The SuperNEMO Demonstrator, designed to search for double beta decay using enriched $^{82}$Se, has been assembled in the Modane Underground Laboratory under the French Alps. Thin foils with radio - purified and enriched $^{82}$Se are installed centrally in the detector. A novel foil fabrication method has been developed, improving the radiopurity achieved in the previous generation experiment. It consists of wrapping standalone selenium pads in raw Mylar, combined with selenium purified by a new reverse-chromatography method. This paper describes the features of these foils, their fabrication process, the characterization results, and the integration of the foils into the SuperNEMO Demonstrator.

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 $^{207}$Bi radioactive source coupled to a plastic scintillator, which 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 $^{207}$Bi 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 $^{82}$Se, with the unique combination of a tracking detector and a segmented calorimeter. This feature allows to detect the two electrons emitted in the decay and measure their individual energy and angular distribution. The SuperNEMO calorimeter consists of 712 plastic scintillator blocks readout by large PMTs. After the construction of the demonstrator calorimeter underground, we have performed its first commissioning using γ-particles from calibration sources or from the ambient radioactive background. This article presents the quality assurance tests of the SuperNEMO demonstrator calorimeter and its first time and energy calibrations, with the associated methods.

The SuperNEMO experiment will search for neutrinoless double-beta decay ($0\nu\beta\beta$), and study the Standard-Model double-beta decay process ($2\nu\beta\beta$). The SuperNEMO technology can measure the energy of each of the electrons produced in a double-beta ($\beta\beta$) decay, and can reconstruct the topology of their individual tracks. The study of the double-beta decay spectrum requires very accurate energy calibration to be carried out periodically. The SuperNEMO Demonstrator Module will be calibrated using 42 calibration sources, each consisting of a droplet of $^{207}$Bi within a frame assembly. The quality of these sources, which depends upon the entire $^{207}$Bi droplet being contained within the frame, is key for correctly calibrating SuperNEMO's energy response. In this paper, we present a novel method for precisely measuring the exact geometry of the deposition of $^{207}$Bi droplets within the frames, using Timepix pixel detectors. We studied 49 different sources and selected 42 high-quality sources with the most central source positioning.

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.

SuperNEMO is a double-$\beta$ decay experiment, which will employ the successful tracker–calorimeter technique used in the recently completed NEMO-3 experiment. SuperNEMO will implement 100 kg of double-$\beta$ decay isotope, reaching a sensitivity to the neutrinoless double-$\beta$ decay ($0\nu\beta\beta$) half-life of the order of $10^{26}$ yr, corresponding to a Majorana neutrino mass of 50–100 meV. One of the main goals and challenges of the SuperNEMO detector development programme has been to reach a calorimeter energy resolution, $\Delta E/E$, around $3\% / \sqrt{E}$ (MeV) $\sigma$, or $7\% / \sqrt{E}$ (MeV) FWHM (full width at half maximum), using a calorimeter composed of large volume plastic scintillator blocks coupled to photomultiplier tubes. We describe the R&D programme and the final design of the SuperNEMO calorimeter that has met this challenging goal.

The BiPo-3 detector, running at the Canfranc Underground Laboratory (Laboratorio Subterráneo de Canfranc, LSC, Spain) since 2013, is a low-radioactivity detector dedicated to measuring ultra low natural radionuclide contaminations of $^{208}$Tl ($^{232}$Th chain) and $^{214}$Bi ($^{238}$U chain) in thin materials. The total sensitive surface area of the detector is 3.6m$^2$. The detector has been developed to measure the radiopurity of the selenium double $\beta$-decay source foils of the SuperNEMO experiment. In this paper the design and performance of the detector, and results of the background measurements in $^{208}$Tl and $^{214}$Bi, are presented, and the validation of the BiPo-3 measurement with a calibrated aluminium foil is discussed. Results of the $^{208}$Tl and $^{214}$Bi activity measurements of the first enriched $^{82}$Se foils of the double $\beta$-decay SuperNEMO experiment are reported. The sensitivity of the BiPo-3 detector for the measurement of the SuperNEMO $^{82}$Se foils is $\mathcal{A}$($^{208}$Tl) $<2$ $\mu$Bq/kg (90% C.L.) and $\mathcal{A}$($^{214}$Bi) $<140$ $\mu$Bq/kg (90% C.L.) after 6 months of measurement.

