NEXT100 Technical Design Report (TDR). Executive summary

Preprint typeset in JINST style - PAPER VERSION NEXT-100 Technical Design Report (TDR). Executive Summary arXiv:1202.0721v1 [physics.ins-det] 3 Feb 2012 A.L. Ferreira, C.A.B. Oliveira, J.F.C.A. Veloso Institute of Nanostructures, Nanomodelling and Nanofabrication (i3N), Universidade de Aveiro Campus de Santiago, 3810-193 Aveiro, Portugal D. Chan, M. Egorov, A. Goldschmidt, T. Miller, D. Nygren, J. Renner, D. Shuman, T. Weber Lawrence Berkeley National Laboratory (LBNL) 1 Cyclotron Road, Berkeley, CA 94720, USA E. Gómez, R. M. Gutiérrez, M. Losada, G. Navarro Centro de Investigaciones, Universidad Antonio Nariño Carretera 3 este No. 47A-15, Bogotá, Colombia F.I.G. Borges, C.A.N. Conde, T.H.V.T. Dias, L.M.P. Fernandes, E.D.C. Freitas, J.A.M. Lopes, C.M.B. Monteiro, H. Natal da Luz, F.P. Santos, J.M.F. dos Santos Departamento de Fisica, Universidade de Coimbra Rua Larga, 3004-516 Coimbra, Portugal P. Evtoukhovitch, V. Kalinnikov, A. Moiseenko, Z. Tsamalaidze, E. Velichev Joint Institute for Nuclear Research (JINR) Joliot-Curie 6, 141980 Dubna, Russia M. Batallé, L. Ripoll, J. Torrent Escola Politècnica Superior, Universitat de Girona Av. Montilivi, s/n, 17071 Girona, Spain J. Hauptman Department of Physics and Astronomy, Iowa State University 12 Physics Hall, Ames, Iowa 50011-3160, USA L. Labarga, J. Pérez Departamento de Física Teórica, Universidad Autónoma de Madrid Ciudad Universitaria de Cantoblanco, 28049 Madrid, Spain E. Ferrer-Ribas, I. Giomataris, F.J. Iguaz IRFU, Centre d’Études de Saclay (CEA Saclay) Gif-sur-Yvette, France J.A. Hernando Morata, D. Vázquez –1– Instituto Gallego de Física de Altas Energías (IGFAE), Universidade de Santiago de Compostela Campus sur, Rúa Xosé María Suárez Núñez, s/n, 15782 Santiago de Compostela, Spain C. Sofka, R. C. Webb, J. White Department of Physics and Astronomy, Texas A&M University College Station, Texas 77843-4242, USA J.M. Catalá, R. Esteve, V. Herrero, J.M. Monzó, F.J. Mora, J.F. Toledo Instituto de Instrumentación para Imagen Molecular (I3M), Universitat Politècnica de València Camino de Vera, s/n, Edificio 8B, 46022 Valencia, Spain V. Álvarez, S. Cárcel, A. Cervera, J. Díaz, P. Ferrario, A. Gil, J.J. Gómez-Cadenas∗, K. González, I. Liubarsky, D. Lorca, J. Martín-Albo, F. Monrabal, J. Muñoz Vidal, M. Nebot, J. Rodríguez, L. Serra, M. Sorel, N. Yahlali Instituto de Física Corpuscular (IFIC), CSIC & Universitat de València Calle Catedrático José Beltrán, 2, 46980 Paterna, Valencia, Spain R. Palma, J.L. Pérez Aparicio Dpto. de Mecánica de Medios Continuos y Teoría de Estructuras, Univ. Politécnica de Valencia Camino de Vera, s/n, 46071 Valencia, Spain J.M. Carmona, J. Castel, S. Cebrián, T. Dafni, H. Gómez, D.C. Herrera, I. G. Irastorza, G. Luzón, A. Rodríguez, L. Seguí, A. Tomás, J.A. Villar Lab. de Física Nuclear y Astropartículas, Universidad de Zaragoza Calle Pedro Cerbuna, 12, 50009 Zaragoza, Spain A BSTRACT: In this Technical Design Report (TDR) we describe the NEXT-100 detector that will search for neutrinoless double beta decay (β β 0ν) in 136 Xe. The document formalizes the ANGEL design presented in our Conceptual Design Report (CDR). The baseline detector is designed to hold a maximum of about 150 kg of xenon at 15 bar, or 100 kg at 10 bar. This option builds in the capability to increase the total isotope mass by 50% while keeping the operating pressure at a manageable level. The ANGEL design calls for an asymmetric TPC, with photomultipliers behind a transparent cathode and position-sensitive light pixels behind the anode. We have chosen the low background R11410-10 PMTs for energy and timing and Hamamatsu MPPCs (S10362-11-050P model) as tracking pixels. Each individual PMT will be isolated from the gas by an individual, pressure resistant enclosure and will be coupled to the sensitive volume through a sapphire window coated with terphenyl-butadiene (TPB) . MPPCs will be arranged in Dice Boards (DB) holding 64 sensors each in an array of 8×8 sensors. The light tube will also be coated with TPB. K EYWORDS : Time Projection Chambers (TPC). ∗ Spokesperson. Email: [email protected] Contents 1. Introduction 2 2. Neutrinoless double beta decay searches 2 3. The NEXT concept 3.1 Development of the NEXT project: R&D and prototypes 3.2 Major subsystems 3 5 6 4. Gas system 7 5. The pressure vessel 12 6. The field cage 15 7. WLS coating 7.1 WLS coating of the tracking plane DBs 7.2 WLS coating of the field cage light tube 17 17 17 8. The energy plane 19 9. The tracking plane 21 10. Front-end electronics and DAQ 10.1 Electronics for the energy plane 10.2 Electronics for the tracking plane 23 23 24 11. Shielding 27 12. NEXT-100 at the LSC 27 13. Radioactive budget 13.1 Sources of background in NEXT 13.2 Contribution of the main materials used in NEXT 29 29 30 14. Expected sensitivity 14.1 Signal and background characterization in NEXT 14.2 The topological signature 34 34 35 –1– 1. Introduction Neutrinoless double beta decay (β β 0ν) is a hypothetical, very slow nuclear transition in which two neutrons undergo β -decay simultaneously and without the emission of neutrinos. The importance of this process goes beyond its intrinsic interest: an unambiguous observation would establish that neutrinos are Majorana particles — that is to say, truly neutral particles identical to their antiparticles — and prove that total lepton number is not a conserved quantity. After 70 years of experimental effort, no compelling evidence for the existence of β β 0ν has been obtained. However, a new generation of experiments that are already running or about to run promises to push forward the current limits exploring the degenerate region of neutrino masses (see [1] for a recent review of the field). In order to do that, the experiments are using masses of β β isotope ranging from tens of kilograms to several hundreds, and will need to improve the background rates achieved by previous experiments by, at least, an order of magnitude. If no signal is found, masses in the ton scale and further background reduction will be required. Only a few of the new-generation experiments can possibly be extrapolated to those levels. The Neutrino Experiment with a Xenon TPC (NEXT) will search for neutrinoless double beta decay in 136 Xe. A xenon gas time projection chamber offers scalability to large masses of β β isotope and a background rate among the lowest predicted for the new generation of experiments [1]. The experiment was proposed to the Laboratorio Subterráneo de Canfranc (LSC), Spain, in 2009 [2], with a source mass of the order of 100 kg. Three years of intense R&D have resulted in a Conceptual Design Report [3] and a Technical Design Report (TDR), summarized in this document, where the final design of the NEXT-100 detector is defined. More detailed reports on the design of the different subsystems will be forthcoming. 2. Neutrinoless double beta decay searches Double beta decay (β β ) is a very rare nuclear transition in which a nucleus with Z protons decays into a nucleus with Z + 2 protons and the same mass number A. The decay can occur only if the initial nucleus is less bound than the final nucleus, and both more than the intermediate one. There are 35 naturally-occurring isotopes that can undergo β β . Two decay modes are usually considered: • The standard two-neutrino mode (β β 2ν), consisting in two simultaneous beta decays, AZX → A − Z+2Y + 2 e + 2 ν e , which has been observed in several isotopes with typical half-lives in the range of 1018 –1021 years (see, for instance, [1] and references therein). A • The neutrinoless mode (β β 0ν), AZX → Z+2 Y + 2 e− , which violates lepton-number conservation, and is therefore forbidden in the Standard Model of particle physics. An observation of β β 0ν would prove that neutrinos are massive, Majorana particles [4]. No convincing experimental evidence of the decay exists to date. The implications of experimentally establishing the existence of β β 0ν would be profound. First, it would demonstrate that total lepton number is violated in physical phenomena, an observation that could be linked to the cosmic asymmetry between matter and antimatter through the process known as leptogenesis [5, 6]. Second, Majorana neutrinos provide a natural explanation to the smallness of neutrino masses, the so-called seesaw mechanism [7 – 10]. –2– Several underlying mechanisms — involving, in general, physics beyond the Standard Model — have been proposed for β β 0ν, the simplest one being the virtual exchange of light Majorana neutrinos. Assuming this to be the dominant one at low energies, the half-life of β β 0ν can be written as: 2 0ν −1 (2.1) ) = G0ν M 0ν m2β β . (T1/2 In this equation, G0ν is an exactly-calculable phase-space integral for the emission of two electrons; M 0ν is the nuclear matrix element of the transition, that has to be evaluated theoretically; and mβ β is the effective Majorana mass of the electron neutrino: mβ β = ∑ Uei2 mi , (2.2) i where mi are the neutrino mass eigenstates and Uei are elements of the neutrino mixing matrix. Therefore, a measurement of the β β 0ν decay rate would provide direct information on neutrino masses [1]. The detectors used in double beta decay experiments are designed to measure the energy of the radiation emitted by a β β source. In the case of β β 0ν, the sum of the kinetic energies of the two released electrons is always the same, and corresponds to the mass difference between the parent and the daughter nuclei: Qβ β ≡ M(Z, A) − M(Z + 2, A). However, due to the finite energy resolution of any detector, β β 0ν events are reconstructed within a non-zero energy range centered around Qβ β , typically following a gaussian distribution. Other processes occurring in the detector can fall in that region of energies, thus becoming a background and compromising drastically the experiment’s expected sensitivity to mβ β [11]. All double beta decay experiments have to deal with an intrinsic background, the β β 2ν, that can only be suppressed by means of good energy resolution. Backgrounds of cosmogenic origin force the underground operation of the detectors. Natural radioactivity emanating from the detector materials and surroundings can easily overwhelm the signal peak, and consequently careful selection of radiopure materials is essential. Additional experimental signatures that allow the distinction of signal and background are a bonus to provide a robust result. The Heidelberg-Moscow experiment set the most sensitive limit to the half-life of β β 0ν so far: 0ν (76 Ge) ≥ 1.9 × 1025 years [12]. In addition, a subgroup of the experiment observed evidence T1/2 of a positive signal, with a best value for the half-life of 1.5 × 1025 years [13], corresponding to a Majorana neutrino mass of about 0.4 eV. The claim was very controversial [14], and still awaits an experimental response. A new generation of β β experiments — already running or about to do so — will push the current limits down to neutrino masses of about 100 meV or better [1]. 