Search

New limits on $\beta^+$EC and ECEC processes in $^{74}$Se have been obtained using a 600 cm$^3$ HPGe detector and an external source consisting of 1600 g of a natural selenium powder. For different $\beta^+$EC and ECEC transitions (to the ground and excited states) obtained limits are on the level $\sim (0.2-4.8)\times10^{19}$ yr at 90\% C.L. In particular, for the potentially resonant transition into the 1204.2 keV excited state of $^{74}$Ge a lower half-life limit of $1.1\times10^{19}$ yr at 90\% C.L. has been obtained. Possibility to increase the sensitivity of such measurements is discussed., Comment: 16 pages, 3 figures. arXiv admin note: text overlap with arXiv:hep-ex/0610046

International audience; Double beta decay (β$^{+}$EC, EC/EC) of $^{74}$Se was investigated at the Modane underground laboratory (LSM, France; 4800 m of water equivalent) using the OBELIX ultralow-background HPGe detector with a sensitive volume of 600 cm$^{3}$ and a sample of natural selenium with a mass of 1.6 kg containing ~0.89% (~14.24 g) of $^{74}$Se. The new experimental limits for β$^{+}$EC and EC/EC decays of $^{74}$Se to ground 0$^{+}$ and excited $2_{1}^{ + },$ 596 keV, and $2_{2}^{ + },$ 1204 keV states of $^{74}$Ge, were obtained from experimental data accumulated over 135 days.

International audience; Investigation of double beta decay processes (β+EC, EC/EC) of 74Se was performed at the Modane underground laboratory (LSM, France, 4800 m w.e.) using ultra low-background HPGe detector Obelix with sensitive volume of 600 cm3 and a sample of natural selenium with a mass of 1.6 kg, containing ∼0.89% (∼14.24 g) of 74Se. New experimental limits (at 90 % CL) on double beta decay transitions of 74Se to the ground and exited states of 74Ge were obtained from the preliminary calculation of experimental data accumulated in 135 days.

International audience; The main limitation in the high-sensitive HPGe gamma-ray spectrometry has been the detector background, even for detectors placed deep underground. Environmental radionuclides such as 40K and decay products in the 238U and 232Th chains have been identified as the most important radioactive contaminants of construction parts of HPGe gamma-ray spectrometers. Monte Carlo simulations have shown that the massive inner and outer lead shields have been the main contributors to the HPGe-detector background, followed by aluminum cryostat, copper cold finger, detector holder and the lead ring with FET. The Monte Carlo simulated cosmic-ray background gamma-ray spectrum has been by about three orders of magnitude lower than the experimental spectrum measured in the Modane underground laboratory (4800 m w.e.), underlying the importance of using radiopure materials for the construction of ultra-low-level HPGe gamma-ray spectrometers.

Double beta decay (β + EC, EC/EC) of 58Ni is investigated at France’s Modane Underground Laboratory (4800 m water equivalent) using the OBELIX ultralow-background HPGe detector with a sensitive volume of 600 cm3 and a natural nickel sample of ~68% 58Ni with a mass of ~21.7 kg. After preliminary analysis of the experimental data accumulated over ~144 days, new experimental limits are obtained for the 2νβ+EC decay of 58Ni to the 0+ ground state and the 2 1 + , 811 keV excited state of 58Fe, and for the 2νEC/EC decay of 58Ni to the 2 1 + , 811 keV and 2 2 + , 1675 keV excited states of 58Fe. The limits are T1/2(β+EC,0→0+) > 1.7 × 1022 yr, T1/2(β+EC,0→2 1 + ) > 2.3 × 1022 yr, T1/2(EC/EC,0→2 1 + ) > 3.3 × 1022 yr, and T1/2(EC/EC,0→2 2 + ) > 3.4 × 1022 yr. Experimental limit T1/2(0νEC/EC–res, 1918 keV > 4.1 × 1022 yr is obtained for resonant neutrinoless radiative EC/EC decay with an energy of 1918.3 keV. All limits are at 90% CL.

