GRoup “Interactions FOndamentales et nature du Neutrino” (GRIFON)
The GRIFON group at LPC Caen is involved in four experimental research activities : LPCTrap: search for exotic couplings in nuclear weak decay processes, MOT: High resolution studies of atomic collisions in a MOT, nEDM: measurement of the neutron electric dipole moment, NEMO-3/SuperNEMO: search for neutrinoless double beta decay.
Each member of the group participates to one or two of these experimental programs. The instrumental and technical know-how and specific skills acquired in these research activities are often shared within the GRIFON group: detection of electrons in the MeV energy range with PMT-based optical modules (LPCTrap/SuperNEMO), electric and magnetic fields computing (nEDM/SuperNEMO/LPCTrap), Monte-Carlo simulations (SuperNEMO/LPCTrap), parallel computing (nEDM/LPCtrap). Most of these research activities are maintained within the framework of internationally recognized collaborations. The experi-mental setups are hosted in first-class facilities :
Institut Laue-Langevin (ILL), Paul Scherrer Institute (PSI), GANIL/SPIRAL,
Laboratoire Souterrain de Modane (LSM),
Center for Nuclear Physics and Astrophysics (CENPA, Seattle), CERN/ISOLDE.
For all these projects, the LPC Caen occupies a prominent position with well identified responsabilities and recognized skills.
G. Ban, C. Couratin*1
, D. Durand, X. Fabian*1
, X. Fléchard, B. Guillon, V. Hélaine*2
, T. Lefort ,Y.
Lemière, A. Leredde*3
, E. Liénard, F. Mauger, O. Naviliat4
Experimental facilities : Institut Laue-Langevin (ILL), Paul Scherrer Institute (PSI), GANIL/SPIRAL, Laboratoire Souterrain de Modane (LSM),
CENPA (Seattle), CERN/ISOLDE.
1LPCTrap, 2nEDM, 3MOT
4NSCL/MSU East Lansing
The LPCTrap setup installed at GANIL/LIRAT has been built to measure with high precision beta-neutrino angular correlation parameters,a, in nuclear beta decays [Ban13]. Such measurements provide the most stringent limits on exotic Scalar (S) or Tensor (T) type contributions to the nuclear weak decay process. Precisions at a level of at least 0.5% are needed to improve the current sensitivity to these exotic weak interaction components. In addition, in the case of a mirror decay, the measurement ofaenables to precisely determine the mixing ratio,ρ, between the Gamow-Teller (GT) and the Fermi (F) components in the transition. Combined with the Ft value, this mixing ratio can be used to determine accurately Vud, the first element of the Cabibbo-Kobayashi-Maskawa (CKM) matrix. Again, measurements ofaat a precision level of at least 0.5% in some selected mirror decays for which the parameters involved in the Ft values are accurately known would enable to reach a precision onVudequivalent to the current precision obtained from the usual pureFtransitions.
In the LPCTrap setup, the radioactive ions are confined in a transparent Paul trap and the correlation parameter, a, is precisely inferred from the time of flight of the low energy recoiling ions detected in coincidence with the beta particles.
Moreover, the recoil ion spectrometer gives access to the charge state distributions of daughter ions stemming from 1+
ions decay. The setup allows a very strong control of the systematic effects.
The6He pureGTdecay is the first transition studied with LPCTrap [Cour13]. The last experiment was performed in 2010.
The recoil spectrometer enabled to measure for the first time the charge state distributions of the recoiling 6Li ions produced after theβdecay of6He1+ions. An electron shake-off probability,Pexp=0.02339(36), was deduced from the data [Cour12]. The value is in perfect agreement with simple quantum mechanical calculations, Pth=0.02322, based on the sudden approximation, which has then been proved to be well suited for such a pure electron shake-off process. A preliminary value has been estimated for the angular correlation parameter,a=-0.3338(26)stat, but the realistic simulations needed to extractahave still to be refined to carefully manage the relevant parameters of the experiment. The number of recorded events, 1.2×106, should enable to determineawith an unprecedented statistical accuracy of 0.0015 (0.45% in relative precision).