3. The NEXT concept An ideal β β 0ν experiment is one characterized by: 1. An arbitrarily large mass of sensitive, 100% enriched target — e.g, target and detector are the same, reconstruction efficiency of the signal is one—. 2. An arbitrarily small radioactive budget (thus, not affected by external backgrounds). –3– 100 mββ (meV) Ge-76 Se-82 Te-130 Xe-136 Nd-150 10 100 1000 exposure (kg year) 10000 Figure 1. Sensitivity of ideal experiments at 90% CL for different β β isotopes, assuming the PMA nuclear matrix element [11]. Since the yields are very similar, the sensitivities of 82 Se, 130 Te and 150 Nd overlap. The asymptotic limit (corresponding to a total exposure of 104 kg · year) on mβ β is of the order of 2 meV for these isotopes, 4 meV for 136 Xe and 10 meV for 76 Ge. Notice that a different set of NME will yield a somewhat different result. The ideal sensitivity scales with the square root of the total exposure. 3. Perfect resolution, necessary to separate the β β 0ν and β β 2ν modes. The sensitivity of such an instrument would depend only of the isotope used for the search, since the total yield depends on the nuclear matrix element and phase space available in the decay (Figure 1), and scales with the square root of the total exposure. Notice that reaching “ultimate” sensitivity (few meV) would require a total exposure of 104 kg · year, even for an ideal experiment. The implication is that a full exploration of the mβ β physics range will require an isotope mass of 10 ton running for one year, or 1 ton running for 10 years. Exploring the inverse hierarchy would require a total exposure of 103 kg · year, or a 100 kg detector running for 10 years. Therefore, mass is a must for the next and next-to-next β β 0ν experiments, even assuming a magic substance immune to backgrounds. This condition, alone, makes some isotopes (and experimental techniques) more suitable than others. Arguably, xenon offers the best deal when it comes to procure a large mass of enriched isotope at a competitive cost. Indeed, there is today about 1 ton of xenon enriched at ∼90% in 136 Xe available at the World, distributed between KamLANDZen (∼800 kg), EXO (200 kg) and NEXT (100 kg). There are two recipes to reduce the radioactive budget of a β β 0ν experiment to very low levels: (a) use of radiopure components, with low contents of uranium and/or thorium, and (b) shielding. All the β β 0ν experiment use a formula that combines both recipes. Of course, no experiment achieves a null radioactive budget. Therefore, resolution and possibly other handles are a must to suppress both intrinsic (e.g, the β β 2ν channel) and external backgrounds. The NEXT experiment will search for β β 0ν in 136 Xe using a high-pressure xenon gas (HPGXe) –4– time projection chamber (TPC). It is instructive to compare NEXT with an ideal detector: 1. Mass: At 10 bar, 100 kg of xenon occupy 2 m3 . NEXT is designed to hold 100 kg of xenon at 10 bar or 150 kg at 15 bar. The total mass of the experiment is then large, and compares well with the mass deployed by EXO (200 kg of enriched gas), CUORE (200 kg of isotope), or the first phase of KamLAND-Zen (330 kg of enriched gas). A future upgrade to a detector holding up to 500–1 000 kg of enriched gas operating at 15–20 bar is conceivable. In the absence of backgrounds, assuming a perfect efficiency, and assuming a 10 year run, NEXT could qualify to explore the inverse hierarchy in its first phase, and to go beyond in a second phase. 2. Radiopurity: NEXT uses the Matrioska principle. The full detector is installed in an underground lab and shielded from the lab radiation by a layer of ultra-pure lead, 30 cm thick. The residual radiation emanating from the lead, as well as the radiation emanating from the pressure vessel (made of a rather radiopure steel alloy) is shielded by an internal shield, 12 cm thick, made of ultra-pure copper. The relevant radioactive budget is the residual radioactivity of the copper (very small) and the contributions of the sensors (PMTs, MPPCs), field cage (itself made of radiopure copper) and electronics. The total radioactive budget is smaller than 1 Bq. 3. Resolution and topology: NEXT offers both good energy resolution — possibly better than 0.5% FWHM at Qβ β — and event topological information that can be used for background rejection and results in one of the smallest background rates of the market. To achieve its target resolution, NEXT uses proportional electroluminescent (EL) amplification of the ionization signal. The detection process is as follows: charged particles propagating in the gas will produce both primary scintillation ultra-violet (UV) light and ionization electrons. Electroluminescence (EL) is a method to amplify the ionization signal, once it has been drifted to the TPC anode. When an ionization electron is accelerated in a moderate electric field, of the order of 3–5 kV/cm/bar, it produces secondary scintillation UV light. The field can be tuned to generate a large number of photons (∼ 103 ) per electron reaching the TPC anode, thus producing a proportional signal. Extremely low fluctuations can be reached with EL, which is crucial for optimal energy resolution. 3.1 Development of the NEXT project: R&D and prototypes During the last three years, the NEXT R&D program has focused in the construction, commissioning and operation of three large prototypes: • NEXT-DBDM (figure 3), a prototype equipped with an array of 19 Hamamatsu R7378A photomultipliers, sensitive to VUV light and pressure resistant (up to 20 bar). The detector can hold 2 kg of xenon at 15 bar. The fiducial volume is a cylinder of 16 cm in length and 16 cm in diameter (a proportion similar to the length to diameter ratio of NEXT-100). The main goal of this prototype was to perform detailed energy resolution studies. The detector is operating at LBNL. –5– Figure 2. The Separated Optimized Functions (SOFT) concept in NEXT TPC. EL light generated at the anode is recorded in the photosensor plane right behind it and used for tracking. It is also recorded in the photosensor plane behind the transparent cathode and used for a precise energy measurement. • NEXT-DEMO, shown in figure 4. This is a larger prototype, operating at IFIC, whose pressure vessel has a length of 60 cm and a diameter of 30 cm. The vessel can withstand a pressure of up to 15 bar. The maximum capacity of the detector is 10 kg but in its current configuration (the fiducial volume is an hexagon of 16 cm diameter and 30 cm length) it holds 4 kg at 15 bar. NEXT-DEMO is also equipped with an energy plane made of 19 Hamamatsu R7378A and a tracking plane made of 300 Hamamatsu MPPCs. The main goals of this prototype are: (a) to demonstrate track reconstruction and the performance of MPPCs (coated with a wavelength shifter, TPB, to make them sensitive to xenon VUV, [15]); (b) to test long drift lengths and very high voltages (up to 50 kV in the cathode and 25 kV in the anode), (c) to understand gas recirculation in a large volume, including operation stability and robustness against leaks; (d) to understand the transmittance of the light tube, with and without TPB. In summary, to demonstrate the technology to be used by the NEXT-100 detector. • NEXT-MM, a prototype initially used to test the Micromegas technology and currently used to explore new gas mixtures. NEXT-MM operates at the University of Zaragoza. The initial results of the prototypes show an excellent energy resolution and tracking capabilities, as illustrated in figure 5. 3.2 Major subsystems Figure 6 shows a sketch of the NEXT-100 detector, indicating all the major subsystems. These are: –6– Figure 3. The NEXT-DBDM prototype. • The pressure vessel (described in section 5), built in stainless steel and able to hold 20 bar of xenon. A copper layer shields the sensitive volume from the radiation originated in the vessel material. • The field cage, electrode grids, HV penetrators and light tube, described in section 6. • The energy plane made of PMTs housed in copper enclosures and connected to a vacuum manifold (section 8). • The tracking plane made of MPPCs arranged into dice boards (DB). The front-end electronics is inside the gas, shielded behind a thick copper plate (section 9). 4. Gas system The gas system must be capable of pressurizing, circulating, purifying, and depressurizing the NEXT-100 detector with xenon, argon and possibly other gases with negligible loss and without damage to the detector. In particular, the probability of any substantial loss of the very expensive enriched xenon (EXe) must be minimized. The general schematic of the gas system is given in figure 7 (the re-circulation compressor, vacuum pump and cold traps are not shown). A list of requirements, in approximate decreasing order of importance, considered during the design is given below: –7– Figure 4. The NEXT-DEMO prototype. From left to right and from top to bottom: (a) The pressure vessel, showing the HVFT and the mass spectrometer, (b) the field cage, which provides 30 cm drift length, (c) the light tube, made of Teflon pannels, showing the honey comb for the PMT plane, (d) the energy plane equipped with 19 Hamamatsu R7378A PMTs, (e) the PMTs to be used in NEXT-100, (f) the tracking plane, equipped with 300 Hamamatsu MPPCs. 1. Pressurize vessel, from vacuum to 15 bar (absolute). 2. Depressurize vessel to closed reclamation system, 15 bar to 1 bar, on fault, in 10 seconds maximum. 3. Depressurize vessel to closed reclamation system, 15 bar to 1 bar, in normal operation, in 1 hour maximum. 4. Pressure relief (vent to closed reclamation system) for fire or other emergency condition. 5. Maximum leakage of EXe through seals (total combined): 100 g/year. 6. Maximum loss of EXe to atmosphere: 10 g/year. 7. Accomodate a range of gasses, including Ar and N2 . 8. Circulate all gasses through the detector at a maximum rate of 200 standard liters per minute (slpm) in axial flow pattern. 