The SuperNEMO detector will search for neutrinoless double beta decay at the Modane Underground Laboratory; the detector design allows complete topological reconstruction of the decay event enabling excellent levels of background rejection and, in the event of a discovery, the ability to determine the nature of the lepton number violating process. In order to demonstrate the feasibility of the full experiment, we are building a Demonstrator Module containing 7 kg of $^{82}$Se, with an expected sensitivity of |$m_{\beta\beta}$| < 0.2 - 0.4 eV after 2.5 yr. The demonstrator tracker is currently being assembled in the UK; the main challenge in the tracker design is the high radiopurity required to limit the background. For this reason the cell wiring is automated and every step of the tracker assembly happens in a clean environment. All components are carefully screened for radiopurity and each section of the tracker, once assembled, is sealed and checked for Radon emanation. We present the detector design, the current status of the construction and present the first results from the surface commissioning of one section of the Demonstrator Module tracker.

We have constructed a GEANT4-based detailed software model of photon transport in plastic scintillator blocks and have used it to study the NEMO-3 and SuperNEMO calorimeters employed in experiments designed to search for neutrinoless double beta decay. We compare our simulations to measurements using conversion electrons from a calibration source of $^{207}$Bi and show that the agreement is improved if wavelength-dependent properties of the calorimeter are taken into account. In this article, we briefly describe our modeling approach and results of our studies.

The possibility to probe new physics scenarios of light Majorana neutrino exchange and right-handed currents at the planned next generation neutrinoless double $\beta$ decay experiment SuperNEMO is discussed. Its ability to study different isotopes and track the outgoing electrons provides the means to discriminate different underlying mechanisms for the neutrinoless double $\beta$ decay by measuring the decay half-life and the electron angular and energy distributions.

The development of BiPo detectors is dedicated to the measurement of extremely high radiopurity in $^{208}$Tl and $^{214}$Bi for the SuperNEMO double beta decay source foils. A modular prototype, called BiPo-1, with 0.8 m$^2$ of sensitive surface area, has been running in the Modane Underground Laboratory since February, 2008. The goal of BiPo-1 is to measure the different components of the background and in particular the surface radiopurity of the plastic scintillators that make up the detector. The first phase of data collection has been dedicated to the measurement of the radiopurity in $^{208}$Tl. After more than one year of background measurement, a surface activity of the scintillators of $\mathcal{A}(^{208}\textrm{Tl})=1.5\mu$ Bq/m$^2$ is reported here. Given this level of background, a larger BiPo detector having 12 m$^2$ of active surface area, is able to qualify the radiopurity of the SuperNEMO selenium double beta decay foils with the required sensitivity of $\mathcal{A}(^{208}\textrm{Tl})<2\mu$ Bq/kg (90% C.L.) with a six month measurement.

Neutrinoless double-beta decay ($\beta\beta (0 \nu)$) is the most sensitive process in the search for leptonic number violation and its discovery would prove that the neutrino is a Majorana particle. From the experience of the NEMO-3 detector construction and data analysis, the NEMO Collaboration proposes a three-year R&D program in order to design a detector (SuperNEMO) sensitive to a $\beta\beta (0 \nu)$ period of few 10$^{26}$ yr coupling track reconstruction and calorimeter.

NEMO-3 results for the double beta decay of $^{150}$Nd to the $0^{+}_{1}$ and $2^{+}_{1}$ excited states of $^{150}$Sm are reported. The data recorded during $5.25$ y with $36.6$ g of the isotope $^{150}$Nd were used in the analysis. For the first time the signal of $2$νββ transition to the $0^{+}_{1}$ excited state is detected with statistical significance exceeding $5$ sigma. The half-life is measured to be $T^{2νββ}_{1/2}(0^{+}_{1})=[1.11^{+0.19}_{−0.14}(stat)^{+0.17}_{−0.15}(syst)]×10^{20}$y. Limits are set on $2$νββ decay to $2^{+}_{1}$ level and on $0$νββ decay to $0^{+}_{1}$ and $2^{+}_{1}$ levels of $^{150}$Sm.

