹⁹⁶Hg. In measurements with HP Ge detector 165 cm³ (Solotvina UL, 1109 h) the limit on the probability of two electron (0ν+2ν) capture in ¹⁹⁶Hg was established:
T_1/2 ≥ 2.5×10¹⁸ yr (the best up-to-date result for this nuclide) [25].

11. Early searches for 2β0ν decays of ⁹⁶Zr, ¹⁰⁰Mo and ¹³⁰Te on the earth level. In the measurements with plastic scintillator-wafer stacks and enriched in ¹³⁰Te sheets of tellurium (1100 h), the limit T_1/2(¹³⁰Te) ≥ 1.2×10²¹ yr (68% CL) was obtained in 1980 [5, 6]. This value was surpassed only in 1992 in the experiment of the Milano group with TeO₂ bolometer. In similar measurements with ¹⁰⁰Mo and ⁹⁶Zr, the limits T_1/2(¹⁰⁰Mo) ≥ 2.2×10²¹ yr (90% CL) [12] and T_1/2(⁹⁶Zr) ≥ 3.0×10¹⁹ yr (68% CL) [9] had been set, also the best at that time.

12. Search for 2β decay of radioactive (β and/or α unstable) nuclides was proposed in [163]. The possible advantage is higher value of Q_2β (up to ~40 MeV) in comparison with «conventional» 2β nuclides (Q_2β ≤ 4.3 MeV) which results in higher 2β0ν decay rates because of dependence T_1/2^-1 ~ Q_2β⁵.

13. Investigation of 4^th forbidden non-unique β decay of ¹¹³Cd. Only three nuclei (also ⁵⁰V and ¹¹⁵In) are known in the nature for which such decay is not hidden by other decay modes; investigations are very laborious because of a big T_1/2 values. Shape of ¹¹³Cd β spectrum and T_1/2 value were measured in the experiment with CdWO₄ (Solotvina UL) as T_1/2 = 7.7±0.3×10¹⁵ yr [59]. Recently they were remeasured with higher accuracy at Gran Sasso UL giving T_1/2 = 8.04±0.05×10¹⁵ yr [189].

14. Search for α decays of natural tungstate isotopes. The limits on probability of α decay of ¹⁸²W, ¹⁸³W, ¹⁸⁴W, ¹⁸⁶W isotopes were set in the measurements with CdWO₄ and ¹¹⁶CdWO₄ scintillators in the Solotvina UL in the range of T_1/2 ≥ (0.8-1.8)×10²⁰ yr [46, 139]. First indications for observation of ¹⁸⁰W α decay were obtained; measured half-life is T_1/2(¹⁸⁰W)=1.1^+0.9_-0.5×10¹⁸ yr [139]. Later it was observed also with CaWO₄ scintillator: T_1/2(¹⁸⁰W)=1.0^+0.7_-0.3×10¹⁸ yr [165].

15. Limit on α decay of ¹⁴²Ce was established in measurements with CeF₃ scintillator (49 g, 2142 h, Gran Sasso UL, Italy) as T_1/2(¹⁴²Ce) ≥ 2.9×10¹⁸ yr [145] (the best value to-date).

16. First observation of α decay of ¹⁵¹Eu was concluded from measurements with CaF₂(Eu) crystal scintillator (370 g, 7426 h, Gran Sasso UL, Italy); measured half-life is T_1/2(¹⁵¹Eu)=5⁺¹¹_-3×10¹⁸ yr [188].

17. Studies of transitions of Cu, Ti and Hg nuclei to super-dense state. The limits on probability of such processes were established in measurements with HP Ge detector of 165 cm³ (Solotvina UL, 1109 h) on the T_1/2 level of 10²¹-10²² yr [25].