As far as the 6He decay is concerned, the team now contributes in a new experiment involving magneto-optical traps (MOT) and installed at CENPA, Seattle [Knec13]. Our main contribution is the development of the recoil ion detector and the installation of the acquisition system FASTER developed at LPC Caen (see section "Administration and technical departments, Instrumentation" of the present report). The goal of the experiment is to reach the precision level of 0.1% in the measurement ofa.
The apparent success of6He experiment has favoured the upgrade of LPCTrap to make it operational with masses heavier than 6 and, in June 2011, the setup has been commissioned with the 35Ar1+beam delivered by SPIRAL. The 35Ar nuclei decay through a mirror transition dominated by theFcomponent (93%). A total of 4×104good events was recorded in 32 hours of data taking. This enabled to measure for the first time the charge state distribution of the recoiling 35Cl ions [Cour13b], which is in excellent agreement with theoretical values (see table 1).
This analysis has highlighted the important role of Auger processes in electronic rearrangement of such ions. The number of35Cl atoms produced during the experiment was deduced from the number of beta particles detected in singles and the overall absolute detection efficiency for ions.
This estimate leads to 72(10)% of neutral 35Cl recoils, which is also consistent with the 73.9% ratio obtained from the theoretical calculations.
Precise correlation measurements in nuclear beta decay
In collaboration with : LPC Caen, CIMAP Caen, GANIL Caen, IKS/KUL Leuven, NSCL/MSU East Lansing,
Univ. Granada, CENPA Seattle, CELIA Bordeaux, Argonne National Laboratory, NCNR Warsaw
Charge Expt. results Calculation with Auger
Calculation without Auger 1
2 3 4
74.75 (1.07) 17.24 (0.44) 5.71 (0.27) 1.58 (0.21) 0.71 (0.18)
74.37 16.98 6.03 1.79 0.82
87.07 11.92 0.95 0.05
Table 1: Experimental ion charge-state relative branching ratios (%) compared to calculations with and without Auger ionizations.
The real experiment with 35Ar was performed in June 2012. The total efficiency of LPCTrap (transmission & trapping:
0.38%), has enabled to record 1.5×106 real coincidences in one week (Fig.1) [Ban13]. This statistics should allow a determination ofawith an absolute uncertainty of 0.0018 (~0.2% in relative precision). Assuming a systematic uncertainty of the same order than the statistical one, the final result would constitute the most precise value ever obtained in a beta-neutrino angular correlation measurement. The analysis is ongoing [Fabi]. As far as the35Ar decay is concerned, the team was also involved in WITCH runs at CERN/ISOLDE [Brei12]. Again our main contribution was dedicated to the recoil ion detector and the installation of the acquisition system FASTER developed at LPC Caen.
In September and October 2013, the commissioning run and a first experiment were performed with 19Ne which mainly decays through a mirror transition to the ground state of 19F (BR=99.9858(20)%). To get rid of the huge contamination of stable molecular ions with mass 19 in 1+ charge state coming from the ECS source of SPIRAL, the beam lines were tuned in 2+ charge state. This enabled to run the experiment, but with a loss of a factor of 3 in the RFQ efficiency. Nevertheless, even with this charge state, the contamination of the beam with19F2+remained important and saturated the RFQ, limiting the effective number of trapped radioactive ions. Finally a total number of 1.3×105coincidences were recorded during 4 days of data taking (Fig. 2), which remains reasonable considering that the half-life of19Ne is ten times larger than in the case of
35Ar. This statistics will enable to determine the precise charge state distribution of the recoiling 19F ions. A comparison between fig. 1 and 2 shows that the higher charge states are less favored for 19F ions, which is consistent with a lower probability of Auger effects in the decay of 19Ne1+ ions, as predicted by the theoretical calculations [Lien12]. This first analysis will be completed in the coming months. The statistics collected during the experiment will also enable to analyze systematic effects in the determination ofain some extreme conditions linked to19Ne as the recoil maximum kinetic energy is the lowest in this case.
