for rare event detection

Transcription

1 A Micromegas detector for rare event detection T. Papaevangelou 1 S. Andriamonje 1 S. Aune 1 H. Brauninger 2 T. Dafni 3 G. Fanourakis 4 E. Ferrer Ribas 1 J. Galán Lacarra 3 T. Geralis 4 A. Giganon 1 I. Giomataris 1 I.G. Irastorza 3 K. Kousouris 4 J. Morales 3 J.P. Mols 5 M. Pivovaroff 6 M. Riallot 1 J. Ruz 1 R. Soufli 6 K. Zachariadou 4 1 IRFU, Centre d Etudes de Saclay, Gif sur Yvette CEDEX, France 2 Max-Planck-Institut for Extraterrestrial Physics, Garching, Germany 3 Instituto de Física Nuclear y Altas Energías, Zaragoza, Spain 4 Institute of Nuclear Physics, NCSR Demokritos, Athens, Greece 5 Institute fur Kernphysik, TU-Darmstadt, Darmstadt, Germany 6 Lawrence Livermore National Laboratory, 7000 East Avenue, Livermore, USA 11th Topical Seminar on Innovative Particle and Radiation Detectors, Siena 2008

2 Outline The micromegas detector Micromegas for solar axion search in CAST A micromegas based TPC for neutrinoless double beta decay Conclusions

3 Micromegas Detector Two-region gaseous detector: Conversion region Primary ionization Charge drift Drift field typical 2-3 V/cm Amplification field typical 4-5 V/cm Amplification gap: 50-0 μm γ Amplification region Charge multiplication Readout layout Strips (1/2 D) Pixels Separated by a Micromesh Very strong and uniform magnetic field Giomataris, Charpak (1996) Mesh signal Pixels / strips signals

4 Micromegas Technologies Conventional technology The pillars are attached to the mesh. A supporting ring or frame is adjusting the mesh on top of the readout plane Typical dimensions: mesh thickness 5 μm, gap 50 μm Bulk technology The pillars are attached to a woven mesh and to the readout plane Typical dimensions: mesh thickness 30 μm, gap 0 μm Microbulk technology The pillars are constructed by chemical process of a kapton foil, that is attached to the mesh and to the readout plane Typical dimensions: mesh thickness 5 μm, gap 50 μm

5 Bulk technology Woven Inox mesh Readout plane + mesh all in one 30 µm vacrel 128 µm Well established technique Readout pads Advantages: Uniformity, it Reachable resolution FWHM, limited i by the thickness of the mesh), very robust, low noise due to lower capacity, easy to construct Disadvantages: higher risetime, sensitivity to pressure variations, stability pad pillar

6 Microbulk technology Readout plane + mesh all in one Micromesh 5µm copper Kapton 50 µm Innovative technique, well developed during the last year Readout pads Advantages: Uniformity, reachable resolution (better than FWHM), stability at long term runs, less sensitivity to pressure variations, background rejection Disadvantages: Higher electronic noise due to higher capacity, complexity of the manufacturing process, fragility Mesh 5 µm of copper Holes 30 µm diameter Pitch 0 µm Pads of 400 μm

7 CAST Experiment (see talk by Annika Nordt) Axion flux on earth 2 GeV g aγγ 1 expect 0.3 events/hour for g aγγ = - GeV -1 and A = 14.5 cm 2 Solar axions may convert to photons inside the field of a prototype LHC magnet via inversed Primakoff effect Low background Low radioactivity Signal: X-Ray excess during tracking (1- kev) materials (plexiglas, Low count rate copper, kapton) Low background Particle recognition Detector stability Shielding CAST phase II different pressure setting(s) X-Ray focusing device in every tracking. Each setting is a new experiment! A background rate ~0 cts/h is essential for the discovery potential

8 Micromegas in CAST Phase I ( ) & He-4 phase ( ) one conventional not shielded Micromegas was used (sunrise) Readout :192 x 192 strips, 350 μm pitch Gas mixture: 95% Ar, 5% 1 bar (flamable) X-Ray detection Threshold: ~06keV ~0.6keV Background ~5-5 events kev -1 s -1 cm -2 Position sensitivity ~0 μm