18. Search for nuclear decay with emission of various clusters. The limits on cluster decays of Hg isotopes were derived from the measurements with HP Ge detector of 165 cm³ (Solotvina UL, 1109 h): for different clusters, from ¹⁴C to ⁶⁹Ni, T_1/2 ≥  ~10²¹ yr [25]. Limits for ¹²⁷I were established on the level of up to ~10²⁴ yr [169] (the most stringent for all known limits on probability of cluster decays); decays of La isotopes were also searched for [170] (T_1/2 ≥  ~10¹⁸-10²¹ yr).

19. Limits on the electron stability and electric charge non-conservation. The results are based on the measurements with the DAMA NaI detectors (87.3 kg, 5364 h) and DAMA liquid Xe detector (6.5 kg, 8336 h) in the Gran Sasso UL, Italy. The determined limits on the electron stability (and, hence, on the electric charge conservation) are equal:

τ(e^- → ν_e+γ) ≥ 2.0×10²⁶ yr [102],
τ(e^- → ν_e+ν_e+ν_e) ≥ 2.4×10²⁴ yr [88],

(the last value, valid also for electron disappearance, is the most stringent to-date and higher ~10 times than those known before). The limits on the electron disappearance with excitation of nuclear levels also were established for ²³Na, ¹²⁷I and ¹²⁹Xe (T_1/2≥1.5×10²³-4.0×10²⁴ yr, the best known values) [89, 90]. The bounds on the charge non-conserving admixtures in the weak interactions were determined (ε_W² ≤ 2.2×10^-26, ε_γ² ≤ 1.3×10^-42) as well as the upper limit on the mass of γ quantum (m_γ ≤ 1.4×10^-14 eV).

20. Charge non-conserving (CNC) β decay of ⁷³Ge, ¹³⁶Xe and ¹³⁹La. To set limit on ⁷³Ge, data accumulated with 952 g HP Ge detector during 15288 h (Baksan UL, Russia) were used. Instead of traditional in this field radiochemical methods, the real-time approach was used for the first time. First limit for ⁷³Ge CNC β decay was set as τ > 2.6×10²³ yr [126]. Limit for ¹³⁶Xe was derived from data of the DAMA/LXe experiment (τ > 1.3×10²³ yr) [160]. Measurements with LaCl₃ crystal scintillator were used to derive limit on CNC β decay of ¹³⁹La: τ > 1.0×10¹⁸ yr [180].

21. CNC β decay of ¹¹⁵In. Limit on the CNC β decay of ¹¹⁵In was set at the first time with the data of the LENS Collaboration where indium sample of 929 g was measured with 4 HP Ge detectors ~225 cm³ each during 2762 h:
τ > 4.1×10²⁰ yr. As a very interesting by-product of this search for exotic process, the first experimental observation of usual β decay of ¹¹⁵In to the first excited level of ¹¹⁵Sn has been obtained. This is beta decay with extremelly low probability of 1.2×10^-6 (what corresponds to T_1/2=3.7×10²⁰ yr) and, possibly, with the lowest known value of Q_β (of ~500 eV) [164, 195].

22. Search for the nucleon, di-nucleon and tri-nucleon decay into invisible channels. A new scheme was proposed to search for disappearance or decay into invisible channels of nucleons in atomic nuclei, when the decay of a daughter nucleus, incorporated into detector, is looked for [104]. Limits were established first on the base of the data with the DAMA liquid Xe detector (6.5 kg, 8336 h, Gran Sasso UL, Italy) [104], and then were improved with the BOREXINO Counting Test Facility (liquid scintillator, 4.2 t, 626 h, Gran Sasso UL, Italy) to the following values:

τ_p ≥ 1.1×10²⁶ yr,
τ_n ≥ 1.8×10²⁵ yr,
τ_pp ≥ 5.0×10²⁵ yr,
τ_nn ≥ 4.9×10²⁵ yr

(the last two were the best known limits) [142]. Reanalysing data of old radiochemical experiment, the limit

τ_np > 2.1×10²⁵ yr

was established (the best to-date) [154]. Tri-nucleon disappearance was looked for at the first time too [181]:

τ_nnp ≥ 1.4×10²² yr,
τ_npp ≥ 2.7×10²² yr,
τ_ppp ≥ 3.6×10²² yr.