As far as the 19Ne decay is concerned, the team was also involved in a precise T1/2measurement performed at GANIL [Ujic12]. Here our main contribution was the installation of the acquisition system FASTER.
In conclusion, we have collected a large amount of data concerning three different transitions. The next year will be dedicated to the complete analysis of these data, to extract precise values of the beta-neutrino angular correlation parameters in the three cases. This will also enable to design an upgraded LPCTrap setup, to continue weak interaction studies with the new beams expected in the upgrade of SPIRAL, first at LIRAT and later in the DESIR hall.
Fig.1: Experimental time-of-flight distributions of 35Cln+ions produced by the βdecay of 35Ar1+ions confined in the Paul trap.
0 5 10
0 10000 20000 30000
~1.1x105 good events
Fig. 2: Raw experimental time-of-flight distributions of 19Fn+ions produced by the βdecay of 19Ne1+ions confined in the Paul trap.
Laser cooling and trapping of atomic samples in a MOT (Magneto-Optical Trap) is now a first step for many exciting and innovating experiments in atomic physics such as Bose-Einstein condensate formation and superfluidity studies, electromagnetically induced transparency, photoassociation, quantum information, etc… Another possible application is the confinement of radioactive atoms produced by nuclear reactions. The cold cloud of trapped exotic atoms constitutes a very clean source to perform precision measurements in nuclearβdecay: using 6 laser beams and a quadrupolar magnetic-field, the atoms are held in a small volume, almost at rest and in a high vacuum, which allows the detection in coincidence of the decay products using surrounding detectors [Knec13]. Moreover, they can be easily polarized using lasers for correlation measurements in the decay of polarized nuclei [Pitc09].
Having for final objective the installation of such a device on the future DESIR facility at SPIRAL2, the LPC has designed and built a MOT for stable rubidium atoms. With stable atoms, the nice properties of a MOT mentioned above can be used to provide a cold atomic target for the study of ion-atom collisions at low energy. In such experiments, the most effective technique is the recoil ion momentum spectroscopy (RIMS) [Dorn00]. It gives access to the main observables of the collision (the Q of reaction and projectile scattering angle) through the measurement of the ionized target recoil momentum. The momentum change due to the collision being very small (a fraction of a.u.), the resolution is usually limited by the temperature of the target. The Rb target provided by the MOT is a cloud of about 107atoms confined in a 1 mm3volume at a temperature below 200 mK. Such a low temperature does not limit the resolution on the momentum measurement and the coupling of a Rb MOT with RIMS (called MOTRIMS) results in very high precision experiments. The MOTRIMS setup designed and built at the LPC is shown in the fig. 3. It includes transverse extraction of the recoil ions with a 3D focusing electrostatic spectrometer, and a fast switch-off of the trapping B-field during data counting [Blie08].
High resolution study of low energy charge exchange collisions with a MOT (magneto-optical trapped) target
In collaboration with CIMAP (Caen) and CELIA (Bordeaux)
Fig. 3: Schematic view of the setup (see text and ref.  for details)
MOTRIMS can in principle be employed, as in conventional RIMS, to probe a multitude of scattering dynamics [Dorn00]. In a first step, we focused on single charge transfer in low energy Na++87Rb(5s,5p) collisions. The performances of the device allowed the detection of weakly populated charge transfer channel (contributing to less than 1% of the total cross sections), and provided accurate relative cross sections for the active channels, along with their associated distributions in projectile scattering angle. The high resolution on the scattering angle measurement (~50 mrad) has even enabled to resolve the predicted diffraction-like oscillations due to the limited range of impact parameters leading to charge exchange process. The results have been then used to test and refine molecular close-coupling (MOCC) calculations performed at the CELIA in Bordeaux with unprecedented precision.
