L esperimento GERDA Balata M., Bellotti E., Benato G., Bettini A., Brugnera R., Cattadori C., Garfagnini A., Hemmer S., Iannucci L., Junker M., Laubenstein M., Nisi S., Pandola L., Pullia A., Riboldi S., Stanco L., Ur C., Zavarise P., Zocca F. GERDA - gruppo INFN L Aquila, Congresso SIF, 30/09/2011 OUTLINE: GERDA Motivations GERDA Status GERDA Future plans 1
The importance of neutrinoless double beta decay Oscillations = Massive neutrinos = ν α = 3 i=1 U αi v i ν α U v i = flavor eigenstate ν e, ν µ, ν τ = 3x3 PMNS (Pontecorvo Maki Nakagawa Sakata) mixing matrix = mass eigenstate i Neutrinos are massive = new open problems Neutrino nature (Majorana or Dirac particle) Absolute mass scale Some answers may come from the observation of Neutrinoless Double Beta (0νββ) decay Schechter Valle theorem: 0νββ = Majorana ν 2
Double beta decay - Diagrammatica Double beta decay = single beta decay forbidden or strongly suppressed 2νββ (by the exchange of light neutrinos) (implicit from now on) 0νββ (by the exchange of light neutrinos) (implicit from now on) (Z, A) (Z + 2, A) + 2e + 2ν e L = 0 = Predicted by s.m. and observed (Z, A) (Z + 2, A) + 2e L = 2 = Physics beyond s.m. Observed? Q = E e1 + E e2 2m e (T1/2 0ν ) 1 = G 0ν (Q, Z) M 0ν 2 mββ 2 m ββ 3 i=1 U2 ei m i (effective mass of electron neutrino) (information on the absolute mass scale) 3
Experimental signature for DBD Measured quantity: sum of the electrons kinetic energies 2νββ: not a constant (a part of the released energy is carried away by neutrinos) 0νββ: constant! Sum of the electrons kinetic energies Sensitivity (with background): T1/2 0ν ln 2 N (nσcl) A Mt n A αɛ B E (α isotopic abundance, ɛ detector efficiency, M detector mass, E energy resolution, B background index, t measuring time, A isotope molar mass) 4
76 Ge experiments HPGe diodes technology (Semiconductor radiation detectors = ionization) 76 Ge natural abundance: 7% = enrichment is required Advantages Ge as source and detector Industrial techniques and facilities available to enrich the material High purity Excellent energy resolution Disadvantages Low Q ββ value (2039 kev) (lower than Tl-208 2614 kev = background) Enrichment is expensive 5
The Heidelberg-Moscow claim HPGe detectors enriched at 86% in 76 Ge Exposure: 71.7 kg y Background: 0.11 counts/(kg y kev) (without pulse shape) Claim! (PL B586 (04) 198) (4.2σ confidence level) T 0ν 1/2 = 1.2(0.7 4.2) 1025 y = m ee = 0.44(0.24 0.58)eV (3σ range) GERDA first goal: Check claim from Klapdor et al., using the already existing enr Ge detectors operated by Heidelberg-Moscow (5 detectors) and by IGEX (3 detectors) 6
GERDA Roadmap Phase I ( 16kg, 1y): Detectors from HDM & IGEX B 0.01cts/(keV kg y) Start 2011 Phase II (+20kg = 36kg, 3y): New GE detectors (BEGe) B 0.001cts/(keV kg y) Start 2013 Phase III: Worldwide collaboration? 7
How to obtain a low background? Underground experiment The GERmanium Detector Array experiment is hosted in the Hall A of the Gran Sasso Laboratory (INFN) Suppression of µ-flux> 106 8
How to obtain a low background? The GERDA Setup Main feature: Naked detectors in LAr Water tank info = 10m H = 9.5m V = 650m3 The water tank acts as an active Cherenkov veto Cryostat info = 2m H = 5.88m LAr volume 63m3 9
Inside the cryostat Detector string Minishroud Detectors are organized in strings. Low mass holders Phase I: 4 detectors x 3 strings 8 enriched + 4 natural There s a fixed Cu-shield, the shroud ( = 76cm), to protect the detectors from Radon convection A string of detector can be surrounded by a Cu-shield, the minishroud ( = 11.5cm) It s possible to set indipendently the voltages of shroud and minishrouds, to change the electric field configuration in the Cryostat 10
How to obtain low background? Filtering events How to understand the nature (signal or background?) of events around Q ββ? First step: we can filter events flagged by a signal from muon-veto from liquid Argon instrumentation (Phase II) Second step: discriminate Single & MultiSite Events SSE: ββ, DEP - MSE: Compton Second step approaches: Anti-coincidence of detectors Pulse shape analysis 11