23. Limit on the proton stability independent on the decay channel. The number of free neutrons, born in big volumes with heavy water, was proposed to use for setting the limit on the proton decay independent on channel, and the best bound was established:
τ_p ≥ 4.0×10²³ yr at 95% CL [114].

24. Limit on the proton decay into invisible channels. Using the data on the number of neutrons born in the SNO solar neutrino detector with 1000 tons of D₂O (Sudbury UL, Canada), the limit on the proton decay into invisible channels was set as:
τ_p ≥ 3.5×10²⁸ yr (three orders of magnitude higher than known before) [138].

25. Upper limit on the photon charge had been established from analysis of observations of extragalactic radiosources: e_γ/e ≤ 3×10^-33 [168]. This limit is the best to-date.

26. Development of the experimental technique and methods of simulation and analysis of experimental data. The super-low background scintillator detector with ¹¹⁶CdWO₄ crystals and improved passive and active shielding was created in the Solotvina UL (in particular, on the base of big - ~200 cm³ - CdWO₄ crystals) with record background rate of 0.03 counts/yr×keV×kg, that is a result of ~18 yr R&D [103, 140, 141]. New data acquisition system (16 independent channels with accumulation of the time, amplitude and shape of scintillation signal). Time-amplitude analysis of data (with possibility to discriminate the chains of decay in the natural radioactive families) [42, 103, 62, 115, 139, 140, 141]. Pulse-shape analysis with discrimination of events from α and β/γ particles, double impulses, etc. [83, 103, 139, 140, 141]. Event generator for description of initial kinematics of events in 2β processes and decays of radioactive nuclides (for ~3000 isotopes): how many particles and of which type were emitted, their energies, directions and times of emission [108]. Programs for simulation (on the base of the GEANT package) of the response function of detectors for α, β and 2β decays. Programs for analysis of data with low statistics. Scintillation properties, pulse-shape discrimination ability and radioactive contamination of crystal scintillators CaWO₄ [165], YAG:Nd [166], ZnWO₄ [167], PbWO₄ [176], CdWO₄ [177], as possible detectors for 2β decay, α decay and dark matter searches.

27. International collaborations.

(1) Since 1998, the physicists from Universitá di Firenze and INFN (Italy) take part in the experiment with ¹¹⁶CdWO₄ detector in the Solotvina UL [83, 103, 139, 140, 141, 177];

(2) Collaboration with the Osaka University (Japan): observation of ¹¹⁶Cd 2β2ν decay with the ELEGANTS-V set-up (Kamioka UL) [40, 50];

(3) NEMO Collaboration (France, Ukraine, Russia, USA, Finland, Czech Republic): measurements of 2β2ν decays of ⁸²Se [84], ⁹⁴Zr [92], ⁹⁶Zr [92], ¹⁰⁰Mo [48] and ¹¹⁶Cd [47, 67] with the NEMO-2 set-up (Frejus UL);

(4) Max Planck Institut für Kernphysik (Heidelberg, Germany):

(4.1) search for ¹⁰⁶Cd 2β decay (Gran Sasso UL) [66];
(4.2) proposal for solar neutrino detector on the base of ¹³¹Xe [73];
(4.3) participation in the GENIUS project (new generation detector for 2β0ν decay search with sensitivity to the neutrino mass on the level of 0.01-0.001 eV): development of the shielding [86, 99] and calculation of backgrounds [99];

(5) Collaboration with Universitá di Roma I, II and INFN (Italy):