9 Micromegas in CAST, Phase II Sunrise line: it has been redesigned for Shielding X-Ray focusing optics A Bulk detector replaced the conventional one Readout :6 x 6 strips, 550 μm pitch Gas mixture: 97.7% Ar, 2.3% 1.44 bar (non flammable increased efficiency) Sunset side: the TPC was replaced by one Bulk and one Microbulk detector t Readout :6 x 6 strips, 550 μm pitch Gas mixture: 97.7% 7% Ar, 2.3% 1.44 bar Under construction at LLNL

10 Micromegas readout Mesh: 1 GHz FADC, 2.5 μsec (MATACQ) Strips: Integrated charge at each strip in groups of 96 (Gasiplex) 1 kev X-Rays: Localized energy deposition (<1 mm) Short risetime and pulse width for the analogue signal Charge in one cluster of few strips per axis Pattern recognition algorithm can be applied to reduce background 55 Fe 55 Fe cosmic 55 Fe cosmic

11 ] Pattern recognition Define signal characteristics for 55 Fe X-Rays (daily calibration runs) Strips: multiplicity, width, topology of clusters Mesh: risetime, width, amplitude, integral of signals Examine distributions Apply cuts in data runs Sequential Multivariate analysis Y-Multiplicity Rise Time / Pulse Time Fe data X-Multiplicity Y-Multiplicity X-Multiplicity 55 Entries Fe data Amplitude_ov_CH_vs_RiseTime_ovPulseTime_run_90031 Neural networks Amplitude / Charge Amplitude / Charge Optimize between efficiency and rejection Data reduction > 3 kev cm -2 dn/de [s Integral 5.911e Calibration_Energy_Mesh_ Entries BackgroundB_Energy_Strips_ Entries Integral Integral ] kev cm -2 dn/de [s -1-1 Rise Time / Pulse Time Fe data Total Energy [kev] Total Energy [kev]

12 ] ] Detector Performance Energy resolution Long term stability of gain -1 kev -1 dn/de [s cm % Isobutane 5 % Isobutane Total_Energy_Mesh_run_7005 Total_Energy_Mesh_run_ χ / ndf / 35 Constant.65 ± 0.94 Mean ± Sigma ± % FWHM -1 kev -1 dn/de [s cm ± Constant Mean ± Sigma ± % FWHM Total Energy [kev] Total Energy [kev] Spatial resolution Panter with X-Ray telescope [mm] Image from a lead foil with pinholes Efficiency [mm]

13 Micromegas background in CAST 2007: Implementation of shielding (Pb+ Cd + Cu + Polyethylene) -1 kev cm -2-1 dn/de [s Background_Energy_Strips_ ] NO shielding: 5.3 cts/h SHIELDED: 1.7 cts/h Total Energy [kev] The background level (after cuts) is reduced by a factor 3 to 5 The new background levels imply 2-3 expected bkg counts per pressure setting (~2800 sec solar tracking per setting) compared to 25 (Micromegas) and 80 (TPC) from 4 He phase (~5700 sec) The combined performance of the three detectors is comparable with the Telescope- CCD system concerning discovery potential Background [ -5 s -1 cm -2 kev -1 ] tracking background mean background Pressure [mbar] Implementation of a telescope in the sunrise side can decrease the background by a factor ~0

14 Neutrinoless Double Beta (0νββ) ββ decay is relevant when the nucleus cannot decay β β ββ 76 Ge 2νββ: Standard process, observed in about isotopes so far e - e - ν ν ββ 0νββ: Only possible if the neutrino has a mass and is Majorana particle. Yet to be seen e - ν 136 Xe Observable: the energy of the two β: Precious information on neutrino properties (mass scale, Majorana/Dirac nature, ) Continuum for the 2ν A narrow peak for the 0ν Good energy resolution is essential!!! 2νββ Q value 0νββ

15 Current generation ββ experiments Source = target Source target NEMO/SUPERNEMO CUORICINO/CUORE GERDA Good E resolution Good scaling-up BUT, modest background discriminstion strong requirements on radiopurity and shielding Event topology information BUT, moderate energy resolution and difficult scaling up

