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.

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.

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.