Investigation of double beta decay processes (β+EC, EC/EC) of 58Ni was performed at the Modane underground laboratory (LSM, France, 4800 m w.e.). A sample of natural nickel, containing ∼68% of 58Ni and a mass of ∼21.7 kg, was measured using ultra low-background HPGe detector Obelix (sensitive volume of 600 cm3) during ∼143.8 days. New experimental limits on 2νβ+EC decay of 58Ni to the ground 0+ and (math), 811 keV excited state of 58Fe, and 2νEC/EC decay of 58Ni to (math), 811 keV and (maht), 1 675 keV excited states of 58Fe were obtained in this measurement. There are -T1/2(β+EC, 0+ → 0+) > 1.7 × 1022 y; (math), (math), (math). For resonant neutrino-less radiative EC/EC decay with energy of 1 918.3 keV a new experimental limit of T1/2 (0νEC/EC – res, 1918KeV) > 4.1 ×1022 y was also obtained. All limits are at 90 % CL.

International audience; Investigation of double beta decay processes (β+EC, EC/EC) of 58Ni was performed at the Modane underground laboratory (LSM, France, 4800 m w.e.) using ultra low-background HPGe detector Obelix with sensitive volume of 600 cm3 and a sample of natural nickel, containing ∼68% of 58Ni with a mass of ∼21.7 kg. New experimental limits on 2νβ+EC decay of 58Ni to the ground 0+ and 21+, 811 keV excited state of 58Fe, and 2νEC/EC decay of 58Ni to 21+, 811 keV and 22+ 1 675 keV excited states of 58Fe were obtained from the preliminary calculation of experimental data accumulated in ∼88d. They are - T1/2(β+ EC, 0+ → 0+) > 1.4 ×1021 y; T1/2(β+EC,0+→21+)>1.7×1022 y, T1/2(EC/EC, 0+→21+)>2.4×1022 y, T1/2(EC/EC, 0+→22+)>2.1×1022 y. For resonant neutrino-less radiative EC/EC decay with energy of 1 918.3 keV a new experimental limit of T1/2 (0νEC/EC − res, 1918keV) > 4.7 ×1022 y was also obtained. All limits are at 90 % CL.

A new low-background spectrometer based on a HPGe detector with a sensitive volume of 600 cm3 is developed to investigate rare nuclear processes such as resonance neutrinoless double electron capture and two-neutrino double beta decay to excited states of daughter nuclei. The spectrometer is installed at the Modane underground laboratory (France) at a depth of 4800 m w.e. The spectrometer background is measured and the spectrometer sensitivity is determined for measuring sources of double beta decay.

A search for the β+β+, β+/EC, and EC/EC decays of 106Cd was performed at the Modane Underground Laboratory (France) located at a depth of 4800 m w.e. using a TGV-2 multidetector germanium spectrometer. A preliminary evaluation is performed of the experimental data accumulated during the measurements (12 900 h) of ∼13.6 g of 106Cd (with an enrichment of 75%) and the spectrometer background without samples and with samples of natural Cd. New limits (at a 90% confidence level) of half-lives are obtained: T 1/2 ⩾ 1.7 × 1020 yr and T 1/2 ⩾ 1.6 × 1020 yr for the 0νEC/EC resonant decay of 106Cd to the 2741 keV and 2718 keV excited states of the daughter nucleus 106Pd and T 1/2 ⩾ 4.2 × 1020 yr for the 2νEC/EC decay to the ground state of 106Pd (0+ → 0+, g.s.). The limits for other branches of the double beta decay of 106Cd with transitions to the ground and excited states of 106Pd are improved.