This joint theoretical/experimental study was published in 2012 [Lerr12].
In a second step, we have used the opportunity to prepare the target with lasers to investigate the case of charge exchange scattering between Na+ ions and oriented Rb(5p±1) atoms. It is known from previous studies on similar systems that the differential cross sections (DCS) in projectile scattering angle for charge exchange exhibit asymmetries related to the coherence of the capture process (Fig. 4). However, most of these previous studies called for more precise measurements in order to reveal the exact angular dependence of the DCS and associated coherence parameters. In this respect, Na++ Rb collisions are particularly challenging since the projectiles are scattered in very forward directions [Lerr12]. To apply the experimental technique to the case of oriented 5p states, several improvements of the setup have been achieved. An additional laser, with circular polarized light (right or left) could be shined on the trapped atoms to prepare the target in a (52p3/2, F=3,mF =+3) state or a (52p3/2,F=3,mF=-3) state when the trapping magnetic-field is switched-off. Helmholtz coils were also added to the setup to provide a weak (4 Gauss), constant and homogeneous magnetic-field that defines a vertical polarization axis. Finally, two different diagnostics were developed to measure the polarization efficiency and the polarization rate. We found that more than 95% of the atoms were oriented within a time interval shorter than 5µs.
Fig. 4: Schematic representation of the experiment. A homogeneous magnetic field Bhdefines the quantization direction (z axis), and optical pumping leads to magnetic sublevels of the target state with well defined hyperfine quantum numbers mF, depending on the handedness of the laser pulse. The initial and final states are therefore quantized in the (x, y, z) reference frame.
The Na+ions impinge on the oriented target with velocity v and impact parameter b, and the scattered projectile distribution is characterized by the spherical angles (q,j) in the (xcol, ycol, zcol) scattering frame.
Experiments were performed with oriented (52p3/2, F=3, mF=±3)≡Rb(5p±1) targets at E=5, 2, and 1 keV. The capture channels of interest Na+ + Rb(5p±1) à Na(nl) + Rb+ were easily identified and selected using the recoil-ion-momentum component parallel to the projectile beam axis. Precise DCS in projectile scattering angles θand ϕ (Fig. 3) were then derived from the transverse momentum components. We present in Figs. 5 (a - f) the weighted DCS sin(θ)sp+1à3p(θ,j) associated with the principal Na+ + Rb(5p±1) to Na(3p) + Rb charge exchange reaction, in terms of its four main contributions to which we refer to as left (ϕ=0), up (ϕ=π/2), right (ϕ=π), and down (ϕ=3π/2) (Fig. 4). These contributions are displayed as functions of Eθ, which is related to the impact parameter b.
We observe in Fig. 5 that the up and down contributions to the DCS are symmetric, whereas the left and right ones exhibit strong asymmetry. As may be seen from Fig. 3, the rotation of the electron flow inherent to the initial oriented state breaks the symmetry of left (y>0) and right (y<0) scatterings while it preserves the up-down symmetry because of reflection symmetry with respect to the (x, y) plane. To proceed more quantitatively, the quantum-mechanical origin of the asymmetry has been investigated using adequate MOCC calculations (Fig. 5). The asymmetries in the DCSs, observed here with unprecedented resolution, were then used to access to the related coherence properties of the capture process in the different charge exchange channels. This work has enabled not only the theoretical description to be improved but also marked out the limits of the single-active-electron and frozen-core approximations. More details about this work can be found in [Lerr12b].
Fig. 5: Weighted DCSs for the charge exchange reaction Na++ Rb(5p+1) à Na(3p) + Rb+at E=1 [(a),(b)], 2 [(c),(d)], and 5 [(e),(f)] keV, as functions of Eθ. The histograms are the measurements, while the continuous
(red) lines correspond to MOCC calculations.