Double-beta decays of $^{100}$Mo from the 6.0195-year exposure of a 6.914 kg high-purity sample were recorded by the NEMO-3 experiment that searched for neutrinoless double-beta decays. These ultra-rare transitions to $^{100}$Ru have a half-life of approximately $7\times10^{18}$ years, and have been used to conduct the first ever search for periodic variations of this decay mode. The Lomb-Scargle periodogram technique, and its error-weighted extension, were employed to look for periodic modulations of the half-life. Monte Carlo modeling was used to study the modulation sensitivity of the data over a broad range of amplitudes and frequencies. Data show no evidence of modulations with amplitude greater than 2.5% in the frequency range of 0.33225y$^{−1}$ to 365.25y$^{−1}$.

The double-beta decay of $^{82}$Se to the 0$^+_1$ excited state of $^{82}$Kr has been studied with the NEMO-3 detector using 0.93 kg of enriched $^{82}$Se measured for 4.75 y, corresponding to an exposure of 4.42 kg y. A dedicated analysis to reconstruct the gamma-rays has been performed to search for events in the $2e2\gamma$ channel. No evidence of a $2\nu\beta\beta$ decay to the 0$^+_1$ state has been observed and a limit of $T^{2\nu}_{ 1/2}(^{82}\text{Se}; 0^+_{\text{gs}}\to 0^+_1) > 1.3 \times 10^{21}$ y at 90% CL has been set. Concerning the $0\nu\beta\beta$ decay to the 0$^+_1$ state, a limit for this decay has been obtained with $T^{0\nu}_{ 1/2}(^{82}\text{Se}; 0^+_{\text{gs}}\to 0^+_1) > 2.3 \times 10^{22}$ y at 90% CL, independently from the $2\nu\beta\beta$ decay process. These results are obtained for the first time with a tracko-calo detector, reconstructing every particle in the final state.

The full data set of the NEMO-3 experiment has been used to measure the half-life of the two-neutrino double beta decay of $^{100}$Mo to the ground state of $^{100}$Ru, $T_{1/2} = \left[ 6.81 \pm 0.01\,\left(\mbox{stat}\right) ^{+0.38}_{-0.40}\,\left(\mbox{syst}\right) \right] \times10^{18}$ y. The two-electron energy sum, single electron energy spectra and distribution of the angle between the electrons are presented with an unprecedented statistics of $5\times10^5$ events and a signal-to-background ratio of $\sim$80. Clear evidence for the Single State Dominance model is found for this nuclear transition. Limits on Majoron emitting neutrinoless double beta decay modes with spectral indices of n=2,3,7, as well as constraints on Lorentz invariance violation and on the bosonic neutrino contribution to the two-neutrino double beta decay mode are obtained.

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 $T_{1/2}^{2\nu} (9.39 \pm 0.17\,\left(\mbox{stat}\right) \pm 0.58\,\left(\mbox{syst}\right)) \times 10^{19}$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.

We report the results of a first experimental search for lepton number violation by four units in the neutrinoless quadruple-$\beta$ decay of $^{150}$Nd using a total exposure of $0.19$ kg$\cdot$y recorded with the NEMO-3 detector at the Modane Underground Laboratory (LSM). We find no evidence of this decay and set lower limits on the half-life in the range $T_{1/2}>(1.1\text{-}3.2)\times10^{21}$ y at the $90\%$ CL, depending on the model used for the kinematic distributions of the emitted electrons.

The NEMO-3 experiment measured the half-life of the 2$\nu\beta\beta$ decay and searched for the 0$\nu\beta\beta$ decay of $^{116}$Cd. Using 410 g of $^{116}$Cd installed in the detector with an exposure of 5.26 y, (4968$\pm$74) events corresponding to the 2$\nu\beta\beta$ decay of $^{116}$Cd to the ground state of $^{116}$Sn have been observed with a signal to background ratio of about 12. The half-life of the 2$\nu\beta\beta$ decay has been measured to be $T^{2\nu}_{1/2} = \left[2.74 \pm 0.04(\textrm{stat}) \pm0.18 (\textrm{syst})\right]\times 10^{19}$ y. No events have been observed above the expected background while searching for 0$\nu\beta\beta$ decay. The corresponding limit on the half-life is determined to be $T_{1/2}^{0\nu}\geq 1.0\times 10^{23}$ y at the 90 C.L. which corresponds to an upper limit on the effective Majorana neutrino mass of $\left < m_\nu \right > \leq 1.4-2.5$ eV depending on the nuclear matrix elements considered. Limits on other mechanisms generating 0$\nu\beta\beta$ decay such as the exchange of R-parity violating supersymmetric particles, right-handed currents and majoron emission are also obtained.