An investigation of double beta decay (β+β+, β+/EC, EC/EC) of 106Cd was performed at the Modane underground laboratory (at a water equivalent depth of 4800 m) using a TGV-2 spectrometer with 32 detectors. Evaluation of the experimental data accumulated over 12 400 h of measuring ∼13.6 g of 106Cd with an enrichment of 75% was performed. New limits (at a confidence level of 90%) on the half-lives of the 0νEC/EC resonant decay of 106Cd to the 2741 keV excited state of 106Pd − T1/2 ≥ 1.5 × 1020 y and on 2νEC/EC decay to the ground state of 106Pd (0+ → 0+, g.s.) − T1/2 ≥ 3.9 × 1020 y were obtained. The limits of the 2νEC/EC decay of 106Cd to the 2+, 512 keV and 01+, 1334 keV excited states of 106Pd and 2νβ+β+- and the 2νβ+EC decays of 106Cd to the ground and excited states of 106Pd were improved.

The studies aimed at decreasing the background of the TGV-2 spectrometer have led to constructive changes in the cryostat and increased the sensitivity of the 32-detector spectrometer. New search for 0νEC/EC, 2νEC/EC) decays of 106Cd was started with ∼13.6 g of 106Cd with enrichment of 75%. The evaluation of the experimental data accumulated for 4800 h was performed. Limits (at 90% CL) on the half-lives of 0νEC/EC resonant decay of 106Cd to the 2741 keV excited state of 106Pd (T1/2 ≥ 6.5 × 1019 y) and on t 2νEC/EC decay to the ground state of 106Pd (0+ → 0+, g.s.) (T1/2 ≥ 1.7 × 1020 y) were obtained. The limits on the other branches of 106Cd decay—2νEC/EC decay to the 2+, 512 keV and 01+, 1334 keV excited states of 106Pd and 2νβ+β+ and 2νβ+EC decays of 106Cd to the ground and excited states of 106Pd were improved.

Search for the β+β+, β+ EC, and EC/EC modes of the 106Cd decay was carried out with the TGV-2 (Telescope Germanium Vertical) low-background multidetector spectrometer installed at the Modane underground laboratory (4800 m w.e.). The measured foil samples ∼50 μm thick and 52 mm in diameter were placed between the entrance windows of the neighboring detectors inside the cryostat. The total measurement time for 10 g of 106Cd enriched to 75% was 8687 h. New limits (at the 90% confidence level, CL) were obtained for the 106Cd half-lives against various branches of the decay to the ground state 0+ and excited states 2+ of the 106Pd daughter nucleus. They are T 1/2(2νβ+β+) ≥ 6.0 × 1019 y, and T 1/2(2νβ+ EC) ≥ 5.9 × 1019 y, and T 1/2(2νEC/EC) ≥ 3.0 × 1020 y for the transitions to the 0+ ground state of 106Pd; T 1/2(2νβ+β+) ≥ 5.7 × 1019 y, T 1/2(2νβ+ EC) ≥ 5.9 × 1019 y, and T 1/2(2νEC/EC) ≥ 4.2 × 1019 y for the transitions to the 2+, 512-keV excited state of 106Pd; and T 1/2(2νEC/EC) ≥ 3.1 × 1019 y for the transition to the 0 1 + , 1334-keV excited state of 106Pd.

International audience; A new experimental run of searching for double beta decay of (106)Cd was performed at the Modane underground laboratory (LSM, France, 4800 m w.e.) using the TGV-2 spectrometer, consisting of 32 planar type HPGe detectors with a total sensitive volume of ~400 cm(3). 16 foils of (106)Cd with an enrichment of 99.57% and a total mass of ~ 23.2 g were inserted between the entrance windows of face-to-face detectors. The limit on 2vEC/EC decay of (106)Cd - T(1/2) > 3.7 × 10(20) y at 90% C.F was obtained from the preliminary calculation of experimental data accumulated for 8198 h of measurement. The limits on the resonance OvEC/EC decay of (106)Cd were obtained from the measurement of ~23.2 g of (106)Cd with the low-background HPGe spectrometer Obelix lasted 395 h -T(1/2) (KF, 2741 keV) > 0.9 × 10(20) y and T(1/2) (KK, 2718 keV) > 1.4 × 10(20) y at 90% C.L.