Towards a new measurement of the neutron Electric Dipole Moment (EDM)
In collaboration with: PTB (Berlin, Germany), LPC (Caen, France), JUC (Crakow, Poland), HNI (Crakow, Poland), JINR (Dubna, Russia), LPSC (Grenoble, France), University of Kentucky (Lexington, UK), KUL
(Leuven, Belgium), CSNSM (Orsay, France), Sussex University (Brighton, SU), PSI (Villigen, Switzerland) and ETH (Zürich, Switzerland).
Searches for permanent electric dipole moment (EDM) of particles are considered to be among the most important particle physics experiments at low energy since a non-zero value may reveal non-standard sources of CP violation and physics beyond the standard model (SM). More than 30 experiments are currently running or planned worldwide aiming at measuring the EDM of fundamental particles or systems such as electrons, neutrons, muons, atoms, molecules, etc... Beside the possible implications on the SM, the discovery of new CP violation sources is also required to explain the baryon asymmetry of the universe (BAU) [Sak67].
In this context, searches for the neutron EDM (nEDM) have been started in the 50’s and have been pursued over more than 60 years lowering the nEDM upper precision limit down to <3×10-26e·cm (90% CL) [Bak06]. Theδ−induced nEDM predicted by the SM is 10-32±1 e·cm while the SM extensions predict a neutron EDM in the range of 10-26-10-28 e·cm. Therefore, the next generation of neutron EDM experiments should be able to validate or exclude such models.
The current limitation on the measurement precision is statistical. As a result, all the new nEDM projects (7 over the world) are coupled to the building of high intensity ultra-cold neutron (UCN) sources. At the Paul Sherrer Institute (PSI) in Switzerland, the nEDM experiment is taking place close to a spallation-induced UCN source which has recently been launched. Very first UCN were delivered in December 2010 and the UCN source commissioning was started in 2011.
nEDM experiment status
During the last two years, UCN beam time has been shared between UCN source studies, nEDM spectrometer tuning and nEDM data taking. The experiment status is the following.
Despite an increase of 40% since 2011, the UCN flux produced by the PSI source remains 15 times lower than initially anticipated. Reasons are not fully understood and studies are still ongoing in order to recover the lack of UCN production. On the other hand, the spectrometer is basically working. The current statistical sensitivity is 2.8×10-25e·cm per dayi.e.the best sensitivity achieved so far with the apparatus. However, the apparatus reliability is not optimal and will be further improved in order to increase the number of data taking days per year. From 2012 and 2013, the integrated statistical sensitivity is at best equal to 5.9×10-26e·cm. The analysis is still ongoing and is part of V. Hélaine’s thesis. Systematic errors are quoted at the level of 4×10−27 e·cmi.e.well below the statistical uncertainty. In these conditions, continuing the data taking for 3 more years will result in a new nEDM measurement at a level comparable to the present limit but with a better control of systematic effects. If the full factor 15 in the UCN source intensity is recovered, we expect to improve the limit by a factor 4.
In order to significantly improve the statistical sensitivity, a new spectrometer (n2EDM) is under study. A statistical precision 5 times better than the former one is foreseen. Combined with the current UCN source performances, a limit of 5×10-27e·cm could be achieved after 4-5 years of data taking, i.e. in about 10 years. Assuming the UCN source will reach its nominal performances, the 10-28e·cm range will start to be explored. Beside the contribution to the running of the experiment itself, the LPC Caen is in charge of the development of a new spin analyzing system, the UCN detection and the 3D magnetic field measurements.
Set-up of a new U-shape Simultaneous Spin Analyser (USSA)
Based on GEANT4 simulations, a U-shape simultaneous spin analyser system has been built and tested at PSI (see the left panel of Fig. 6). The aim of such a new device is to perform UCN counting and to be able to simultaneously measure both UCN spin components. It will replace the current sequential spin analyser which induces UCN losses and depolarizations. A second NANOSC detector and a dedicated FASTER acquisition have been developed for the USSA. This project is part of the V. Hélaine’s thesis.
