The performance of Hamamatsu 8" photomultiplier tubes (PMTs) of the type used in the SuperNEMO neutrinoless double-beta decay experiment (R5912-MOD), is investigated as a function of exposure to helium (He) gas. Two PMTs were monitored for over a year, one exposed to varying concentrations of He, and the other kept in standard atmospheric conditions as a control. Both PMTs were exposed to light signals generated by a Bi-207 radioactive source that provided consistent large input PMT signals similar to those that are typical of the SuperNEMO experiment. The energy resolution of PMT signals corresponding to 1 MeV energy scale determined from the Bi-207 decay spectrum, shows a negligible degradation with He exposure; however the rate of after-pulsing shows a clear increase with He exposure, which is modelled and compared to diffusion theory. A method for reconstructing the partial pressure of He within the PMT and a method for determining the He breakdown point, are introduced. The implications for long-term SuperNEMO operations are briefly discussed.

The SuperNEMO experiment is searching for neutrinoless double-beta-decay of Se-82, with the unique combination of a tracking detector and a segmented calorimeter. This feature allows us to detect the two electrons emitted in the decay and measure their individual energies and angular distribution. The SuperNEMO Demonstrator's calorimeter consists of 712 plastic scintillator blocks read out by large PMTs. Having constructed the calorimeter underground, we performed its first commissioning using gamma-rays from calibration sources or from the ambient radioactivity background. This article presents quality assurance tests of the SuperNEMO Demonstrator's calorimeter, and its first time and energy calibrations with gamma-rays, along with the associated methods. A time alignment of about 120 ps and a time resolution around 615 ps have been achieved. Concerning the energy, an alignment of 7.5% has been obtained. These results will be further improved when associating the tracking detector and detecting electrons from calibration sources.

The III phase of experiment TGV-2 to search for β+β+, β+EC, EC/EC decay of 106Cd was performed at the Modane underground laboratory (LSM, France, 4800 m w.e.). 16 foils (∼23.2 g) of enriched 106Cd were measured using the 32-detector low background HPGe spectrometer TGV-2 during 42500 h. New limit on 2νEC/EC decay of 106Cd to the ground 0+ state of 106Pd - T1/2 > 7.2 × 1020 y at 90% C.L was obtained. The limits on 2νβ+β+, 2νβ+EC decay of 106Cd, and 2νECEC decay of 106Cd to excited states of 106Pd were significantly improved in comparison with previous phase II of the TGV-2 experiment.

Abstract 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.

New limits on β+EC and ECEC processes in 74Se have been obtained using a 600 cm3 HPGe detector and an external source consisting of 1600 g of a natural selenium powder. For different β+EC and ECEC transitions (to the ground and excited states) obtained limits are on the level ∼(0.2−4.8)×1019yr at 90% C.L. In particular, for the potentially resonant transition into the 1204.2 keV excited state of 74Ge a lower half-life limit of 1.1×1019yr at 90% C.L. has been obtained. Possibility to increase the sensitivity of such measurements is discussed.

Double beta decay (β+EC, EC/EC) of 74Se 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 cm3 and a sample of natural selenium with a mass of 1.6 kg containing ~0.89% (~14.24 g) of 74Se. The new experimental limits for β+EC and EC/EC decays of 74Se to ground 0+ and excited $$2_{1}^{ + },$$ 596 keV, and $$2_{2}^{ + },$$ 1204 keV states of 74Ge, were obtained from experimental data accumulated over 135 days.

Abstract A radiochemical method for producing 82Se sources with an ultra-low level of contamination of natural radionuclides (40K, decay products of 232Th and 238U) has been developed based on cation-exchange chromatographic purification with reverse removal of impurities. It includes chromatographic separation (purification), reduction, conditioning (which includes decantation, centrifugation, washing, grinding, and drying), and 82Se foil production. The conditioning stage, during which highly dispersed elemental selenium is obtained by the reduction of purified selenious acid (H2SeO3) with sulfur dioxide (SO2) represents the crucial step in the preparation of radiopure 82Se samples. The natural selenium (600 g) was first produced in this procedure in order to refine the method. The technique developed was then used to produce 2.5 kg of radiopure enriched selenium (82Se). The produced 82Se samples were wrapped in polyethylene (12 μm thick) and radionuclides present in the sample were analyzed with the BiPo-3 detector. The radiopurity of the plastic materials (chromatographic column material and polypropylene chemical vessels), which were used at all stages, was determined by instrumental neutron activation analysis. The radiopurity of the 82Se foils was checked by measurements with the BiPo-3 spectrometer, which confirmed the high purity of the final product. The measured contamination level for 208Tl was 8–54 μBq/kg, and for 214Bi the detection limit of 600 μBq/kg has been reached.

Investigation of double beta decay processes (beta+EC, EC/EC) of Se-74 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 cm(3) and a sample of natural selenium with a mass of 1.6 kg, containing similar to 0.89% (similar to 14.24 g) of Se-74. New experimental limits (at 90 % CL) on double beta decay transitions of Se-74 to the ground and exited states of Ge-74 were obtained from the preliminary calculation of experimental data accumulated in 135 days.

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.