International audience; A new experiment devoted to searches for double electron capture in Cd-106 decay is being performed at the Modane underground laboratory (4800 mwe) with the 32-detector TGV-2 spectrometer. The limit T (1/2)(2 nu EC/EC) > 2.0x10(20) yr at a 90%confidence level (C.L.) was obtained from a preliminary analysis of data obtained over 2250 h of measurements with about 23.2 g sample enriched in the isotope Cd-106 to 99.57%. The limits T (1/2)(KL, 2741 keV) > 0.9 x 10(20) yr and T (1/2)(KK, 2718 keV) a parts per thousand << 1.4 x 10(20) yr at a 90% C.L. on the neutrinoless decay of Cd-106 were obtained from measurements performed with the Obelix low-background spectrometer from high-purity germanium (HPGe spectrometer) for a sample of mass about 23.2 g enriched in the isotope Cd-106.

International audience; A new experimental run of searching for EC/EC decay of Cd-106 was performed at the Modane underground laboratory (4800 m w.e.) using the TGV-2 spectrometer and similar to 23.2 g Cd-106 with enrichment of 99.57%. The limit on 2 nu EC/EC decay of Cd-106 -T-1/2(2 nu EC/EC) > 3.1x10(20) y, at 90% C. L was obtained from the preliminary calculation of experimental data accumulated for 7018 h of measurement. The limits on the resonance neutrino-less double electron capture decay of Cd-106 were obtained from the measurement of similar to 23.2 g of Cd-106 with the low-background HPGe spectrometer OBELIX lasted 395 h -T-1/2(KL, 2741 keV) > 0.9x10(20) y and T-1/2(KK, 2718 keV) > 1.4x10(20) y at 90% C.L.

Investigations of double beta decay processes to excited states of daughter nuclei were performed at the Modane underground laboratory (LSM, France, 4800 m w.e.) using the high sensitivity spectrometer OBELIX [1], which is a common activity of JINR Dubna, IEAP CTU in Prague and LSM. The spectrometer is based on the HPGe detector with the sensitive volume of 600 cm3 and relative efficiency of 160%. Investigation of resonant neutrino-less double electron capture of 106Cd was performed with ∼23.2 g of 106Cd (enrichment of 99.57%) during ∼17 days. The experiment with natural Ni (∼21.7 kg of mass) was also carried out during ∼47 days. The preliminary experimental limits for 0νEC/EC resonant decay to the excited states of 106Pd and different modes of β β decay 58Ni are presented.

An ultra-low background coaxial HPGe detector for gamma-ray spectrometry with a relative efficiency of 160%, corresponding to a 600 cm3 Ge crystal, was installed at the Laboratoire Souterrain de Modane, France (4800 m.w.e). To reduce the instrinsic detector background, all parts involved in the detector cryostat were selected for their low radioactivity contamination. A shielding, composed of an inner layer of roman lead and an external layer of regular lead was installed, together with a system to reduce the Rn level inside the sample chamber. The shielding was designed to allow the measurement of Marinelli-shaped samples. We present the constructional details which lead to a remarkable low detector background of 73 cts/kg·d in [40, 3000] keV. Measured samples showed that sensitivities about 100 μBq/kg in 226Ra and 228Th are reached for samples of some kg and 30 days of lifetime.

The SPT collaboration has been investigating the applicability of pixel detectors in the detection of two neutrino double electron capture (2νEC/EC) in106Cd. The collaboration has proposed a Silicon Pixel Telescope (SPT) where a pair of Si pixel detectors with enriched Cd foil in the middle forms the detection unit. The Pixel detector gives spatial information along with energy of the particle, thus helps to identify and remove the background signals. Four units of SPT prototype (using 0.5 and 1 mm Si sensors) were fabricated and installed in the LSM underground laboratory, France. Recent progress in the SPT experiment and preliminary results from background measurements are presented.

The low-background multi-HPGe spectrometer TGV II installed in the Modane Underground Laboratory (France) is described in detail and the results of the background measurements are reported. The spectrometer is focused on the double beta decay measurements with two isotopes—106Cd ( 2 ν EC / EC mode) and 48Ca ( β β mode). A basic summary of the physics of β β decay (especially EC/EC mode) is also given.