9. Purify EXe continuously. Purity requirements: < 1 ppb O2 , CO2 , N2 , CH4 –8– Cs-137 Photoelectric electron 150 0.20 y (mm) 0.18 Cs-137 energy spectrum Only one blob 100 0.16 0.14 0.12 50 0.10 0.08 0.06 0 Xe 35 keV X-ray 0.04 0.02 -50 -50 0 50 100 0.00 X (mm) Figure 5. (Left): Energy spectrum measured by NEXT-DBDM using a 137 Cs radioactive source. The energy of the photoelectric peak is 1% FWHM. This energy resolution extrapolates to ∼0.5% FWHM at the energies of 136 Xe decay. (Right):The topological signature of a photoelectric electron produced at 660 keV (the energy of the 137 Cs source used by NEXT-DEMO prototype) is a single-blob at one end of the track (Bragg peak) and a separated satellite cluster due to the fluorescence emission of xenon. EL mesh planes Cathode Tracking Plane, SiPM Cu Shield EL HV F.T. Main Cylindrical Vessel Torispheric Heads Energy Plane, PMTs Cu Shield Vac. Manifold PMT FTs HV Cable HV/Press. relief/Flow/Vac. Ports Reflectors Field Cage Rings F.C. Insulator Cu Shield Bars Shielding, External, Cu on Pb Figure 6. The NEXT-100 apparatus. The most vulnerable component of the gas system is the re-circulation compressor, that must have sufficient redundancy to minimize the probability of failure and leakage. Furthermore, to preserve the purity of the gas all seals must be metal-to-metal. The Collaboration has chosen a –9– Check Valve Shielding Bursting Disk Regulator Recirculation Pump Servo Valve Xe Xe Line Manual Valve Service line Dome Loaded Reg Overpressure Relief Pressure Reference P-x Underpressure Relief V1 BPR1 V2 DLR1 R1 V3 Pressure Reference Value typically P=15 bar C1 VM1 Reduce to 1 bar RP1 Getter BD2 Two parallel Getters and a bypas V16 Gas Analyzer V17 Rn Trap 2 Evacuation Alarm UPR1 V18 OPR1 V21 R4 V21 V23 V22 V20 V19 Emerg Expanssion Dump Emerg Cold Dump Xe Cold Recov Xe Xe Figure 7. Schematic of the NEXT-100 gas system. compressor manufactured by SERA R . This compressor is made with metal-to-metal seals on all the wetted surfaces. The gas is moved through the system by a triple stainless steel diaphragm. Between each of the diaphragms there is tha sniffer port to monitor for gas leakages. In the event of a leakage, automatic emergency shutdown can be initiated. MicroTorr cold getter model number MC4500-902FV has been chosen as the purification filter for the Xe gas. Capable of removing electron negative impurities to less than 1 ppb, the model chosen has a nominal flow rate of 200 standard liters per minute, well in excess of the required – 10 – flow rates for NEXT-100, thus offering sufficient spare capacity. The gas system will contain two such getters in parallel with a bypass. This configuration has been developed and used by the smaller gas systems operating at the Universidad de Zaragoza and IFIC. The second spare getter is placed in parallel allowing uninterrupted running in the event of accidental contamination of one of the getters. Also, the ability to bypass the getters will allow the testing of the purification of the gas and aid in diagnostic and monitoring of the gas system. While cold getter technology is capable of reaching the required purity levels in water and oxygen, a hot getter can also remove nitrogen and methane. In that regard, we foresee to upgrade to a hot getter technology for the enriched xenon run. An automatic recovery system of the expensive EXe will be needed to evacuate the chamber in case of an emergency condition. A 30-m3 expansion tank will be placed inside the laboratory to quickly reduce the gas pressure in the system. Additionally, we will implement a similar solution to that proposed by the LUX collaboration, where a permanently chamber cooled by liquid nitrogen will be used. Two primary conditions to trigger automatic evacuation are foreseen: • An over-pressure, that can potentially cause an explosion. Because the gas system for NEXT100 will be operated in a closed mode the overpressure condition could occur only under two possible scenarios: a problem during the filling stage of the operation or a thermal expansion of the gas due to laboratory fire. In the case of overpressure an electromechanical valve, activated by a pressure switch, will open a pipe from the chamber to a permanently cold recovery vessel. This will then cryo-pump xenon into the recovery vessel, causing the gas to freeze in the recovery tank. In the event of the electromechanical valve failing, a mechanical spring-loaded relief valve, mounted in parallel to the electromechanical valve, would open and allow the xenon to be collected in the recovery vessel. A bursting disk will also be mounted in parallel to the electromechanical and spring-loaded valves as a final safety feature. • An under–pressure, indicating a leak in the system. Such condition would require evacuation of the chamber to prevent losses of gas. If this happens an electromechanical valve sensing under-pressure will open and evacuate the xenon into the recovery vessel. We have also considered the scenario in which xenon could leak through some of the photomultipliers enclosures (leaking can). If this happens the use of a cold trap would permit to recover the gas. To insure the cleanliness of the chamber and the Xe gas system prior to the introduction of Xe both the chamber and the Xe gas system need to be vacuum evacuated to as low pressure as possible. A reasonably good vacuum is in the range of 10−4 to 10−5 mbar. To achieve this, the turbo-molecular pump needs to be positioned as close as possible to the vessel being evacuated. For that reason, the turbo-molecular pump station will be directly connected as close as possible to the NEXT-100 vessel through a large conductance valve rated for vacuum and pressure. However, many internal structures of the NEXT 100 detector, such as the light pipe surrounding the active volume, will not allow good conductance for vacuum evacuation. Therefore, instead of evacuating the system from a single point, the vacuum manifold will be connected to several points simultaneously, and the system heated to 200 ◦ C to remove water. Also, flushing with argon several times – 11 – Figure 8. Pressure Vessel/Detector: side cross section view. might help in the cleaning process. Finally, continuous gas re-circulation through the getters will clean the Xe gas system. 5. The pressure vessel The pressure vessel (PV) consists of a cylindrical center section (barrel) with two identical torispheric heads on each end, their main flanges bolted together. The vessel orientation is horizontal, so as to minimize the overall height; this reduces the outer shielding cost and allows essentially unlimited length on each end for cabling and service expansion. Each head has four axial nozzles, the central nozzle of each head is for services (power and signal cabling) to the PMT array (energy plane) and to the SiPM array (tracking plane). The two auxiliary nozzles on each side of this are for gas flow and pressure relief; at present only one on each head is used. The fourth nozzle, located furthest from the vessel axis is used for the high voltage feedthrough of the cathode plane. To keep heads identical, one will be present, but capped off on the tracking head, to allow for future reconfiguration. All these axial nozzles are located on the vertical midplane of the vessel; this allow the two lead shielding walls to come together at this midplane by making semicircular cutouts on their mating surfaces. A longitudinal cross section of the PV is shown in figure 8. There are also a ring of eight radial nozzles, located at the EL gap. Two of these, (one on top, and one at a 45 deg angle) are for the EL gate high voltage feedthrough (2 possible locations) and the other 6 are inspection ports for the EL gap; they may prove useful for EL diagnostics and in-situ cleaning or repair. The PV supports a number of internal components on the main flanges; nothing is supported directly on the 10mm thick shell. The barrel flanges have an inside flange containing a circle of – 12 – Table 1. NEXT-100 pressure vessel parameters. Maximum operating pressure (absolute) Maximum allowable pressure (absolute) Minimum allowable Pressure (external) Inner diameter, barrel Outer diameter, barrel and heads Outer diameter, main flanges Length, head to head, inside Thickness, barrel and head wall Thickness, main flanges (each side) Number of bolts, main flanges Bolt diameter, main flanges Bolt length, main flanges Mass, pressure vessel Mass, internal copper shielding (incl. heads) Mass, energy plane Mass, field cage Mass, tracking plane Mass, NEXT-100 total 15.0 bar 16.4 bar 1.5 bara 136 cm 138 cm 147 cm 228 cm 10 mm 4.0 cm 140 14 mm 11 cm 1 200 kg 10 000 kg 750 kg 250 kg 300 kg 12 500 kg 240 M8 threaded holes. The internal copper shield (ICS) bars attach on each end to these internal flanges; in turn these bars have machined features which then support the field cage/EL structure, the PMT array on its carrier plate, and the SiPM array. The heads also have an internal flange; to these are fastened the internal copper shield plate(s). The cathode plane high voltage feedthrough is integrated into the energy head and makes contact with the cathode plane when the head is assembled. The vessel will be made of stainless steel, specifically the low-activity 316Ti alloy, unless similarly low activity 304L or 316L alloy can be found which will allow use of roll forgings for the flanges; these promise better leak tightness and mechanical integrity. Measurements by the XENON collaboration show that it is possible to secure 316Ti with an activity at the level of 0.2 mBq/kg for the thorium series and 1.3 mBq/kg for the uranium series [16]. The mass of the PV is 1200 kg, resulting in an activity of about 1.6 Bq due to the uranium series. To shield this activity we introduce an inner copper shield (ICS) made of radiopure copper, with an activity of about 10 µBq/kg. The ICS will attenuate the radiation coming from the external detector (including the PV and the external lead shield) by a factor of 100. After the ICS the residual activity due to the PV is about 0.02 Bq. One needs to add the residual activity of the ICS itself which is, taking into account self-shielding, of the order of 0.03 Bq. Thus, the resulting activity of the whole system is ∼0.05 Bq. The basic parameters and dimensions of the pressure vessel are shown in table 1. All pressure sealing flange