16 Gas Xe TPCs for ββ? Can they be competitive in the race towards ton or multiton scale exp s? Gas TPCs offer in principle the advantages of both previous approaches: topological signature & scaling-up But also: Xe easy to enrich No long lived isotope to activate Very weak 2νββ mode (still to be measured!) Single homogeneus medium (no surfaces/boundaries) 870 kev e- in the MUNU TPC 1 e - events and 2 e - events have different topologies. This can be used to reject gamma background (1 e - ) Gothard demostrated that this can be done. They achieved a 96.5% efficiency in rejecting single e - events. We may do better. A gas TPC would have an extra handle to reduce background by a factor of at least 2 (most probably more?). 2 e-

17 A micromegas based TPC for ββ? TPC s not presently comtemplated in present projects (EXO: liquid TPC) Why?: Energy resolution: Fano factor, gain stability homogeneity, equalization, ballistic deficit, Complex detectors. Specially for large V needed. traditional limitations of TPCs read by wires are overriden by recent developemnts (micropattern detectors) No mechanical challenge. Standard PCB technology replaces plane of wires. Spatial resolution achievable much higher. h Manufacturing process of large surfaces/volumes is much simpler, and cheaper. Robustness Impact on electronics (more freedom in readout design) All MICROPATT TERN And specially: Energy resolution close to the Fano limit Enhanced stability and homogeneity Specifica ally MΜ

18 A new experiment: NEutrino Xenon TPC (NEXT) Goal is 1. To define all technical aspects of a 0 kg Xe gas TPC by doing some R&D, and building a demonstrator of kg, NEXT, to operate underground in a timescale of 2-3 years. Pressure vessel cathode HV Xe PMTs 2. To build a 0 kg detector, NEXT0, already with physics interest. (in a timescale of 5 years) Readout plane Readout planes 3. To assess the option of scaling up to 1 ton, NEXT00, and eventually build it. A sensitivity down to 60 mev (for NEXT-0) and 20 mev (for NEXT-00) is a priori reachable if Low enough resolution is achieved (~1% FWHM) Low enough background after topology cuts (i.e. not background limited) Expression of Interest to the Canfranc Underground Laboratory received positively Collaborating groups: Barcelona, Berkeley, Saclay, Valencia, Zaragoza Hall A of new Canfranc Lab

19 High energy resolution in Micromegas: ongoing g tests Measurement of E resolution at high energies: Highpressure Ar+Isob small setup, read by new generation Micromegas readout (microbulk) non-pixelized anode Mixtures tests: Ar + Iso 2%, Ar + Iso 5% Pressures tested: from 1 to 5 bar Americium alpha source: 5.5 MeV alpha μm P exhaust

20 Energy resolution Measurements Energy resolution tested with Am alpha source (5.5 MeV) Best resolution obtained: % (FWHM) in a wide range of parameters (mesh and drift V, P, etc ) Landau deconvolution analysis indicate possible intrinsic Micromegas energy resolution of 0.7 % FWHM. Setup upgraded recently for same measurement in Xe (recirculation system). First (very priliminary) measurements indicate: at 2 bars: 2.8 % FWHM at 4 bars: 4.5 % FWHM Am 5.5 MeV α Hellaz TPC Equipped with MM Recirculation pump E res TPC

21 CONCLUSIONS The micromegas detector has several advantages in rare event detection: Low intrinsic background due to Low radioactivity materials Particle discrimination Spatial resolution Stability in long term runs Good energy resolution During ~6 year operation in CAST, micromegas has shown a low and stable background level, contributing significantly in the achieved results. Recent upgrades are leading to better performance and a dominant contribution to the future results is expected. These established characteristics, combined with the achieved and prospective improvements in energy resolution makes the option of a gas Xe TPC for ββ very promising ii

22 END

23 Neutrinoless Double Beta (0νββ) (A,Z) (A,Z+2) + 2 e - Lepton number violation (ΔL =2) Neutrino must be Majorana (equal to its antiparticle) Decay rate: Phase space factor Nuclear Matrix Element Region being explored by present experiments quasi degeneracy m 1 m 2 m 3 Effective neutrino mass is the underlying quantity (assuming light neutrino exchange as fundamental process) Cosmological disfavoured Region Inverse hierarchy Δm 2 12= Δm 2 atm Direct hierarchy Δm 2 12= Δm 2 sol IGEX ( 76 Ge) <m ν > < ev PRD65(02) NEMO-3 ( 0 Mo) <m ν > < ev PRL95(05) & TAUP07 expected soon < ev CUORICINO ( 130 Te) <m ν > < ev PRL95(05)142501