A search for double electron capture of 106Cd was performed at the Modane Underground Laboratory (4800 m w.e.) using a low-background and high-sensitivity multidetector spectrometer TGV-2 (Telescope Germanium Vertical). New limits on β+/EC, EC/EC decays of 106Cd were obtained from preliminary calculations of experimental data accumulated for 4800 h of measurement of 10 g of 106Cd with enrichment of 75%. They are \(T_{{1 \mathord{\left/ {\vphantom {1 2}} \right. \kern-\nulldelimiterspace} 2}}^{2\nu \beta ^ + EC} \) > 9.1 × 1018 yr, \(T_{{1 \mathord{\left/ {\vphantom {1 2}} \right. \kern-\nulldelimiterspace} 2}}^{2\nu {{EC} \mathord{\left/ {\vphantom {{EC} {EC}}} \right. \kern-\nulldelimiterspace} {EC}}} \) > 1.9 × 1019 yr for transitions to the first 2+, 511.9 keV excited state of 106Pd, and \(T_{{1 \mathord{\left/ {\vphantom {1 2}} \right. \kern-\nulldelimiterspace} 2}}^{2\nu \beta ^ + EC} \) > 1.3 × 1019 yr, \(T_{{1 \mathord{\left/ {\vphantom {1 2}} \right. \kern-\nulldelimiterspace} 2}}^{2\nu {{EC} \mathord{\left/ {\vphantom {{EC} {EC}}} \right. \kern-\nulldelimiterspace} {EC}}} \) > 6.2 × 1019 yr for transitions to the ground 0+ state of 106Pd. All limits are given at 90% C.L.

A 48CaCO3 powder sample containing 20.18 g of 48Ca was measured for 797 h with a 400 cm3 low-background HPGe detector. New limits on decays of 48Ca were obtained. For single beta transitions to 48Sc the limits are equal to 0.71×1020 y, 1.1×1020 y, and 0.82×1020 y for transitions to the ground state, excited 5+ and 4+ states, respectively. The new limits on double beta decay to excited states of 48Ti are equal to 0.47×1020 y, 1.1×1020 y, and 0.90×1020 y for transitions to the first 2+, second 2+ and first 0+ excited states, respectively. All limits are given at the 90% CL.

Present status of the experiment TGV II is given. The experiment TGV II is devoted to the measurement of double-beta decay of 106Cd and 48Ca. The new HPGe multi-detector TGV spectrometer has been constructed and installed in the Modane underground laboratory (in France). Preliminary results of the first background measurement are presented.

The TGV (Telescope Germanium Vertical) collaboration is interested in the measurement of double-beta decay of 48Ca (TGV I) and 106Cd (TGV II). The background in the experiment TGV I has been suppressed by several methods. One of them was based on distinguishing between electrons and γ-rays due to the different rise time of the signals obtained from semiconductor detector. Two experimental setups have been tested, the first one used a charge-sensitive analog-to-digital converter, while the second one utilized digital oscilloscope. The reduction of the background counting rate (due to γ-rays) in single HPGe detector by a factor of (1.4-3.8) was reached.

Present status of experiments TGV I and TGV II is given. The TGV I collaboration has studied the double-beta decay of 48Ca with a low-background and high sensitivity Ge multi-detector spectrometer TGV (Telescope Germanium Vertical). The preliminary results of $${\text{T}}_{{\text{1/2}}}^{{\text{2}}\nu \beta \beta } = (4.2_{ - 1.3}^{ + 3.3} ) \times 10^{19} $$ years and $${\text{T}}_{{\text{1/2}}}^{0\nu \beta \beta } >1.5 \times 10^{21} $$ years (90% CL) for double-beta decay of 48 Ca has been found after the processing of experimental data obtained after 8700 hours of measuring time using approximately 1 gramme of 48Ca. The aim of the experiment TGV II is the development of the experimental methods, construction of spectrometers and measurement of the ββ decay (β+β+, β+/EC, EC/EC) of 106Cd particularly the 2νEC/EC mode. The theoretical description and performance of the TGV II spectrometer are also given.