joints that are exposed to atmosphere on the outside are sealed using – 13 – double O-rings in grooves, for both sealing reliability, and to minimize the flange and bolt sizes. The inner O-ring is for pressure sealing; the outer O-ring serves not only as a backup, but also to create a sealed annulus which can be continuously monitored for leakage by pulling a vacuum on it with an RGA monitor (sense port). This is the only way to monitor for leakage, however xenon will permeate through these O-rings and will need to be recovered in a cold trap, the total amount is estimated to be <200 gram/year for butyl or nitrile O-rings, this includes the nozzle flanges and PMT enclosure O-ring leakage. The use of metal C-ring gaskets (special low force design) is being explored. The standard sealing force for these gaskets is very high and greatly increases flange thickness and OD. The pressure vessel flanges are being designed with some reserve capacity for the higher bolt force that would be required for these low force metal gaskets. The head to vessel flange bolts are Inconel 718; this is the highest strength noncorrosive bolting material allowed in the ASME code. Flange thickness and outer diameter are substantially minimized by using the highest possible strength bolting; this mostly compensates for the lower radiopurity of this material (relative to Inconel 625, the next best alloy) and saves on external shielding cost. All nozzles will be as short as possible, with simple flat-faced flanges. This is to reduce vessel damage risk, to ease fabrication, and to preserve flexibility in length; extension spools are used to bring services out through the shielding. Nozzle flanges, although being flat-faced (with the double O-rings on the spools and caps) use standard CF flange bolt hole pattern; this is to preserve the possibility of using CF components (pressure rated). These could be pre-assembled to double O-ring/CF adapter plates, then pressure and leak tested prior to assembling on the nozzle. The vessel will be built strictly to ASME pressure Vessel Design Code, sec VIII. It has been designed almost in full by the collaboration, however the outside fabricator will be required to supply additional details of fabrication such as weld design for approval, as these can be dependent on the in-house capabilities. Fabricator is also responsible for pressure integrity, under ASME rules, and will likely perform their own calculations to verify soundness of design. The Collaboration will be specifying many aspects (and approving every aspect) of the design and fabrication, such as: cleaning of joints, weld preparation methods, inspection methods, fabrication sequence, post-weld heat treatments, etc. This is to assure that the unusually high tolerances on dimensions and radiopurity are met, without needing rework; most pressure vessel manufacturers do not typically deal with these. Our preferred fabricator at this time has experience in vessels for food and pharmaceutical processing, which also typically require a great deal of oversight by the client. Saddle supports will be welded to the barrel wall, as is standard practice. The barrel will be bolted through the shielding floor to the seismic platform below. The head/shield assemblies will first be attached to a precisely adjustable 6-strut support fixture, also known as a hexapod, or Stewart Platform. This is a cradle support ring for the head which is connected to a baseplate by adjustable length turnbuckle struts, forming a kinematic mount. This fixture is attached to a set of linear ball bearing carriages that slide on precision rails that are bolted the shielding floor when needed. There is 1 m carriage travel to allow each head to be retracted far enough to clear the Central Manifold inside. The 6-strut fixture is easily adjusted by hand to mate the head flanges to the barrel flanges with very high precision, whereby they can be bolted together without stress. The adjustments are essentially independent for the three degrees of translation and for roll and yaw about the vessel axis. The pitch adjustment is coupled only with the Z translation (along the axis) which is easy to deal with since this is also the rail motion direction. This 6-strut fixture will – 14 – Table 2. NEXT-100 EL grid parameters E/P Drift field Pressure EL grid gap Drift length Gate grid voltage Anode grid voltage Cathode voltage Optical gain 3.0 kV cm−1 bar−1 0.3 kV cm−1 15.0 bar 0.5 cm 127 cm -22.5 kV 0 -58 kV 2500 photons/e− also be used to assemble the PMT and SiPM arrays to their attachment points in the barrel. 6. The field cage The main body of the Field Cage (FC) will be a high density polyethylene (HDPE) cylindrical shell with a 2.5 cm wall thickness. The drift region will consist of OFHC copper strips connected with low background resistors. There is also a buffer region between the cathode and PMTs which will be used to degrade the high cathode voltage safely to ground. Using the parameters in Table 2, the electric field varies <3% across the fiducial volume. The light tube (LT) will cover the inner part of the field cage. The presence of a strong electric field demands, therefore, non conductive material as the substrate for the wavelength shifter. We have chosen the same materials studied by the ArDM collaboration. The LT substrate will be ESR (VikuitiTM Enhanced Specular Reflector foil) from the company 3MTM and Tetratex (TTX) from the company Donaldson Membranes. The ESR foil is a multilayer specular reflecting polymer measured to be highly radiopure. Its appearance is that of a polished metal although the material is non conducting. It has a specular reflection coefficient of practically 100% in a large region of the optical spectrum. TTX is an aligned polytetrafluoroethylene (PTFE) fibrous cloth and is nearly a 100% diffuse Lambertian reflector. It is also radiopure (typically less than 1 ppb, which, given the very low mass involved, gives a negligible contribution to the radioactive budget) and has low degassing. The measured reflectivity of the ESR + TTX foils is about 97% at 430 nm. The radiopurity of polyethylene has been measured by several collaborations (see Section 13). The XENON collaboration has measured a total activity of ∼0.3 mBq/kg. The mass of the polyethylene is of the order of 100 kg, and thus its contribution to the radioactive budget is or the order of 0.03 Bq. Adding low background resistors and copper rings we estimate that the contribution of the field cage will be 0.05 Bq. The cathode high voltage feedthrough (HVFT) will be constructed using a compression seal approach as illustrated in Figure 9. A metal rod is pressed into a plastic tube (Tefzel or FEP, which have a high dielectric strength) which is then clamped using plastic ferrules from both the pressure side and air side. A sniffer port is placed between the seals to assure that xenon is not leaking. The – 15 – Figure 9. Cathode high voltage feedthrough (HVFT) designed for up to 100 kV operating voltage. Figure 10. EL grids used in NEXT-DEMO. feedthrough will be attached to a flange located on the energy plane end-cap. A shielded cable will be connected to the feedthrough and placed through the PMT support plate. The unshielded portion of the cable, with an additional resistive coating, will then run along the inside of the buffer field rings and mate with the cathode via a spring loaded junction. This approach, with the exception of the resistive coating, has been used in NEXT-DEMO, where a cathode voltage of 45 kV has been achieved. A smaller prototype was tested to 100 kV in vacuum and 70 kV in nitrogen at 3 bar. It has been demonstrated to be leak tight at 10 bar xenon and 10-7 mbar vacuum. Figure 10 shows the electroluminescent (EL) grids built for the NEXT-DEMO prototype, and Table 2 show the electric parameters. The grids were constructed using a stainless steel mesh with a 0.5 mm pitch and a 30 µm wire diameter, which results in an open area of 88%. The grids are – 16 – formed by clamping in a ring with a tongue and groove to hold the mesh and using a tensioning ring that is torqued with set screws to achieve the optimum tension. One important issue is that for the large diameter required in NEXT-100, preliminary estimates show that electrostatic attraction will cause the EL grids to bow considerably. This can be remedied by using a larger gauge wire. For example, a wire mesh is available with 90 micron wire diameter made from titanium with a similar open area. We are also investigating the use of titanium or copper grid frames to minimize the radioactive budget. However, the total mass of the EL and HVFT system is small and the use of low-background steel will be sufficient to keep the radioactive budget acceptable. 7. WLS coating Xenon scintillates in the VUV range, with a peak at ∼175 nm. On the other hand, the PDE of MPPCs peaks in the blue region and they have a very low PDE below 200 nm. Furthermore the NEXT PMTs will be enclosed in cans coupled to the gas through sapphire windows which are very transparent to the visible light but not to UV (UV grade sapphire is extremely expensive). Last but not least, the reflectivity of the light tube (made of TTX, a Teflon cloth) is almost 100% in the visible spectrum and no better than 50% in the VUV region. Consequently, our strategy in NEXT is to shift the VUV light emitted by xenon to the blue region using a wavelength shifter molecule, specifically Tetraphenyl-butadiene (TPB) of ≥ 99% purity grade. TPB absorbs light in a wide UV range and re-emits it in the blue with an emission peak around 430 nm. The molecule can be applied by vacuum evaporation, and other techniques from crystalline form directly onto surfaces. 7.1 WLS coating of the tracking plane DBs A TPB procedure to deposit a thin layer of TPB on flat (relatively small) surfaces, such as daughter boards (DBs) and the sapphire windows of the PMT cans has been developed at ICMOL and IFIC. A second procedure to coat large surfaces, such as the NEXT-100 light tube has also been developed at IFIC. The technique to deposit thin layers in flat surfaces —including glass, quartz, sapphire and teflon— has been developed at the coating facility of the Instituto de Ciencias de Materiales (ICMOL). Figure 11 shows the evaporator used. As an illustration of the procedure, Figure 12 shows a glass-slice (left) and a prototype DB (right) coated with TPB and illuminated with UV light at 240 nm, clearly re-emitting in the blue. The coated glass-slices and DB samples have been tested and characterized at IFIC and ICMOL with different UV light sources. The procedure and results have been submitted to publication [15]. 