NEXT-100 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

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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

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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].

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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) (T1/2 ) = G0ν M 0ν m2β β . 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: (2.2) mβ β = ∑ Uei2 mi , 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).

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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)

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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.

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entering the region of intense field (E/P ≈ 3 kV/cm.bar) between the transparent EL grids. This light is recorded by an array of silicon photomultipliers (SiPM) located right behind the EL grids and used for tracking measurement. It is also recorded in the PMT plane behind the cathode for energy measurement. The primary scintillation recorded by PMTs gives the start-of-event time t0 . The EL scintillation recorded by SiPMs, provides the transversal coordinates (x, y) of the track’s trajectory and the longitudinal coordinate (z) from the time t of the signal.

Figure 1. The Separated Optimized Functions (SOFT) concept in NEXT TPC. EL light generated at the 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 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. photosensor plane behind the transparent cathode and used for a precise energy measurement.

Several NEXT prototypes with up to 1 kg of pure gaseous xenon at 10-15 bar, were recently built. In the NEXT-DBDM prototype [2], the energy of the events from EL signals was measured • NEXT-DEMO, shown in figure 4. This is a larger prototype, 137 operating at IFIC, whose preswith a near 1% FWHM resolution from the 662 keV gamma rays of Cs, using an array of UV sure vessel lengthtracking of 60 plane cm and diameterforofthe30NEXT-DEMO cm. The vessel can [1],[3], withstand a sensitive PMTs.has Thea SiPM first adeveloped prototype pressure ofreconstruct up to 15 bar. The maximum capacity of the detector is 10that kg abut in its current will allow to the tracks of these gamma ray events and demonstrate large-mass 136 configuration (the fiducial volume is an hexagon of 16 cm diameter and 30 cm length) it gaseous xenon TPC, enriched with Xe and EL readout, would provide a possible pathway for a robust experiment. holdsdouble-beta 4 kg at 15 decay bar. NEXT-DEMO is also equipped with an energy plane made of 19 HamaSiPMs or Multi Pixel Photon Counters (MPPC) haveHamamatsu been chosen in NEXT for many matsu R7378A and a tracking plane made of 300 MPPCs. Thetheir main goals of outstanding features for tracking purposes. SiPMs offer comparable detection capabilities as stanthis prototype are: (a) to demonstrate track reconstruction and the performance of MPPCs dard small PMTs and APDs with the additional advantages of ruggedness, radio-purity and cost(coated with a wavelength shifter, TPB, to make them sensitive to xenon VUV, [15]); (b) effectiveness, essential for a large-scale radiopure detector. Their main drawback however is their to test long drift lengths and very high voltages (up to 50 kV in the cathode and 25 kV in poor sensitivity in the emission range of the xenon scintillation (peak at 175 nm, see reference [5]). the makes anode), (c) to understand recirculation in a large volume,theincluding This necessary the use of agas wavelength-shifter (WLS) to convert UV light operation into visiblestability and robustness against leaks;optimal (d) tophoton understand theefficiency transmittance light, where these sensors have their detection (PDE). of the light tube, with

and without TPB. In summary, to demonstrate the technology to be used by the NEXT-100 detector. –2–

• 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:

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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:

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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

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Cs-137 Photoelectric electron 150 0.20

y (mm) Cs-137 energy spectrum

0.18

Only one blob 100

0.16 0.14 0.12

50

0.10 0.08

0

0.06

Xe 35 keV X-ray

0.04 0.02 -50

-50

0

50

0.00

100

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. NEXT-100 Pressure Vessel

Detector Overall Cross Section

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 D. Shuman (LBNL)

Reflectors Field Cage Rings F.C. Insulator Cu Shield Bars Shielding, External, Cu on Pb

NEXT-100 Pressure Vessel, Nov. 1, 2011

November 2, 2011

13 / 20

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

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P-x Dome Loaded Reg BPR1 Overpressure Relief

Underpressure Relief VM1

Check Valve

Shielding

RP1 Bursting Disk Regulator

BD2 V16

Recirculation Pump

Servo Valve Xe

Xe Line

Manual Valve

UPR1

Service line Dome Loaded Reg

OPR1 V21

Overpressure Relief

Pressure Reference P-x

V23 V1

BPR1

V21

V2

DLR1 R1

V3 C1

VM1

Reduce to 1 bar RP1

Underpressure Relief V22

Getter

BD2

Pressure Reference Bursting Disk Value typically P=15 bar

Emerg

Emerg

Dump

Dump

Pump Expanssion Recirculation Cold

V20

Xe Cold Recov

Two parallel Getters and a bypas

V16

Gas Analyzer Xe Line

X V17

Rn Trap 2

Service line 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. R compressor manufactured by SERA . 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

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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

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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

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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

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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