Fig. 6: left panel: picture of the simultaneous spin analyser (USSA);
right panel: neutron frequency measurement performed with the USSA.
The USSA has been tested and compared to the current system for standard nEDM runs. A preliminary measurement of the nEDM statistical sensitivity is very promising showing an increase of 18.2(6.1)% with respect to the sequential device. As a result, the USSA has been installed below the nEDM spectrometer for the future nEDM data taking. Such results have to be further confirmed in 2014.
Development of a new UCN gas detector
Investigations about a novel UCN gas detector have been started. Two approaches are under study: a Micro-Pattern Gaseous Detectors (MPGD) based on the GEM (Gas Electron Multiplier) technology and a scintillating gas detector using one or two PhotoMultipliers Tubes (PMT). The aims are three fold: decrease the background sensitivity, increase the detection efficiency and handle large counting rates up to106counts/s.
A generic detector chamber has been built (see the left panel of Fig. 7). The vacuum tightness has been tested and a pressure limit down to 1.5×10-7mbar has been reached. The GEM version of the detector has been characterized using an alpha source for two gases: ArCO2and CF4. A Maximum pulse duration of 150 ns has been measured, which fulfills the counting rate requirement. No discharge was observed for gains up to 8×103.
The scintillating version of the detector has been tested with a CF4/4He gas mixture with a partial4He pressure varying from 0% up to 10%. Using an alpha source and one photomultiplier, about 50 photons have been collected for a deposited energy of 6 MeV. The next step is the design of a new detector with two PMTs for the light readout. The goals are two fold:
increase the photon collection efficiency and suppress the background by coincidence counting technique.
Magnetic field mapping
Aprecise knowledge of the 3D magnetic field inside the nEDM apparatus is of crucial importance for correcting some systematics effects (see the previous progress report for more details). In order to measure the field components, we use either a 3D fluxgate or a vector caesium magnetometer. They are positioned on a new mapping device designed and built at LPC as shown in Fig. 8. To avoid field induced by eddy currents, this mapper is made almost fully out of non-conducting materials (PEEK, POM and ceramics). This device allows mapping a cylindrical volume by moving the probe inside the nEDM vacuum tank (radial, vertical and azimuthal motions). Special care has been taken to ensure mechanical reproducibility.
A mapping campaign has been performed during the winter 2013 with this device. One of the measured maps is shown in Fig. 8 where a magnetic anomaly is seen at the front left side of the nEDM spectrometer. Further maps will be performed in 2014 looking for such magnetic pollution in order to remove them.
Fig. 7: left panel: detector chamber picture; right panel: identification chart for the scintillating version of the detector.
Fig. 8: left panel: picture of the new mapper into the vacuum chamber; right panel: field map of the spectrometer inside.
Many extensions of the Standard Model provide a natural framework for neutrino masses and lepton number violation. In particular the see-saw mechanism, which requires the existence of a Majorana neutrino, naturally explains the smallness of neutrino masses. The existence of Majorana neutrinos would also provide a natural framework for the leptogenesis process which could explain the observed baryon-antibaryon asymmetry in the Universe. The observation of neutrinoless double-β decay (0νββ) would prove that neutrinos are Majorana particles and that lepton number is not conserved. The isotopes for which a single-βis energetically forbidden or strongly suppressed are most suitable for the search of this rare radioactive process. The experimental signature of 0νββdecays is the emission of two electrons with total energy (ETOT) equal to the Q-value of the decay (Qββ). The NEMO-3/SuperNEMO international collaboration has maintained an experimental program of research of the (0νββ) process for about 20 years. Currently, the LPC NEMO group is involved in two projects: the NEMO-3 experiment and the SuperNEMO project.
NEMO3 final analysis
After 8 years of data collection, the NEMO-3 detector has been stopped in february 2011 and dismantled at the Modane Underground Laboratory (LSM, Fig. 9). The NEMO collaboration now does the final analysis with the full statistics: ~35 kg⋅y of
100Mo and ~4.5 kg⋅y of82Se.