7.2 WLS coating of the field cage light tube The light tubes will consist of thin sheets of TTX, fixed over a 3MTM . The TTX will be vacuum coated with TPB. The NEXT collaboration has acquired the large evaporation chamber developed by the ArDM collaboration, which is currently installed at the IFIC (Figure 13). This is a stainless steel vacuum chamber large enough to house reflector sheets of 120×25 cm2 which have the right size for the NEXT-100 detector. The apparatus consists mainly of two parts, a horizontal tube with pumping – 17 – Figure 11. The Evaporator where one can distinguish from bottom to top, the crucibles and the depositionsensors, the shutter, the sample-holder supported by a spinning disk and the vacuum chamber which is closed down during evacuation and coating. Figure 12. Illumination with 240 nm UV light of a glass-slice (left) and a 5-SiPM board (right) both coated with TPB. connection on its closed end and a slide-in array of 13 crucibles mounted onto a Viton-sealed access flange. The crucibles are electrically connected in series for better uniformity and lower total supply current (∼10A). The reflector sheets are supported by 100µm wires in a crescent arrangement for constant distance to the crucibles. An evaporation cycle is started by filling the crucibles with TPB powder and positioning the TTX reflector sheet on its support. After this preparation the sheets were inspected optically with a UV lamp (Figure 14) showing the characteristic re-emission in the blue. – 18 – evaporator Figure 13. The large evaporation chamber, details. Figure 14. Response of the light tubeTTX foil (ESR+TTX) hining on 3M foil after withcoating, TPB showing the characteristic blue light emission when illuminated by a UV lamp. In the upper part of the picture the same foil without coating. 8. The energy plane The energy measurement in NEXT is provided by the detection of the electroluminescence light by an array of photomultipliers, the energy plane, located behind the cathode. Those PMTs will also record the scintillation light that indicates the start of the event. – 19 – Figure 15. The Hamamatsu R11410-10 photomultiplier. This is a large PMT, 3 inches in diameter, with an average radioactivity of 3.3 mBq for the uranium chain and 2.3 mBq for the thorium chain. A total of 60 low-background, high-QE PMTs, model R11410-10 from Hamamatsu (figure 15), covering 32.5% of the cathode area constitute the energy plane. These are large tubes, with a 3” photocathode and low levels of activity of the order of 2.5 mBq per unit in the uranium and 2.5 mBq per unit in the thorium series, the only relevant backgrounds for NEXT. The QE of the R11410-10 model is around 25% both in the VUV and in the blue region. PMTs are sealed into individual pressure resistant, vacuum tight copper enclosures (cans) coupled to sapphire windows. The cans are all connected via individual pressure resistant,vacuum tight tubing conduits to a central manifold. The PMT cans are maintained at vacuum well below Paschen minimum, avoiding sparks and glow discharge across PMT pins. The PMT coverage is a compromise between the need to collect as much light a possible for energy resolution and the measurement of t0 and the need to minimize the number of sensors, to reduce cost, complexity and radioactive budget. To gain a quantitative idea, let’s first consider the detection of scintillation light. The number of photons that arrive to the PMT housing windows depends on the properties of the reflector as well as the transparency of the EL grids. Our simulation shows that a light tube coated with TPB and providing 90% diffuse reflectivity will transfer 9% of the photons. Assume now that an event is produced near the EL grids (the worst scenario for the detection of primary scintillation light with the cathode PMTs). A recent measurement of the energy needed to produce a scintillation photon, carried out within the context of the NEXT R&D [17] yields: Ws = 76 ± 6 eV (8.1) or 13 158 scintillation photons per MeV. Then the number of photoelectrons (pes) detected by the PMTs at the cathode is: (13158 photons/MeV) ×C × TR × TW × QE where C is the cathode coverage, TR is the reflector transfer function, TW is the housing window transfer function and QE is the PMT quantum efficiency. Monte Carlo simulation yields TW = – 20 – HV Cable/Receptacle HV Feedthrough Octagon Feedthrough/vac ports Nozzle Central Manifold Conduits Pressure Vessel Head Copper Shield Pressure Relief Nozzle Module Carrier Plate Internal PV Flange PMT Modules HV Cathode Contactor Figure 16. The full energy plane. 0.75 for a sapphire window coated with TPB. Then, setting C = 0.325, TR = 0.09 and QE = 0.25 we obtain: (13158 photons/MeV) × 0.325 × 0.09 × 0.75 × 0.25 ∼ 72 (pes/MeV) which implies a comfortable ∼ 7 pes for 100 keV, allowing the detection of primary scintillation in the full chamber range up to energies well below 100 keV. This is important, not only to study the lower part of the β β 2ν spectrum, but also to trigger in low energy gammas sources for detector calibration. Consider now EL light. As discussed in [18], we need at least 10 pes per primary electron to achieve an optimal resolution. Our optical gain is 2.5 × 103 . The number of pes per electron, is, therefore: 2.5 × 103 (photons/electron) × 0.325 × 0.09 × 0.75 × 0.25 ∼ 14 (pes/electron) which, again, is a very comfortable figure. 9. The tracking plane The tracking function in NEXT-100 will be provided by a plane of multi-pixel photon counters (MPPCs) operating as sensor pixels and located behind the transparent EL meshes. The chosen MPPC is the S10362-11-050P by Hamamatsu. This device has an active area of 1 mm2 , 400 cells per sensor and high particle detection efficiency (PDE) in the blue region. We have measured – 21 – Sapphire window Enclosure Screw-down Ring Antirotation washer PEEK Ring Kapton Shim PMT O-ring Conduit Fitting Backplate Heat Spreader Retaining Ring Potting Base Spring Band Clamp Clamp Mount Optical Coupling Pad Figure 17. A photomultiplier inside its enclosure. the spread in gain between the sensors to be less than 4%, while the average in gain is (2.27– 2.50) × 105 . These values provide a homogeneous response of the plane, and ensure the correct resolution for the reconstruction of β β events. Last but not least, MPPCs are very cost effective and its activity is very low, given its composition (mainly silicon) and very light mass. The level of the dark current is at about 5 photoelectrons per microsecond, less than 3% of the total charge collected. In NEXT-100 a digital threshold at the 6 p.e. level should lead to an insignificant noise rate. An automatic system to compensate temperature differences by adjusting the bias voltage has also been tested successfully and will be implemented in NEXT-100. The MPPCs will be mounted in Dice Boards (DB). These are square boards made of cuflon (PTFE fixed to a copper back plane). Figure 18 shows one of the DB of the NEXT-DEMO prototype, holding 4×4 pixels. NEXT-100 DB will be similar, but containing 8×8 pixels. The pitch of the NEXT-100 tracking planes is a compromise between several constraints imposed by physics: 1. An electron moving in dense gas does not behave exactly as a minimum ionizing particle. Instead, it loses a significant part of its energy by the emission of discreet delta rays and bremsstrahlung radiation. The “photon cloud” associated to the electron has an rms distribution of the order of 1 cm. 2. Transverse diffusion in pure xenon is large, and the typical rms of the charge distribution for electrons produced in the center of the chamber is of the order of 1 cm. – 22 – Figure 18. Dice Board containing 16 (4×4) MPPCs. 3. Identifying low-energy photons (e.g, x-rays of 35 keV) nearby the electron track is an extra handle to label background events (e.g, photoelectric events in the 214 Bi peak). Photon-track separation is directly proportional to pitch. A pitch significantly smaller than 1 cm is not useful due to charge dispersion and the photon cloud. Conversely, as the pitch increases, the background rejection capability decreases. Monte Carlo studies show that a reasonable tradeoff may be found for pitches between 1 cm and 1.5 cm. While physics performance appears not to degrade too much with pitch in that region, the number of pixels decreases with the square of the pitch. A reasonable compromise appears to be 1.1 cm, which in turn requires about 7 000 pixels. 10. Front-end electronics and DAQ The DAQ follows the modular architecture described in [3], named the Scalable Readout System (SRS). At the top of the hierarchy, a PC farm running the DAQ software, DATE, receives event data from the DAQ modules via GbE (gigabit Ethernet) links. The DATE PCs (Local Data Concentrators, LDCs) assemble incoming fragments into sub-events, which are sent to an additional PC (Global Data Concentrator, GDC). The GDC builds the complete event and stores it to disk for offline analysis. The DAQ modules used are Front-End Concentrator (FEC) cards, which serve as the generic interface between the DAQ system and application-specific front-end modules. The FEC module can interface different kinds of front-end electronics by using the appropriate plug-in card. The FEC card and the overall SRS concept have been developed within the framework of the CERN RD51 Collaboration. Three different FEC plug-in cards are used in NEXT-100. The cards responsible for the energy plane readout digitization and for the trigger generation are described in Sec. 10.1, where the energy plane analog front-end is also described. The tracking plane readout, described in Sec. 10.2, uses a third type of plug-in card. 10.1 Electronics for the energy plane The front-end (FE) electronics for the PMTs in NEXT-100 will be very similar to the system de- – 23 – veloped for the NEXT-DEMO and NEXT-DBDM prototypes. The first step in the chain is to shape and filter the fast signals produced by the PMTs (less than 5 ns wide) to match the digitizer and eliminate the high frequency noise. An integrator is implemented by simply adding a capacitor and a resistor to the PMT base. The charge integration capacitor shunting the anode stretches the pulse and reduces the primary signal peak voltage accordingly. Our design uses a single amplification stage based on the fully differential amplifier THS4511, √ which features low noise (2nV/ Hz) and provides enough gain to compensate for the attenuation in the following stage, based on a passive RC filter with a cut frequency of 800 kHz. This filtering produces enough signal stretching to allow acquiring many samples per single photo-electron at 40MHz. The front-end circuit for NEXT-DEMO was implemented in 7 channel boards and connected via HDMI cables to 12-bit 40-MHz digitizer cards. These digitizers are read out by the FPGA-based DAQ modules (FEC cards) that buffer, format and send event fragments to the DAQ PCs. As the FEC card, also the 16-channel digitizer add-in card has been designed as a joint effort between CERN and the NEXT Collaboratio within the RD-51 R&D program. These two cards are edge mounted to form a standard 6U×220 mm eurocard. The energy plane readout system for NEXT-100 will use 4 FEC cards to read 60 PMT channels. An additional FEC module with a different plug-in card is used as trigger module. Besides forwarding a common clock and commands to all the DAQ modules, it receives trigger candidates from the DAQ modules, runs a trigger algorithm in the FPGA and distributes a trigger signal. The trigger electronics accepts also external triggers for detector calibration purposes. 