(5.1) search for ¹⁰⁶Cd 2β decays (Gran Sasso UL) [87];
(5.2) studies of 2β processes in ⁴⁰Ca, ⁴⁶Ca (Gran Sasso UL) [91];
(5.3) experiments with NaI and liquid Xe detectors to search for processes related with the electron instabilities (Gran Sasso UL) [88, 89, 90, 102];
(5.4) search for nucleon, di-nucleon and tri-nucleon decays into invisible channels (Gran Sasso UL) [104, 181];
(5.5) experiments with CaF₂(Eu) detectors to study β and 2β decays in ⁴⁸Ca [128];
(5.6) search for 2β processes in ¹³⁶Ce, ¹³⁸Ce and α decay of ¹⁴²Ce with CeF₃ detector [145];
(5.7) search for cluster decays of ¹²⁷I [169] and La isotopes [170];
(5.8) search for CNC β decay of ¹³⁶Xe [160];
(5.9) search for CNC β decay of ¹³⁹La [180];
(5.10) first observation of α decay of ¹⁵¹Eu [188];
(5.11) measurement of β decay of ¹¹³Cd with high accuracy [189];

(6) Collaboration with the Milano University and INFN (Italy):

(6.1) testing a small (58 g) CdWO₄ crystal as a bolometer at ~20 mK, and measurements of 4^th forbidden β decay of ¹¹³Cd (Gran Sasso UL, 340 h) [36, 38];
(6.2) participation of the italian physicists in the CAMEO project [105, 113];

(7) BOREXINO Collaboration (Italy, USA, Canada, France, Germany, Russia, Hungary, Poland, Ukraine): search for nucleon and di-nucleon instabilities [142], violation of Pauli exclusion principle [156], investigation of neutrino properties [143, 144], production of cosmogenic ¹¹C in an organic liquid scintillator [178] and search for antineutrino component in the Solar neutrino flux [179];

(8) XMASS Collaboration (Japan, USA, Ukraine): development of a massive liquid Xe detector for solar neutrinos, 2β decay and dark matter investigation [135, 136, 137];

(9) Collaboration with the JINR (Dubna) - INR (Moscow) to search for CNC β decay of ⁷³Ge [126]; CARVEL experiment [162]; investigation of properties of CaWO₄ [165] and PbWO₄ scintillators [176]; participation in the TGV2 experiment [182, 184];

(10) LENS Collaboration: search for ¹¹⁵In CNC β decay and the first observation of β decay ¹¹⁵In → ¹¹⁵Sn^* [164, 195];

(11) SuperNEMO Collaboration: participation in research and development (event generator, development of scintillator) [183, 185, 200];

(12) Participation in the EURECA project for development of massive (ton) bolometric detector for search and investigation of dark matter and rare decays in nuclear and particle physics.

28. Large-scale proposals in the neutrino physics and 2β research developed by the LPD group:

(1) CAMEO project for a high sensitivity study of ¹⁰⁰Mo and ¹¹⁶Cd neutrinoless 2β decay [105, 113]. With 100 kg of ¹¹⁶CdWO₄ crystals placed in the liquid scintillator of the BOREXINO Counting Test Facility, working in the Gran Sasso UL, the expected T_1/2 limit on ¹¹⁶Cd 2β0ν decay is ≈10²⁶ yr, that corresponds to upper limit of m_ν ≤ 0.06 eV;

(2) in the GEM project [116] one ton of naked HP Ge detectors operating in high-purity nitrogen is used for the search for ⁷⁶Ge 2β0ν decay and dark matter investigations (similar to the GENIUS proposal, however with simpler technical realization). Expected sensitivities are:
T_1/2 ≥ 10²⁷-10²⁸ yr and
m_ν ≤ 0.015-0.05 eV;

(3) the GSO scintillators for double β searches and solar neutrino investigations are discussed in [62, 115]. With 2 tons of the GSO crystals placed in the BOREXINO or SNO set-up, the sensitivity for ¹⁶⁰Gd 2β0ν decay of T_1/2 ≥ 2×10²⁶ yr (m_ν ≤ 0.07 eV) could be reached;

(4) the CARVEL proposal is developed for 2β0ν decay of ⁴⁸Ca with ⁴⁸CaWO₄ crystal scintillators with sensitivity of
T_1/2 ≥ 10²⁷ yr and
m_ν ≤ 0.04-0.09 eV [162];

(5) solar neutrino detector on the base of ¹³¹Xe is proposed in [73].