We present results from a search for neutrinoless double-$\beta$(0$\nu\beta\beta$) decay using 36.6 g of the isotope $^{150}$Nd with data corresponding to a live time of 5.25 y recorded with the NEMO-3 detector. We construct a complete background model for this isotope, including a measurement of the two-neutrino double-$\beta$ decay half-life of $T^{2\nu}_{1/2} = \left[9.34 \pm 0.22(\textrm{stat})+0.62-0.60 (\textrm{syst})\right]\times 10^{18}$ y for the ground state transition, which represents the most precise result to date for this isotope. We perform a multivariate analysis to search for 0$\nu\beta\beta$ decays in order to improve the sensitivity and, in the case of observation, disentangle the possible underlying decay mechanisms. As no evidence for 0$\nu\beta\beta$ decay is observed, we derive lower limits on half-lives for several mechanisms involving physics beyond the standard model. The observed lower limit, assuming light Majorana neutrino exchange mediates the decay, is $T_{1/2}^{0\nu}\geq 2.0\times 10^{22}$ y at the 90% C.L., corresponding to an upper limit on the effective neutrino mass of $\left < m_\nu \right > \leq 1.6-5.3$ eV.

The NEMO-3 experiment at the Modane Underground Laboratory investigates the double-beta decay of $^{48}$Ca. Using 5.25 yr of data recorded with a 6.99 g sample of $^{48}$Ca, 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 $^{48}$Ca is measured to be $T^{2\nu}_{1/2} = \left[6.4 ^{+0.7}_{-0.6}(\textrm{stat})^{+1.2}_{-0.9} (\textrm{syst})\right]\times 10^{19}$ yr. A search for neutrinoless double-beta decay of $^{48}$Ca yields a null result, and a corresponding lower limit on the half-life is found to be $T_{1/2}^{0\nu}\geq 2.0\times 10^{22}$ at 90% confidence level, translating into an upper limit on the effective Majorana neutrino mass of $\left < m_{\beta\beta} \right > \leq 6.0-26$ eV, with the range reflecting different nuclear matrix element calculations. Limits are also set on models involving Majoron emission and right-handed currents.

The NEMO-3 detector, which had been operating in the Modane Underground Laboratory from 2003 to 2010, was designed to search for neutrinoless double-$\beta$(0$\nu\beta\beta$) decay. We report the final results of a search for 0$\nu\beta\beta$ decays with 6.914 kg of $^{100}$Mo using the entire NEMO-3 data set with a detector live time of 4.96 yr, which corresponds to an exposure of 34.3 kg yr. We perform a detailed study of the expected background in the 0$\nu\beta\beta$ signal region and find no evidence of 0$\nu\beta\beta$ decays in the data. The level of observed background in the 0$\nu\beta\beta$ signal region $[2.8–3.2]$ MeV is $0.44\pm0.13$ counts/yr/kg, and no events are observed in the interval $[3.2–10]$ MeV. We therefore derive a lower limit on the half-life of 0$\nu\beta\beta$ decays in $^{100}$Mo of $T_{1/2}(0$\nu\beta\beta$)> 1.1\times 10^{24}$ yr at the 90% confidence level, under the hypothesis of decay kinematics similar to that for light Majorana neutrino exchange. Depending on the model used for calculating nuclear matrix elements, the limit for the effective Majorana neutrino mass lies in the range $\left < m_\nu \right > \leq 0.33-0.62$ eV. We also report constraints on other lepton-number violating mechanisms for 0$\nu\beta\beta$ decays.