From 2007, the NEMO group at LPC Caen has been in charge of the quality survey of the NEMO-3 calorimeter energy calibration using the laser system. This task has been completed in 2012 and the final set of quality parametershas been delivered to the collaboration, corresponding to the individual survey of 2034 photomultiplier tubes (PMT) from 2003 to 2011.
This deliverable is now used by the collaborators responsible of the analysis. This work is critical because the search for the (0νββ) process is very sensitive to the stability of the energy measurement. During 8 years of running, the majority of the 1940 PMTs have shown a very good stability (<1%) of their gain. However, a few percents of the PMTs had been observed with gain fluctuations larger than what was acceptable (>2%). The laser survey system has been used to identify the PMTs with such a problematic behaviour. This approach allows to reject these PMTs from the analysis and leads to elaborate a safe dataset, particularly for the search of the (0νββ) process at high energy, where one wants to achieve the best signal/background ratio.
The limit obtained on the half-life of the (0νββ) process for100Mo is T1/2(0nbb)>1.1×1024y [Bong13]. This is the best result ever obtained for thisββisotope. In 2013, the group has been actively involved in the writing of the publication dedicated to the final NEMO3 results of (0νββ) with100Mo.
Search for neutrinoless double beta decay
Collaboration NEMO3/SuperNEMO LPC (Caen, France), IPHC (Strasbourg, France), LAL (Orsay, France), Idaho National Laboratory (Idaho Falls, U.S.A.), ITEP (Moscow, Russia), UCL (London, UK), University of Manchester (Manchester, UK), JINR (Dubna, Russia), CPPM (Marseille, France), CENBG (Gradignan, France), LAPP (Annecy-le-Vieux, France), IEAP (Prague, Czech Republic), University of Texas (Austin, U.S.A.), LSM (Modane, France), University of Warwick Coventry, (UK), Osaka University (Osaka, Japan), Saga University (Saga, Japan), FMFI (Bratislava, Slovakia), LSCE (Gif-sur-Yvette, France), Imperial College (London,UK), Institut Universitaire de France (Paris, France), Jyväskylä University (Jyväskylä, Finlande), MHC (South Hadley, Massachusetts, U.S.A.), Institute for Nuclear Research (Kyiv, Ukraine), Charles University (Prague, Czech Republic).
Fig. 9: Left: the NEMO-3 detector within the radon-free tent at LSM (2004). Right: a neutrinoless double beta decay candidate event in the NEMO-3 detector.
Fig. 10: The ETOTdistribution for 100Mo in the NEMO-3 detector after 34.7 kg⋅y exposure.
In the [2.8-3.2] MeV range (Qbb=3.034 MeV), 15 events have been observed. Low radioactivity measurements, dedicated analysis and Monte-Carlo simulations have been used to predict 18 background events. The NEMO-3 experiment shows no evidence of
neutrinoless double beta decay.
Fig. 11: Left: Exploded view of the SuperNEMO demonstrator module: the central planar source frame is surrounded by two tracking chambers (2034 open drift cells working in Geiger regime) and two calorimeter walls (520 optical modules with low-radioactivity 8” PMTs). Right: the SuperNEMO demonstrator
will be installed in the LSM cavity in place of the NEMO-3 detector.
SuperNEMO experiment construction status
The SuperNEMO detector is the next generation experiment designed to search the neutrinoless double beta decay process at the 1026y sensitivity level (sensitivity to the effective Majorana neutrino mass: ~50 meV). The design of the SuperNEMO experiment reuses and improves the NEMO-3 technology: it consists in 20 planar modules, each hosting about 5 kg of ββ enriched isotopes. After a R&D program from 2005 to 2011 in which the LPC Caen has been strongly involved (BiPo1 and Bipo2 prototype detectors for the measurement of the radioactivity of the source foils, DAQ development, analysis and simulation software, electronics development), the IN2P3 Scientific Council has validated a first phase of the project in 2011: the SuperNEMO demonstrator module [CSIN2P3]. This module is now in construction (Fig. 11). The data collection will start in the second semester of 2015 for 2.5 years. It will accomodate ~7 kg of82Se and should reach a sensitivity of T1/2(0νββ)~6.5×1024y.