Double beta decay (ββ) experiments are challenging frontiers in contemporary physics. These experiments have the potential to investigate more about neutrinos (eg. nature and mass). The main challenge for these experiments is the reduction of background. The group at IEAP, CTU in Prague is investigating a new approach using pixel detectors Timepix. Pixel detector offer background reduction capabilities with its ability to identify the particle interaction (from the 2D signature it generates). However, use of pixel detectors has some challenges such as the presence of readout electronics near the sensing medium and heat dissipation. Different aspects of pixel setup (identification of radio-impurities, selection of radio-pure materials) and proposed experimental setup are presented. Also, results of preliminary background measurements (performed on the surface and in the underground laboratories) using the prototype setups are presented.

New limits on 48Caβ− and β−β− decays to excited states of 48Ti have been obtained using a 400 cm3 low-background HPGe detector and an external source of 24.558 g enriched CaF2 powder (9.822 g of 48Ca). The limits for β− decay to the 6+ ground state and excited 5+ and 4+ states in 48Sc are 1.6×1020 yr, 2.5×1020 yr, and 1.9×1020 yr at the 90% confidence level. For the β−β− decay to 48Ti, the limits to the first 2+, second 2+, and first 0+ excited states are 1.8×1020 yr, 1.5×1020 yr, and 1.5×1020 yr, again at the 90% confidence level.

The collaboration TGV has finished measurement of 2νEC/EC decay of 106Cd with 10 g (Phase I) and 13.6 g (Phase II) samples (with enrichment of 75%), respectively. The times of measurements were 8687h (Phase I) and 12900h (Phase II). The half‐life limits (at 90% CL) were obtained for 2νEC/EC decay of 106Cd to the ground state of 106Pd as T1/2>3.0×1020 y (Phase I) and T1/2>4.2×1020 y (Phase II). The collaboration decided to continue with the study of 2νEC/EC decay of 106Cd using 23 g of an isotope enriched to 98.4%. We plan to use the TGV II spectrometer for this measurement and also Si pixel detectors for further study of double beta decay (ββ) processes.

Investigation of double electron capture in 106Cd was performed at the Modane underground laboratory (4800 m.w.e.) using the multi-detector spectrometer TGV-2. In Phase I of the experiment, ∼10 g of 106Cd with an enrichment of 75% was measured during 8687 hours. In Phase II, the TGV-2 background was significantly suppressed in comparison with Phase I and the 106Cd mass was increased to ∼13.6 g. New half-life limits (at 90% CL) were obtained for 2νEC/EC decay of 106Cd to the ground state of Pd 106 – T 1 / 2 > 3.0 × 10 20 y (Phase I) and T 1 / 2 > 3.6 × 10 20 y (Phase II, 9000 hours), and for 0νEC/EC decay of 106Cd to the 2741 keV excited state of Pd 106 – T 1 / 2 > 1.1 × 10 20 y (Phase II).

International audience; Currently, the TGV collaboration is investigating the two-neutrino double electron capture (2 nu EC/EC) of (106)Cd at the Modane underground laboratory. The study is performed with low-background multi-HPGe detector TGV II, which has been constructed for measurements of the rare processes. The half-life limits of T(1/2) > 2.6 x 10(20) years (for Phase I, 8687 hours) and T(1/2) > 3.6 x 10(20) years (for Phase II, 9003 hours) were obtained for the ground state to ground state 2 nu EC/EC of (106)Cd. The results already allow to rule out some of the previous nuclear structure calculations.