10.2 Electronics for the tracking plane The tracking plane will have ∼ 7 000 channels. Passing all those wires across feedthroughs, as it has been done for NEXT-DEMO, is possible but challenging, and probably not optimal. Consequently we are developing a new in-vessel FE electronics that reduces the total number of feedthroughs to an acceptable level. Here we present the new electronics readout architecture. Since the electronics will be inside the PV, it must necessarily be very low power to minimize the heat dissipated inside the vessel. Our design consists of a very simple Front-End Board (FEB, Fig. 19) to be placed inside the detector. The 64-ch FEB takes the input of a single DB (transmitted via low-crosstalk kapton ribbon cables) and includes the analog stages, ADC converters, voltage regulators and an FPGA that handles, formats, buffers and transmits data to the outer DAQ. LVDS clock and trigger inputs are also needed. A total of 110 FEBs are required. The low power consumption is achieved by using a passive RC circuit and ultra low power amplifiers, rather than gated integrators and power-hungry devices as was done in NEXT-DEMO. Figure 20 shows our design for the analog stage. It is a three-stage circuit, with a gain of 10 in each stage. The first two stages are based on the AD8012 (two amplifiers per package, very low noise) and the last one on the AD8005 (ultra low power, 400 µA quiescent current). The total gain is (Rt is the input termination resistance) 103 × Rt = 5 × 104 , as the first stage is a transimpedance amplifier with gain of 10 × Rt = 5 × 102 . A passive, 2 µs time-constant RC circuit (200 pF, 10 kΩ between the second and the third stage) acts as the circuit integrator. This gain will result in a 1 V output for a 250-pe dynamic range. Total electronic noise in the amplifier circuit is very low according to the simulations: 1.7 mV rms. A preliminary estimation for the power dissipation due to the analog stage yields 30 mW per channel, or 210 W in total. Additional power dissipation in the FEB comes – 24 – Figure 19. Functional blocks in the FEB card. Figure 20. This low power amplifier circuit for NEXT-100 features only 30 mW power, 4 mV/pe gain and 1.7 mV rms noise. from drop in voltage regulators, FPGA (data handling and multiplexing) and transmission circuits required to reduce the number of feedthroughs in the TPC vessel. For the tracking plane readout digitization two alternatives are currently under study: • A single-channel 1 MHz (or 3 MHz) low-power 12-bit ADCs (like AD7476), requiring only two lines (data and chip select) for readout. This solution is used in NEXT-DEMO, and would correspond to a total heat dissipated by the in-vessel electronics of 315 W. • A fast (40 MHz or higher) multi-channel ADC and an analog switch for multiplexing. This solution can lead to a reduced power consumption, though the effect of the noise induced by hundreds of switches inside the vessel has to be studied. FEB size can be 15×15 cm, leaving 3.5 cm2 board area per channel. This can easily accommodate the three amplifying stages and ADC per channel plus associated SMD passive components in one board side. The FPGA, voltage regulators and I/O connectors can sit in the opposite layer. – 25 – In addition to power consumption, another key figure of merit is FEB throughput. Data from the 6 800 SiPM channels must be sent across the PV. Minimizing the number of vacuum feedthroughs is a must, and this number is directly proportional to the aggregated throughput. To this end, we envisage the FEB readout to be zero suppressed and triggered. Zero suppression implies that every microsecond (the ADC sampling period), only channels with a charge readout above an adjustable threshold will send digitized and timestamped data to the DAQ module, where the data are stored in a circular buffer. Raw data mode of operation, where no zero suppression occurs at the online level, will also be supported for testing purposes. Additionally, FEB data will be read in triggered mode. For a 10 Hz trigger rate and a 2 ms event duration, a triggered FEB readout may further reduce the FEB throughput by a factor of 50 with respect to a continuous readout. In continuous readout mode, no trigger exists at the front-end level, and (zero-suppressed) data are sent continuously to the DAQ module, every microsecond. Once a timestamped trigger arrives to the DAQ module, the right data time interval is read from the DAQ module circular buffer and sent to the DATE online system for event building. In triggered readout mode, on the other hand, FEB data are sent to the DAQ module only in the presence of a trigger. The reduced throughput of the triggered mode comes at the cost of increased complexity, as circular (ring) buffers are needed at the FEB level. Quantitatively, 7 000 channels produce approximately 20 MByte/event in raw data mode (no zero suppression). A 10 Hz FEB trigger rate implies therefore a 200 MByte/s, or 1,6 Gb/s, throughput. This is an acceptable number. The necessary FEB circular buffer size has also been estimated, and possible solutions have been identified. One full event (2 000 samples) requires 2.000 × 64 × 12 bit, that is 1.5 Mb buffer size. For zero-dead-time operation the buffer must be large enough to be continuously filled. A buffer of about 10 ms is threfore needed, assuming a 200 Mb/s link speed to read the buffer. A conservative estimate for the FEB buffer size is therefore 7.5 Mb. This is feasible with Xilinx Artix FPGAs (XC7A200T), that has an internal 13.5 Mb memory. A trade-off between readout speed and buffer size must be found in order to minimize cost and power: data links must a have a low speed for the sake of reduced power consumption, cheaper electronics, reduced FEB noise and enhanced signal integrity. Careful evaluation of copper and optical link solutions will provide the right compromise between buffer size, power dissipation, reliability and cost. The number of FECs and of Local Data Concentrator (LDC) PCs for the tracking partition of the DAQ is determined by the tracking plane throughput and by the speed of the links (from the in-vessel electronics to the FEC card, and from the FEC card to the LDC PC). As discussed above, the tracking plane will produce 1.6 Gb/s data at most (10 Hz triggered mode, no zero suppression). Assuming 400 Mb/s as a comfortable working point for the gigabit Ethernet links between the FECs and the LDCs, 4 LDCs are required. Assuming 200 Mb/s link speed (LVDS over copper) from the in-vessel electronics to the DAQ (the same speed and technology used in NEXT-DEMO for the SiPM plane readout), the existing 16-link LVDS add-in card for the FEC module can be used. The 110 links coming from the vessel require then 110/16=7 FEC cards. We therefore need 7 FECs and 4 LDCs in the tracking DAQ partition, as shown in Fig. 21. This is approximately the size of the full NEXT-DEMO DAQ system. – 26 – Figure 21. The NEXT-100 SiPM plane can be read out with 4 LDCs and 7 FECs. 11. Shielding A relatively simple lead castle shield has been chosen for simplicity and cost-effectiveness. The lead wall has a thickness of 20 cm and is made of four layers of staggered lead bricks and copper or steel sheets. The lead bricks have standard dimensions (200×100×50 mm3 ), and, by requirement, an activity in uranium and thorium lower than 0.4 mBq/kg. The detector itself is placed on top of a seismic structure called the detector pedestal, DP, independent of the working platform (WP), and of the lead castle (LC). The DP is anchored to the ground and supports all the weight of the detector and the LC. The WP is designed to stand a uniform load of 300 kg/m2 and a concentrated load of 200 kg/m2 .The platform is anchored to the hall ground and walls. Notice that the WP and the DP structured are independent to allow seismic displacements in the event of an earthquake. The platform floor tiles are made of galvanized steel and have standard dimension to minimize cost. All beams and pillars of the WP are designed with IPE and HEB cross sections, IPE-100 for pillars, HEB-240 for seismic beams and HEB-500 for main seismic beams. The movable lead castle has and open and a close position. The open position is used for the installation and service of the pressure vessel. The closed position is used normal operations. The LC itself is made of two halves mounted on a system of wheels that move on tracks with the help of an electric engine. The system includes a lifting device and a lock for each wheel. The lifting devices elevates all the wheels from 5 to 10 mm above the guides. Once in the desired position the lock fixes the movable lead castle to the seismic structure, both in the open and in the closed position, to avoid seismic displacements during an eventual earthquake. 