GERDA Software Setup GELATIO: a general framework for modular analysis of high-purity Ge detector signals JINST 6 (2011) P08013 Written in C++ / ROOT Managing different data sources in a common way Modular design Signal processing features Fully integrated with a database The graphical user interface Event viewer Event analyzer 12
Calibrations Energy calibration is performed by using 7 prominent lines, in the window 511keV-2614keV Energy resolution of GTF detectors is 3.6keV - 6keV (FWHM at 2.6MeV) 13
First GERDA Run (June 2010): surprise! Unexpected strong signal at 1524.7 kev! 14
Radioactivity in argon We are seeing the effects of two radioactive Argon isotopes: 39 Ar and 42 Ar 39 Ar 39 K β decay, Q = 565keV, T 1/2 = 269y Expected, clearly visible, and not a background for GERDA! (Q < Q ββ ) 42 Ar 42 K 42 Ca The 1524.7keV line is not itself a concern, but the β emitted in the decay of 42 K is a possible background! I1524 observed >> I expected 1524 (upper limit 43µBq/kg at 90% CL in literature) 42 Ar Production: Nuclear explosions: 40 Ar(n, γ)(n, γ) 42 Ar Cosmic rays: 40 Ar(α, 2p) 42 Ar better model? 15
Play with the electric fields Effect of mini-shroud Effect of voltage configuration The initial decay 42 Ar 42 K produces the daughter in a charged state, which can drift close to the detectors under the action of electric fields Background source only if 42 K comes very close to the detectors. Minishrouds limit the drift The problem can be strongly mitigated by canceling the electric fields in the surrounding of the detectors or by applying counter-fields to repel 42 K ions Should not be an issue for the Phase I background goal, but potentially more relevant for Phase II 16
Muon induced events Spectrum of muon-induced events in Germanium Exposure=2.09 kg y, 3 nat Ge detectors Black=data, Blue=Monte Carlo... excellent agreement! Estimate of muon veto efficiency for events which cause a signal in Ge: ε = 98.7% Estimate of muon-induced background at Q ββ : B µ < 2.0 10 4 cts/(kg kev y) 95% CL 17
Background from other isotopes HDM vs GERDA. HDM exposure: 71.7 kg y. GERDA exposure: 1.6 kg y Background measured with a nat Ge string at Q ββ ± 200keV range: 0.06 ± 0.02cts/(kg kev y) Most likely from a combination of Compton continuum from Th/U, residual 42 K, degraded α, cosmogenic isotopes 18
Enriched detectors (from June 2011): low energy spectrum Data obtained with string of 3 enriched 76 Ge detectors 39 Ar 1.01Bq/kg from WARP (NIM A574, 2007) 2νββ T1/2 2ν = 1.74 1021 y from HDM (NIM A522, 2004) 42 K Normalized to peak assuming homogeneous distribution Very good description of the data! 19
Pulse shape discrimination - BEGe detectors Phase II detectors have been chosen to ensure a superior PSD. We tested the point-contact BEGe from Canberra, and found them OK for Phase II - JINST 4 P10007 PSD based on A/E ratio 20
Liquid argon instrumentation We are testing LAr instrumentation for Phase II in a parallel facility Combining the superior PSD of BEGe with the LAr veto, we measured a suppression factor at Q ββ 0.5 10 4 around Q ββ for a 228 Th calibration source. 21
The collaboration 22
Conclusions GERDA started its commissioning phase with nat Ge in June 2010 Encountered 42 Ar - 42 K unexpected intensity. 42 K is produced charged and we learned how to move it in the LAr by appling Efield Background from Th and U is much lower than in previous experiments (HDM & IGEX): GERDA concept validated In June 2011 deployed first string of enr Ge in the best electrostatic and shielding configuration found so far 2νββ spectrum well visibile. Spectrum well reproduced by the sum of the 39 Ar, 2νββ 76 Ge, 42 Ar contributions By end 2011 all the 8 enr Ge Phase I detectors will be deployed Phase II in advanced state of preparation. Features: BEGe detectors (enhanced PSD), Veto by LAr scintillation light... 23