International audience; Search for double beta decay (β+β+, β+/EC, EC/EC) of 106Cd was performed at the Modane underground laboratory (4800 m w.e.) using a spectrometer TGV-2 with 32 HPGe detectors. New limits on the half-lives of 0vEC/EC resonant decay - T1/2 ≥ 1.6 × 1020 y, and on 2vEC/EC decay of 106Cd − T1/2 ≥ 4.1 × 1020 y (at 90% CL) were obtained from preliminary calculations of experimental data accumulated for 12900 h of measurement of ~13.6 g of 106Cd with enrichment of 75%. The limits on 2vEC/EC decay of 106Cd to the 2+,512 keV and 0+1,1334 keV excited states of 106Pd and on 2vβ+β+ and 2vβ+/EC decay of 106Cd were improved

The TGV II (Telescope Germanium Vertical) facility is a low background spectrometer operated in Modane Underground Laboratory (France). It aims at the study of double beta electron capture of 106Cd. The spectrometer is composed of 32 HPGe planar detectors (total mass of Ge ∼3 kg) interleaved with thin-foil samples made of enriched 106Cd. In 2007 the Phase I of the experiment (1 year measurement) was finished and the new limit on half-life for two-neutrino double electron capture (g.s. → g.s.) of 106Cd was obtained as 3.2 × 1020 years. This limit is significantly higher (by almost three orders of magnitude) than those already published. From December 2007 the Phase II of the experiment continues with more enriched material (13.6 grams instead of 8 grams) and with significantly suppressed background of the cryostat.

International audience; The TGV II (Telescope Germanium Vertical) facility is a low background spectrometer operated in Modane Underground Laboratory. It aims at the study of double electron capture of 106Cd. The spectrometer is composed of 32 HPGe planar detectors interleaved with thin‐foil samples made of Cd‐106 enriched to 75% (total mass about 10 g). In 2006, the main run of phase I (1 year duration) was terminated yielding a new limit on half‐life for two‐neutrino double electron capture (g.s.→g.s.) in 106Cd as 2.0×1020 years. This limit is significantly higher (by almost three orders of magnitude) than those already published. © 2007 American Institute of Physics

Present status of the experiment TGV II which is devoted to the measurement of double-beta decay of 106Cd is given. The low background spectrometer TGV II is installed in the Modane Underground Laboratory and has been running from February 2005 with approx 10 grams of 106Cd enriched at 75%. After an analysis of 3736 hours of experimental data the new improved half-life limit for 2νEC/EC decay of 106Cd (0 g.s. + → 0 g.s. + ) is given as T 1 2/2 > 4.8 × 1019 years (90% CL). The search for 2νEC/EC decay of 106Cd to the excited states of 106Pd allows to determine the limits of the half-lives T 1 2/2 (0 g.s. + → 2 1 + ) > 3.9 × 1019 years (90% CL) and T 1 2/2 (0 g.s. + → 0 1 + ) > 5.8 × 1019 years (90% CL).

This Letter describes a collaborative TGV (Telescope Germanium Vertical) study of the double beta decay of 48Ca with a low-background and high sensitivity Ge multi-detector spectrometer. The results of T1/22νββ=(4.2+3.3−1.3)×1019 years and T1/20νββ>1.5×1021 years (90% CL) for double beta decay of 48Ca were found after processing experimental data obtained after 8700 hours of measuring time, using approximately 1 gramme of 48Ca. The features of a TGV-2 experiment are also presented.

A high sensitivity double beta spectrometer TGV (Telescope Germanium Vertical) has been developed. It is based on 16 HPGe detectors of volume 1200 × 6 mm3 each in the same cryostat. The TGV spectrometer was proposed for the study of ultrarare nuclear processes (e.g. 2νββ, 0νββ, 2νEC/EC). Details of the TGV spectrometer construction are described, the principles of background suppression, the results of Monte Carlo simulations and the results of test background measurements (in Dubna and Modane underground laboratory) are provided.

New limits (at 90% C.L.) on double beta decay of Cd 106 − T 1 / 2 ( 0 ν EC / EC ) > 1.7 × 10 20 yr and T 1 / 2 ( 2 ν EC / EC ) > 4.2 × 10 20 yr were obtained in a preliminary calculation of data accumulated for 12900 h on the TGV-2 spectrometer.