We report the results of a search for the neutrinoless double-$\beta$ decay (0$\nu\beta\beta$) of $^{100}$Mo, using the NEMO-3 detector to reconstruct the full topology of the final state events. With an exposure of 34.7 kg y, no evidence for the 0$\nu\beta\beta$ signal has been found, yielding a limit for the light Majorana neutrino mass mechanism of $T_{1/2}^{0\nu}> 1.1\times 10^{24}$ years (90% C.L.) once both statistical and systematic uncertainties are taken into account. Depending on the nuclear matrix elements this corresponds to an upper limit on the Majorana effective neutrino mass of $\left < m_\nu \right > < 0.3-0.9$ eV (90% C.L.). Constraints on other lepton number violating mechanisms of 0$\nu\beta\beta$ decays are also given. Searching for high-energy double electron events in all suitable sources of the detector, no event in the energy region [3.2–10] MeV is observed for an exposure of $47 \textrm{kg}\cdot\textrm{y}$.

Double beta decay of $^{100}$Mo to the excited states of daughter nuclei has been studied using a 600 cm$^3$ low-background HPGe detector and an external source consisting of 2588 g of 97.5% enriched metallic $^{100}$Mo, which was formerly inside the NEMO-3 detector and used for the NEMO-3 measurements of $^{100}$Mo. The half-life for the two-neutrino double beta decay of $^{100}$Mo to the excited $0_1^+$ state in $^{100}$Ru is measured to be $T_{1/2} = \left[7.5 \pm 0.6(\textrm{stat})\pm 0.6 (\textrm{syst})\right]\cdot 10^{20}$ yr. For other ($0\nu + 2 \nu$) transitions to the $2_1^+$, $2_2^+$, $0_2^+$, $2_3^+$ and $0_3^+$ levels in $^{100}$Ru, limits are obtained at the level of $~(0.25-1.1)\cdot 10^{22}$ yr.

We report results from the NEMO-3 experiment based on an exposure of 1275 days with 661 g of $^{130}$Te in the form of enriched and natural tellurium foils. The $\beta\beta$ decay rate of $^{130}$Te is found to be greater than zero with a significance of 7.7 standard deviations and the half-life is measured to be $T^{2\nu}_{1/2} = \left[7.0 \pm 0.9(\textrm{stat})\pm 1.1 (\textrm{syst})\right]\times 10^{20} \textrm{yr}$. This represents the most precise measurement of this half-life yet published and the first real-time observation of this decay.

Using 9.4 g of $^{96}$Zr isotope and 1221 days of data from the NEMO-3 detector corresponding to 0.031 kg y, the obtained 2$\nu\beta\beta$ decay half-life measurement is $T^{2\nu}_{1/2} = \left[2.35 \pm 0.14(\textrm{stat})\pm0.16 (\textrm{syst})\right]\times 10^{19} \textrm{yr}$.Different characteristics of the final state electrons have been studied, such as the energy sum, individual electron energy, and angular distribution. The 2$\nu$ nuclear matrix element is extracted using the measured 2$\nu\beta\beta$ half-life and is $M^{2\nu}=0.049\pm0.002$. Constraints on 0$\nu\beta\beta$ decay have also been set.

The half-life for double-$\beta$ decay of $^{150}$Nd has been measured by the NEMO-3 experiment at the Modane Underground Laboratory. Using 924.7 days of data recorded with 36.55g of $^{150}$Nd, we measured the half-life for 2$\nu\beta\beta$ decay to be $T^{2\nu}_{1/2} = \left[9.11 ^{+0.25}_{-0.22}(\textrm{stat})\pm0.63 (\textrm{syst})\right]\times 10^{18} \textrm{yr}$. The observed limit on the half-life for neutrinoless double-$\beta$ decay is found to be $T_{1/2}^{0\nu}>1.8\times 10^{22}$ yr at 90% confidence level. This translates into a limit on the effective Majorana neutrino mass of $\left < m_\nu \right > <4.0-6.3$ eV if the nuclear deformation is taken into account. We also set limits on models involving Majoron emission, right-handed currents, and transitions to excited states.

In the double beta decay experiment NEMO 3 a precise knowledge of the background in the signal region is of outstanding importance. This article presents the methods used in NEMO 3 to evaluate the backgrounds resulting from most if not all possible origins. It also illustrates the power of the combined tracking-calorimetry technique used in the experiment.