12. NEXT-100 at the LSC NEXT 100 detector and ancillary systems are intended to commence for installation at the LSC in the second quarter of 2012. Figure 23 shows an image of Hall A, future location of NEXT 100. The pool-like structure is intended to be a catchment reservoir to hold Xe or Ar1 gas in the event 1 ArDM will be the neighbor of NEXT 100 in Hall A. – 27 – Figure 22. The NEXT-100 lead castle shield. Figure 23. View of Hall A at the LSC prior to any equipment installation. of a catastrophic leak. Therefore, for reasons of safety all experiments bust preclude any personnel working bellow the level of the top of the catchment reservoir. An elevated working platform will be built prior to the installation of NEXT-100. Figure 24 shows the placement of Next-100 systems and components on the platform as well as the dimensions. Additionally due to mild seismic activity of the part of the Pyrenees where the LSC is located – 28 – Figure 24. Left: Intended location of the components and subsystems for the operation of NEXT 100 on the working platform: (a) NEXT-100; (b) the lead castle in its open configuration; (c) gas purification system; (d) working platform; (e) seismic platform; (f) emergency gas vent tank; (g) data acquisition system; (h) other systems. Right: Top view showing the dimension of the working platform. the heavy lead castle and NEXT 100 will be placed on seismically isolated platform. Work is underway, in coordination with LSC staff to refine and complete the design of each relevant element, and the integration of all the systems. 13. Radioactive budget 13.1 Sources of background in NEXT 214 Bi and 208 Tl The β β 0ν peak of 136 Xe is located in the energy region of the naturally-occurring radioactive processes. The half-life of the parents of the natural decay chains, of the order of the age of the universe, is, however, very short compared to the desired half-life sensitivity of the new β β 0ν experiments (∼ 1026 years). For that reason, even small traces of these nuclides create notable event rates. The only significant backgrounds for NEXT are the high energy gammas produced in the β -decays of the isotopes 208 Tl and 214 Bi, found in the thorium and uranium series, respectively. The daughter of 208 Tl, 208 Pb, emits a de-excitation photon of 2614 keV with a 100% intensity. The Compton edge of this gamma is at 2382 keV, well below Qβ β . However, the scattered gamma can interact and produce other electron tracks close enough to the initial Compton electron so they are reconstructed as a single object falling in the energy region of interest (ROI). Photoelectric electrons are produced above the ROI but can loose energy via bremsstrahlung and populate the window, in case the emitted photons escape out of the detector. Pair-creation events are not able to produce single-track events in the ROI. After the decay of 214 Bi, 214 Po emits a number of de-excitation gammas with energies above 2.3 MeV. The gamma line at 2447 keV (intensity: 1.57%) is very close to Qβ β . The photoelectric peak infiltrates into the ROI for resolutions worse than 0.5%. The gamma lines above Qβ β have low intensity and their contribution is negligible. The contribution of pair-creation events is also insignificant. – 29 – Radon Radon constitutes a dangerous source of background due to the radioactive isotopes 222 Rn (half-life of 3.8 d) from the 238 U chain and 220 Rn (half-life of 55 s) from the 232 Th chain. As a gas, it diffuses into the air and can enter the detector. 214 Bi is a decay product of 222 Rn, and 208 Tl a decay product of 220 Rn. In both cases, the radon suffers from an alpha decay into polonium, producing a negative ion which is drifted towards the anode by the electric field of the TPC. As a consequence, 214 Bi and 208 Tl contaminations can be assumed to be deposited on the anode surface. Radon may be eliminated from the TPC gas mixture by recirculation through appropriate filters. There are also ways to suppress radon in the volume defined by the shielding, whether this is a water tank or a lead castle. Radon control is a major task for a β β 0ν experiment, and will be of uppermost importance for NEXT. Cosmic rays and laboratory rock backgrounds Cosmic particles can also affect our experiment by producing high energy photons or activating materials. This is the reason why double beta decay experiments are conducted deep underground. At these depths, muons are the only surviving cosmic ray particles, but their interactions with the rock produce neutrons and electromagnetic showers. Furthermore, the rock of the laboratory itself is a rather intense source of 208 Tl and 214 Bi backgrounds as well as neutrons. The above backgrounds can be reduced below those intrinsic to the detector by shielding, as described in the previous section. In addition, given the topological capabilities of NEXT the residual muon and neutron background do not appear to be significant for our experiment. 13.2 Contribution of the main materials used in NEXT Information on radiopurity of the materials expected to be used in the construction of NEXT100 has been compiled, performing specific measurements and also examining data from the literature for materials not yet screened. In this executive summary we just present a brief summary of our detailed data base. A full report can be found online at the NEXT database web page2 . NEXT has the structure of a Matrioska, or russian doll. The flux of gammas emanating from the LSC walls is drastically attenuated by the lead castle (LC), and the residual flux, together with that emitted by the lead castle itself and the materials of the pressure vessel is further attenuated by the inner copper shield (ICS). The ICS also attenuates the flux emitted by the tracking plane FE electronics (FEE), which sits behind it. One then needs to add the contributions of the “inner elements” in NEXT: field cage (FC), energy plane (EP), and the elements of the tracking plane (TP) not shielded by the ICS. Thus, the basic ingredients in the NEXT background model recipe are: 1. The radiation emanating from the Matrioska, which includes the residual radiation emanating from the LSC walls, from the LC, from the PV and from the FEE, after being attenuated by the ICS. 2 http://next.ific.uv.es/cgi-bin/DocDB/public/ShowDocument?docid=76 – 30 – Table 3. Activity (in mBq/kg) of the most important materials used in NEXT. The numbers are selected from the detailed report available at the NEXT webpage. Material Pb Cu Steel (316Ti) Inconel 718 Inconel 625 Peek Capacitors (Tantalum) SMD Resistors, Finechem (per pc) Polyethylene TTX TPB PTFE (Teflon) PMT (R11410-MOD per pc) PMT (R11410-MOD per pc) Sapphire window CUFLON Kapton cable NEXT system Shield ICS PV PV PV FC/EP/TP FC/EP/TP FC FC LT LT/EP/TP EP/TP/DB EP EP EP TP TP/EP Reference Cometa (supplier) Luvata (supplier) XENON [16] LSC LSC Unizar Unizar [19] Unizar XENON [16] ArDM [20] A. Chemicals [21] GERDA [22] XENON [16] LUX [23] EXO [24] ICPMS [25] KAPPA [19] 238 U 232 Th 0.37 <0.012 < 1.9 <5.6 <2.4 36 320 0.022 0.23 12.4 1.63 0.025 < 2.5 < 0.4 0.31 0.36 14 0.07 < 0.004 <1 < 13.8 <6.0 11.7 1.23 × 103 0.048 <0.14 1.6 0.47 0.031 < 2.5 < 0.3 0.12 0.28 39 2. The residual radiation emanating from the ICS, after being attenuated (self-shielding) by the ICS itself. 3. The radiation emanating from the inner elements, namely the FC, the EP and the TP. The only relevant backgrounds for NEXT are the photons emitted by the 208 Tl line (2614.5 keV) and the 214 Bi line (2448 keV). These lines sit very near Qβ β and the interaction of the photons in the gas can fake the β β 0ν signal, as we will discuss in the next section. Our first step, therefore, is to quantify the number of photons crossing the gas emanating from the different sources enumerated above. The activity for each material is given in Table 3 in terms of 238 U or 232 Th decays per unit of time and mass. The high energy photons relevant for NEXT should then be computed by taking into account the branching ratio of the mother nucleus decaying into 214 Bi or 208 Tl (that is 1 and ∼ 1/3 respectively), plus the branching ratio of the daughter nucleus itself into the dangerous gamma, plus a geometric factor which reflects the fact that photons coming from nuclear decays are emitted in all directions and not all of them get to reach the gas. The second and the third aspect are included in the rejection factor of background calculated through Monte Carlo (see Section ??). Thus, in the following, the numbers of photons per year are actually the numbers of photons due to a decay of 214 Bi or 208 Tl (the branching ratio of thorium into thallium is already included), before – 31 – the correction for the branching ratio and geometric factor. The only exception is the contribution from the LSC walls, which has been measured. Contribution from the Matrioska In the following, the contribution to the total activity of each one of the shells of the Matrioska is described and the photon rates are shown in Table 4. The flux of photons emanating from the LSC walls is[26]: 1. 0.71 ± 0.12 γ/cm2 /s from the 238 U chain. 2. 0.85 ± 0.07 γ/cm2 /s from the 232 Th chain. These measurements include all the emissions in each chain. The flux corresponding to the line at 2614.5 keV and the flux corresponding to the 214 Bi line at 1764.5 keV were also measured (from this last measurement it is possible to deduce the flux corresponding to the 2448 keV line). The results are: 208 Tl 1. 0.13 ± 0.01 γ/cm2 /s from the 208 Tl line. 2. 