The search for double beta decay of 106Cd was performed at the Modane underground laboratory (France, 4800 m.w.e.) using the multi-detector spectrometer TGV-2. 16 samples (~13.6 g) of 106Cd with an enrichment of 75% were installed between neighbouring HPGe detectors and measured during 12900 h. New half-life limits (at 90% CL) were obtained for 2νEC/EC decay of 106Cd to the ground state of 106Pd - T1/2 > 4.2 × 1020 y, and for OνEC/EC resonant decay of 106Cd to 2741 keV and 2718 keV excited states of 106Pd - T1/2 > 1.8 × 1020y and T1/2 > 1.6 × 1020y respectively.

The double beta decay (ββ) is very challenging subject of today's physics. It can be used as a powerful tool to test neutrino properties (e.g. Dirac or Majorana type of neutrino) and lepton number conservation. However, these experiments demand very high sensitivity and very strong background reduction. We have been performing intensive R&D towards the use of pixel detector Timepix in the ββ decay experiments. The Timepix device, operated in Time Over Threshold (TOT) mode, provides spectroscopic capabilities in each individual pixel. The main advantage of such detector is its ability to identify and reject background signals (e.g. tracks made by electrons, alpha particles, muons). It would efficiently recognize the signal of ββ decay processes. Two pixel detectors, Si (pixel size 55 × 55 μm2) and CdTe (pixel size 110 × 110 μm2), were tested in the surface laboratory as well as in the underground laboratories, from the point of view of intrinsic background and the results are presented. Low background materials for construction of the setup were also identified. The first prototype of Silicon Pixel Telescope (SPT) has been tested and future improvements are also presented.

A novel experimental approach to search for double electron capture (EC/EC) is dis- cussed in this article. R&D for a new generation EC/EC spectrometer based on silicon pixel detec- tors (SPDs) has been conducted since 2009 for an upgrade of the TGV experiment. SPDs built on Timepix technology with a spectroscopic readout from each individual pixel are an effective tool to detect the 2nEC/EC signature of the two low energy X-rays hitting two separate pixels. The ability of SPDs to indentify a=b=g particles and localize them precisely leads to effective back- ground discrimination and thus considerable improvement of the signal-to-background ratio (S/B). A multi-SPD system, called a Silicon Pixel Telescope (SPT), is planned based on the experimental approach of the TGV calorimeter which measures thin foils of enriched EC/EC-isotope sandwiched between HPGe detectors working in coincidence mode. The sources of SPD internal background have been identified by measuring SPD radiopurity with a low-background HPGe detector as well as by long-term SPD background runs in the Modane underground laboratory (LSM, France), and results of these studies are presented.

The TGV II (Telescope Germanium Vertical) facility is a low background spectrometer operated in Modane Underground Laboratory. It aims at the study of double electron capture of 106Cd. The spectrometer is composed of 32 HPGe planar detectors interleaved with thin-foil samples made of 106Cd enriched to 75%. In 2006, the main run of phase I (1 year duration) was terminated yielding a new limit on half-life for two-neutrino double electron capture in 106Cd as 2.0x1020y. The new limit is significantly higher (by almost three orders of magnitude) than those already published.

A short description of experiment TGV (double beta decay of48Ca) is given. The measurement started in the Modane underground laboratory in August 1996. The first result ofT 1 2/0 ≥ 4.6 × 1020 years [90% CL] after 2545 hours is presented.

High-precision direct gamma -ray and K X- gamma coincidence measurements have been performed in the electron-capture decay of 158Tb. A revised level scheme of 158Gd is proposed, with high-precision energies and intensities. The Q value determined for this decay using K capture to the high-lying levels in 158Gd shows that a K capture to the 1187 keV level is excluded. In addition the authors' K X- gamma coincidence measurements do not show any such event with an upper limit of 2*10-5 (90% CL).

High-precision K x- gamma coincidence measurements have been performed in the electron-capture decay of 158Tb in order to measure the K-capture probability to the high-lying levels in 158Gd. Results show that the Q-value of this decay does not allow an ultra-low-energy capture decay to the 1187 keV level as predicted previously. In addition, the K x- gamma coincidence measurements do not show any such event within the statistical errors.