Using data from the NEMO-3 experiment, we have measured the two-neutrino double beta decay ($2\nu\beta\beta$) half-life of $^{82}$Se as $T_{1/2}^{2\nu} = \left[ 9.39 \pm 0.17\,\left(\mbox{stat}\right) \pm 0.58\,\left(\mbox{syst}\right)\right] \times 10^{19}$ y under the single-state dominance hypothesis for this nuclear transition. The corresponding nuclear matrix element is $\left|M^{2\nu}\right| = 0.0498 \pm 0.0016$. In addition, a search for neutrinoless double beta decay ($0\nu\beta\beta$) using 0.93 kg of $^{82}$Se observed for a total of 5.25 y has been conducted and no evidence for a signal has been found. The resulting half-life limit of $T_{1/2}^{0\nu} > 2.5 \times 10^{23} \,\mbox{y} \,(90\%\,\mbox{C.L.})$ for the light neutrino exchange mechanism leads to a constraint on the effective Majorana neutrino mass of $\langle m_{\nu} \rangle < \left(1.2 - 3.0\right) \,\mbox{eV}$, where the range reflects $0\nu\beta\beta$ nuclear matrix element values from different calculations. Furthermore, constraints on lepton number violating parameters for other $0\nu\beta\beta$ mechanisms, such as right-handed currents, majoron emission and R-parity violating supersymmetry modes have been set.

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.

A new ultra low-background spectrometer based on a HPGe detector with a sensitive volume of 600 cm3 was developed to investigate rare nuclear processes, such as resonant neutrino-less double electron capture (0νEC/EC) and double beta decay processes (2ν2β−, 2νβ+EC, 2νEC/EC) to the excited states of daughter nuclei. The spectrometer was installed at the Modane underground laboratory (LSM, France, 4800 m w.e.). Sensitivity of the spectrometer and its background were tested. A new method for the efficiency calibration in measurements of low-active samples was developed. The spectrometer was used for the measurements of low active materials and samples. Results obtained in 395 h investigation of resonant 0νEC/EC decay of 106Cd to the 2718 keV and 2741 keV excited states of 106Pd with ∼23.2 g of enriched 106Cd and 2ν2β− decay of 100Mo sample with a mass of 2588 g to the 0+, 1130 keV and 2+, 539.5 keV excited states of 100Ru are presented.

A new ultra-low-background spectrometer based on a HPGe detector with a sensitive volume of 600 cm 3 was developed to investigate rare nuclear processes, such as resonant neutrino-less double electron capture (0νEC/EC) and double beta decay processes (2ν2β -, 2νβ + EC, 2νEC/EC) to the excited states of daughter nuclei.The spectrometer was installed at the Modane underground laboratory (LSM, France, 4800 m w.e.).Sensitivity of the spectrometer and its background were tested.A new method for the efficiency calibration in measurements of low-active samples was developed.A spectrometer was used for the measurements of low active materials and samples.Results obtained in 395 h investigation of resonant 0νEC/EC decay of 106 Cd to the 2718 keV and 2741 keV excited states of 106 Pd with ~23.2 g of enriched 106 Cd, β + EC, EC/EC decays of 58 Ni in measurements of ~21.7 kg sample of natural nickel and 2ν2β -decay of 100 Mo sample with the mass of 2588 g to the 0 + , 1130 keV and 2 + , 539.5 keV excited states of 100 Ru are presented.

Investigation of double beta decay processes (beta(+) EC, EC/EC) of Ni-58 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 cm(3) and a sample of natural nickel, containing similar to 68% of Ni-58 with a mass of similar to 21.7 kg. New experimental limits on 2 nu beta(+) EC decay of Ni-58 to the ground 0(+) and 2(1)(+), 811 keV excited state of Fe-58, and 2 nu EC/EC decay of Ni-58 to 2(1)(+), 811 keV and 2(2)(+), 1 675 keV excited states of Fe-58 were obtained from the preliminary calculation of experimental data accumulated in similar to 88d. They are -T-1/2(beta(+) EC, 0(+) -> 0(+)) > 1.4 x10(21) y; T-1/2 (beta(+) EC, 0(+) -> 2(1)(+) > 1.7 x10(22) y, T-1/2 (EC/EC, 0(+) -> 2(1)(+)) > 2.4 x10(22) y, T-1/2 (EC/EC, 0(+) -> 2(2)(+)) > 2.1 x 10(22) y. For resonant neutrino-less radiative EC/EC decay with energy of 1 918.3 keV a new experimental limit of T-1/2 (0.EC/EC - res, 1918keV) > 4.7 x10(22) y was also obtained. All limits are at 90 % CL.

The NEMO-3 experiment at the Modane Underground Laboratory investigates the double-beta decay of Ca-48. Using 5.25 yr of data recorded with a 6.99 g sample of Ca-48, approximately 150 double-beta decay candidate events are selected with a signal-to-background ratio greater than 3. The half-life for the two-neutrino double-beta decay of Ca-48 is measured to be T-1/2(2v) = [6.4(-0.6)(+0.7)(stat)(-0.6)(+0.7)(syst)] x 10(19) yr. A search for neutrinoless double-beta decay of Ca-48 yields a null result, and a corresponding lower limit on the half-life is found to be T-1/2(0v) > 2.0 x 10(22) yr at 90% confidence level, translating into an upper limit on the effective Majorana neutrino mass of < m(beta beta)> < 6.0-26 eV, with the range reflecting different nuclear matrix element calculations. Limits are also set on models involving Majoron emission and right-handed currents.

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.