This intermediate step is necessary to prove that the NEMO technology will be able to reach the target sensitivity with 20 modules and a 100 kg source of82Se isotope. Several points will be addressed with the demonstrator:
the calorimeter energy resolution should be validated at the level of 8% FWHM at 1 MeV,
the source radiopurity should be measured at the level of 10 mBq/kg for214Bi and 2 mBq/kg for208Tl, the radon (222Rn) contamination of the tracking chamber should be measured at the level of 0.15 mBq/m3.
The LPC Caen is involved in severalworking packagesof the SuperNEMO project.
Readout and trigger electronics
The LPC Caen NEMO group leader is the scientific coordinator of the SuperNEMO electronics work package. He is assisted by a senior engineer from LAL. This implies the management of five engineering and development teams: LPC Caen (FEAStraduction enterinerT ASIC), LAL Orsay (calorimeter front end board and integration), University of Manchester (tracker front end board), University of Osaka (DAQ) and LAPP Annecy. The group is therefore strongly involved in the elaboration and design of the specifications of various core components of the SuperNEMO demonstrator front end electronics:
the specifications and design of the tracker front end board's ASIC (FEAST).
the specifications of the calorimeter front end board (with LAL) the trigger system and strategy (with LAL)
the readout system and data format (with LAL)
In addition, the LPC Caen participates to the specification and architecture design of:
the DAQ system (with Osaka)
the Control and Monitoring System (CMS, with LAPP) various interfaces (cabling, mechanics...)
Several tasks have been achieved in the 2012-2013 period:
the hardware architecture and specifications of the front end electronics integration scheme has been finalized. This implies: 52 calorimeter front end boards for >700 channels, 57 tracker front end boards (>6000 channels), 6 control boards (CB), 6 crates with their custom backplanes, 1 trigger board (TB), dedicated unified bus and protocols, interfaces (DAQ, CMS, etc...),
the final batch of 150 FEAST chips has been delivered,
a new test bench had been produced to perform exhaustive tests of the FEAST ASIC in real readout conditions,
the specifications of the trigger system for both calorimeter and tracker front end board, as well as for the control board is completed,
parts of the specifications and design of the readout have been done.
The NEMO group has also a strong involvement in the development of the SuperNEMO simulation and data processing off-line software. This task is organized in two main parts: the design and implementation of the generic multi purpose Bayeux C++ library and the development of the software tools that are specific to the SuperNEMO project (SuperNEMO demonstrator, BiPo3 detector): the Falaise C++ library.
The Bayeux library
Aset of C++ library modules and companion applications has been designed to perform various core tasks of interest in the making of nuclear and particle physics simulation applications: these contributions have been packaged in the Bayeux library. This software package contains the following generic components in charge of:
data modelling, generic serialization and advanced I/O system, object factories, data selection and data processing mechanisms(pipeline),
generic geometry modelling (compatible with GDML/Geant4), generic vertex generation for Monte-Carlo inputs,
generic event generation for Monte-Carlo inputs (radioactivy,ββprocesses, etc...), generic electro-magnetic field modelling,
generic Monte-Carlo simulation engine (based on Geant4).
Bayeux has been designed with genericity in mind and thus can be used in the context of many different experimental setups (Fig. 12). It is now a rather mature and stable library.
Fig. 12: A simple detection setup modeled with the Bayeux library and used through the Geant4 engine to simulate the emission of 1 MeV electrons from a radioactive source. This simulation framework is fully parameterized by a set of human friendly configuration files (geometry, vertex generator, event generator, management of Geant4 session), without the need to write a single line of C++ code.