The double beta decay of $^{100}$Mo to the $0_1^+$ and $2_1^+$ excited states of $^{100}$Ru is studied using the NEMO 3 data. After the analysis of 8024 h of data the half-life for the two-neutrino double beta decay of $^{100}$Mo to the excited $0_1^+$ state is measured to be $T^{2\nu}_{1/2} = \left[5.7 ^{+1.3}_{-0.9}(\textrm{stat})\pm0.8 (\textrm{syst})\right]\times 10^{20} \textrm{y}$. The signal-to-background ratio is equal to 3. Information about energy and angular distributions of emitted electrons is also obtained. No evidence for neutrinoless double beta decay to the excited $0_1^+$ state has been found. The corresponding half-life limit is $T_{1/2}^{(0\nu)}(0^+ \to 0_1^+)>8.9\times 10^{22}$ y (at 90% C.L.). The search for the double beta decay to the $2_1^+$ excited state has allowed the determination of limits on the half-life for the two neutrino mode $T_{1/2}^{(2\nu)}(0^+ \to 2_1^+)>1.1\times 10^{21}$ y (at 90% C.L.) and for the neutrinoless mode $T_{1/2}^{(0\nu)}(0^+ \to 2_1^+)>1.6\times 10^{23}$ y (at 90% C.L.).

The NEMO-3 tracking detector is located in the Fréjus Underground Laboratory. It was designed to study double beta decay in a number of different isotopes. Presented here are the experimental half-life limits on the double beta decay process for the isotopes $^{100}$Mo and $^{82}$Se for different majoron emission modes and limits on the effective neutrino–majoron coupling constants. In particular, new limits on “ordinary” majoron (spectral index 1) decay of $^{100}$Mo $(T_{1/2}>2.7\times 10^{22}\textrm{yr})$ and $^{82}$Se $(T_{1/2}>1.5\times 10^{22}\textrm{yr})$ have been obtained. Corresponding bounds on the majoron–neutrino coupling constant are $\left < g_{ee} \right > <0.4-1.8\times 10^{-4}$ and $<0.66-1.9\times 10^{-4}$.

The NEMO 3 detector, which has been operating in the Fréjus underground laboratory since February 2003, is devoted to the search for neutrinoless double beta decay ($\beta\beta$0$\nu$). The half-lives of the two neutrino double beta decay ($\beta\beta 2 \nu$) have been measured for $^{100}$Mo and $^{82}$Se. After 389 effective days of data collection from February 2003 until September 2004 (Phase~I), no evidence for neutrinoless double beta decay was found from $\sim$7 kg of $^{100}$Mo and $\sim$1 kg of $^{82}$Se. The corresponding lower limits for the half-lives are $4.6 \times 10^{23}$ years for $^{100}$Mo and $1.0 \times 10^{23}$ years for $^{82}$Se (90\% C.L.). Depending on the nuclear matrix element calculation, the limits for the effective Majorana neutrino mass are $\langle m_{\nu} \rangle < 0.7-2.8$ eV for $^{100}$Mo and $\langle m_{\nu} \rangle < 1.7-4.9$ eV for $^{82}$Se.

The development of the NEMO 3 detector, which is now running in the Fréjus Underground Laboratory (L.S.M. Laboratoire Souterrain de Modane), was begun more than ten years ago. The NEMO 3 detector uses a tracking-calorimeter technique in order to investigate double beta decay processes for several isotopes. The technical description of the detector is followed by the presentation of its performance.

The background induced by radioactive impurities of $^{208}\rm Tl$ and $^{214}\rm Bi$ in the source of the double beta experiment NEMO-3 has been investigated. New methods of data analysis which decrease the background from the above mentioned contamination are identified. The techniques can also be applied to other double beta decay experiments capable of measuring independently the energies of the two electrons.

Most currently, viable double beta decay experiments require highly enriched isotopic sources. These sources must be extraordinarily free of radioactive contamination. The double beta decay experiment NEMO 3 will study $^{100}$Mo, for which physical and chemical purification techniques have been investigated. The success of the chemical purification process is discussed in the context of ultra-low background, high-purity germanium spectrometer measurements.