0.006 ± 0.001 γ/cm2 /s from the 214 Bi line at 2448 keV. The photon flux through the surface of the lead castle is 4.3 × 1010 and 9.3 × 1011 photons per year for 214 Bi and for 208 Tl respectively. The LC is made of radiopure lead. The thickness of the shield is 20 cm and the total mass of the castle is 60 tons, the supplier being most likely COMETA. The activity of the other components of the lead castle (e.g, external steel support infrastructure) after self-shielding from the lead is negligible compared with the contribution of the lead itself. Notice that the photon rate emitted by the LC is much larger than the residual contribution from the LSC walls in the 214 Bi contamination and about the same in 208 Tl. The PV is made of radiopure 316Ti alloy and its total mass is 1 200 kg. We assume the activity measured by the XENON collaboration (measurements of 316Ti samples are currently under way at the LSC). The PV does not act like a shield, but it still respects the Matrioska principle in the sense that the background rate that it adds is of the same order of magnitude (actually slightly larger) than the residual background from the outer shell. Other elements of the PV such as the Inconel screws add a small contribution. Their specific activity adds a 12.5% (20%) extra background rate in 214 Bi (208 Tl) to the PV itself. The inner copper shield is the most radiopure shell of the Matrioska. It is made of radiopure copper and its total mass is 1 200 kg. We consider the activity measured by the supplier (Luvata). The ICS attenuates the radiation coming from the outer shell by two orders of magnitude, and contributes with a background rate that is almost identical to the residual background. The final rate is 106 214 Bi photons a year and about one order of magnitude less in 208 Tl. Contribution from the inner elements The contribution to the total activity from the inner elements is summarized in Table 5. The tracking plane is made of the supporting structure for the DBs (a honeycomb made of PTFE), and the DB themselves, 110 boards made of Cuflon, each weighting about 250 grams. Each – 32 – Table 4. Number of photons per year outgoing each shell of the Matrioska. The numbers in bold font are the residual photons after all the shells. from previous vol after passing through current vol LC PV ICS from current vol (self shielded) total 214 Bi 208 Tl 214 Bi 208 Tl 214 Bi 208 Tl 8.8 × 104 1.2 × 107 5.6 × 105 1.9 × 106 1.6 × 106 9.8 × 104 3.2 × 107 4.5 × 107 5.2 × 105 2.1 × 106 7.8 × 106 5.9 × 104 3.2 × 107 5.7 × 1.12 × 107 1.0 × 106 4.0 × 106 9.4 × 1.2 × 106 1.6 × 105 Table 5. Number of photons per year emitted by the inner elements. TP EP FEE FC 214 Bi 208 Tl 3.2 × 105 8.6 × 105 7.2 × 105 8.2 × 105 8.2 × 104 2.8 × 105 5.7 × 105 1.7 × 105 DB has 64 MPPCs (glued to the DB with silver epoxy). Each DB has 1 resistor and 2 capacitors. The dominant contribution comes from the DBs themselves, while the addition due to resistors and capacitors is more than two orders of magnitude lower, thus negligible. The contribution of the support structure is also small. The energy plane is composed of 60 individual modules plus a vacuum manifold. Each module consists of: (a) one PMT; (b) one PMT base; (c) the can, including its window, support structures, etc. The Hamamatsu R11410-MOD PMTs have been measured by the XENON and the LUX collaborations, and a new measurement is under way at the LSC. Since both measurements are compatible (both are upper limits) we take the lowest limit (see Table 3). The tracking plane front end electronics (FEE) sits behind the ICS and thus its contribution to the radioactive budget is attenuated by a factor 100. The bulk of the FEE are the 110 front end boards (FEBs). Each FEB will have about 750 resistors and 1100 capacitors. The dominant source of background in the field cage is the polyethylene used as an insulator and support of the field cage rings. The FC support has a mass of 110 kg. The contribution of the metal wires (made of radiopure copper) as well as those of the resistors is negligible compared with that of the polyethylene. Total radioactive budget Notice that the leading contribution to the budget (e.g, in the 214 Bi contamination) of the Matrioska (including LC, PV and ICS), energy plane, field cage and FEE are very similar. An important consequence of this is that a substantial reduction of the radioactive budget can only be achieved if – 33 – -150 Y (mm) -200 -250 -300 0 50 100 X (mm) 150 200 Figure 25. Double beta decay events have a distinctive topological signature in HPGXe: a ionization track, of about 20 cm length at 15 bar, tortuous because of multiple scattering, and with larger depositions or blobs in both ends. it happens in all the subsystems at the same time. In its current configuration the NEXT radioactive budget is quite balanced. The total radioactive budget is 3.7 × 106 photons per year due to 214 Bi and 1.3 × 106 photons per year in the 208 Tl channel. 14. Expected sensitivity 14.1 Signal and background characterization in NEXT Double beta decay events have a distinctive topological signature in HPGXe (Figure 25): an ionization track, of about 20 cm length at 15 bar, tortuous because of multiple scattering, and with larger depositions or blobs in both ends. As the track propagates in the dense gas it emits δ -rays and bremsstrahlung radiation. Those are typically low-energy gammas with a mean free path below 1 cm. The convolution of emission of electromagnetic energy with the effect of diffusion (about 1 cm for a drift of 1 m) results in a track for β β 0ν events that looks more like a wiggly, wide "stripe" of energy deposition, about 1-2 cm wide, than like a well defined “wire”, as usual when tracking high energy muons, for example. The implications are quite clear: 1. Space resolution is not an issue in NEXT. A pitch of about 1 cm is sufficient, given the combined effect of radiation and diffusion, that blur the track into a stripe. Our final design parameter is 1.1 cm. 2. Instead, identification of low energy gammas nearby the track (at distances of a few cm) is important. This is due to the difference between signal (two electrons of average energy Qβ β /2) and the dominant background (one electron of energy Qβ β ). In the second case the – 34 – probability of radiation is higher and the mean free path of the gamma is longer. Topologies with one or more isolated clusters of energy, corresponding to relatively high energy gammas flying distances of 2 or more cm are therefore more likely for the background than for the signal. Therefore, identifying low-energy satellite clusters is important in NEXT. This requires a sensor with low energy threshold, as the MPPCs. NEXT has three powerful handles to separate β β 0ν events from backgrounds. These are: 1. Signal events appear with equal probability in the target, that is, the gas that fills the PV. Defining a fiducial volume, separated from the PV walls by a few cm of active target permits eliminating events in which a high energy gamma is accompanied by charged particles that exit the PV walls. The requirement that the events are strictly contained in the active fiducial volume guarantees that any event with charged activity is eliminated. Notice that, from the point of view of rejecting backgrounds, an HPGXe behaves in a complementary way than a LXe. While liquid xenon has excellent self-shielding properties, xenon gas, even at (moderately) high pressures, is very transparent to gammas. 2. Signal events have all the same energy. Imposing that the events are in the ROI (taken as 0.5% FWHM) eliminates substantially the backgrounds. 3. Signal events have a distinctive topological signature that can be exploited to further suppress the background. Our calculations yield a rejection factor of 20 at moderate efficiency cost. This is one of the strongest points offered by NEXT technology. 14.2 The topological signature The initial processing of the event allows the “voxelization” of the track, which is formed in terms of connected 3D “voxels” formed using the 2D position given by the tracking plane and the third dimension given by time information. The initial voxels are cubes of 1 cm3 in volume, corresponding to the pitch between the MPPCs and twice the EL grid distance. The size of isolated photons is typically one voxel, while a β β 0ν “track” will consist of about 20 voxels. An event is accepted as a β β 0ν candidate if: 1. The event is fully contained in the fiducial volume 2. Only one reconstructed track: That is the BFS algorithm finds only one connected set of voxels. Events with more than one object (tracks or disconnected voxels) are rejected. 3. Energy in the ROI: The event is required to be within 1 FWHM of Qβ β . 4. β β 0ν signature: the unique track ends in two blobs of high energy, as expected for a β β event. This is translated into a cut Eth = 0.4 − 0.55 MeV and R = 2.5 − 3.0 cm, that allows a suppression factor between 10 and 20 for the background at the expense of selection efficiency between 78% and 55%. To estimate the performance of NEXT-100 β β 0ν and background data samples have been generated with the NEXUS Monte Carlo simulation. Signal events are simulated in the volume – 35 – Table 6. Suppression of the 214 Bi events by the selection cuts. Selection criteria Fraction of events surviving cuts Events analyzed Fiducial, 1 track ROI Topology 108 6 × 10−5 2.2 × 10−6 1.9 × 10−7 Table 7. Suppression of the 208 Tl events by the selection cuts. Selection criteria Fraction of events surviving cuts Events analyzed Fiducial, 1 track ROI Topology 108 2.4 × 10−3 1.9 × 10−6 1.8 × 10−7 Table 8. Signal efficiency. Selection criteria Fraction of events surviving cuts Events analyzed Fiducial, 1 track ROI (1 FWHM) Topology 106 0.48 0.33 0.25 inside the vessel. Backgrounds (208 Tl and 214 Bi decays) are simulated in the vessel and in the readout planes. The events thus generated are passed through the selection procedure. Tables 6, 7 and 8 summarize the rejection factors for 214 Bi and 208 Tl backgrounds as well as the signal efficiency. The background rate of the experiment is obtained multiplying by the radioactive budget computed in the previous section. Multiplying 3.7 × 106 photons per year due to 214 Bi by 1.9 × 10−7 rejection factor yields 0.7 events a year in the 214 Bi chain. Multiplying 1.3 × 106 photons per year due to 208 Tl by 1.9 × 10−7 rejection factor yields 0.23 events a year in the 208 Tl chain. Thus the expected background number is 1 event a year. Divide 1 count by the mass (100 kg) and by the ROI width (12.5 keV) to obtain a background rate of 8. × 10−4 counts/(keV · kg · y). If 50 extra kilograms can be found the background rate could be even lower, 5. × 10−4 counts/(keV · kg · y). The combination of good resolution, radiopurity and small background rate results in an excellent expected sensitivity. For a total exposure of 10 000 kg·year (which appears feasible, in – 36 – particular if we can run with 150 kg) the sensitivity expected using the PMA NME ([11]) is 35 meV, enough to fully cover the degenerate hierarchy and partially explore the inverse hierarchy. This sensitivity can match or outperform that of the best experiments in the field. Acknowledgments The NEXT Collaboration acknowledges support by the Spanish Ministerio de Ciencia e Innovación under grants CONSOLIDER-Ingenio 2010 CSD2008-0037 (CUP) and FPA2009-13697-C04-04. References [1] J. J. Gómez-Cadenas, J. Martín-Albo, M. Mezzetto, F. Monrabal, and M. Sorel, The search for neutrinoless double beta decay, Riv. Nuovo Cim. 35 (2012) 29–98, [arXiv:1109.5515]. [2] NEXT Collaboration, F. 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