GERDA GERmanium Detector Array Search for neutrinoless decays

GERDA. GERmanium Detector Array Search for neutrinoless decays of 76 Ge A. Bettini Università di Padova, Dipartimento di Fisica G. Galilei; INFN - Sezione di Padova You are here 20/02/2021 A. Bettini. INFN 1

The GERDA Collaboration 20/02/2021 A. Bettini. INFN 2

Double beta decays For even-even nuclei energy conservation forbids beta decay 2 n 2 decay may happen 2 nd order WI process very long lifetimes 0 n 2 decay happens if Lepton number violated Neutrinos are Majorana particles Neutrinos are massive (they are) 76 Ge 20/02/2021 is a candidate A. Bettini. INFN Experimentally Ge diodes Source = detector Measure sum energy with calorimetric techniques 3

Decay Rates and Matrix Elements 1/t = G(Q, Z) |Mnucl Mee |2 Large uncertainty in nuclear matrix elements Mnucl Phase space factor G(Q, Z) Q 5 2 Cancellations are possible due to the phase factors We measure t not Mee Feruglio, Strumia, Vissani te a r Experimental sensitivity with background index b, sensitive mass M, live time T and energy resolution ∆E inverse normal de cosmology e n ge If b 0 in a few DE @ Mee Define 1 y. DRU 1 c/(kg ke. V yr) 20/02/2021 A. Bettini. INFN 4

Why 76 Ge + “continuous” fluctuating background + lines background • Source=detector • Highest energy resolution: 3 ke. V FWHM @ 2039 ke. V • Extreme (radio)purity of Ge crystals • Mature HP-Ge technology • Industrial techniques for isotopic enrichment (8% 88%) • Single Site (SSE: signal) vs. Multiple Site (MSE: background) discrimination with available techniques (AGATA, . . ) • KK evidence must be tested on Ge E. Fiorini et al. Phys. Lett 25 B (1967) 602 20/02/2021 A. Bettini. INFN 5

Background Discrimination with Ge Distinguish SSE from MSE Pulse Shape Analysis Close packing Segmentation E SS E MS 20/02/2021 A. Bettini. INFN 6

The Heidelberg Moscow Evidence H. V. Klapdor-Kleingrothaus, et al. 214 Bi [Phys. Lett. B 586: (2004)198] MT = 10. 9 kg (86% 76 Ge) x 13 yr Exposure (analysis): 50. 57 kg y b = 0. 11 y. DRU before PSA Resolution on 8 years DE = 3. 27 ke. V 0 214 Bi Claim: 4. 2 signal Expected @ Q = 2039. 006± 0. 05 ke. V T 1/2= 1. 2 1025 yr (± 3 s 0. 69 -4. 18 yr) Mee ≈ 440 me. V (100 -900) KK Matr. El. Mee≈700 me. V Rodin et al. Matr. El. 2 2 Pulse Shape Analysis to select SSE 20/02/2021 A. Bettini. INFN 7

A closer look 214 Bi 0 214 Bi “ 0 2 ” line moves to lower energy (? ? ) 20/02/2021 A. Bettini. INFN 8

GERDA HP water in 10 m diameter SS vessel Neutron moderator Cherenkov medium for µ veto HP LAr in 4. 2 m diameter SS cryostat 10 -15 cm thick Cu shield Enriched Ge diodes Naked crystals in ultra-pure liquid proposed by G. Heusser, Ann. Rev. NPS 45 (1995) 54 20/02/2021 A. Bettini. INFN 9

GERDA Locations at LNGS Underground detector laboratory (LAr. Ge-facility) 20/02/2021 A. Bettini. INFN 10

Background Evaluation and Materials Screening Detector Fabrication Zone ref. & Crystal Pulling • Analytical calculations • Example: Cosmogenic activation of 68 Ge during detector production Purification Detailed evaluation of background sources by • Monte Carlo simulations: MAGE = frame (GEANT 4) and database developed in cooperation with Majorana • Screening of the materials and components in different sites, depending on the issue • g spectroscopy @LNGS, Baksan, MPIK, GEEL • ICP-MS (Inductively Coupled Plasma Mass Spectroscopy) @ LNGS and Frankfurt U. 20/02/2021 A. Bettini. INFN 11

Background Suppression & Discrimination Source g’s from external environment 208 Tl(232 Th). 214 Bi(226 Ra), … Actions • Shield with hyperpure liquids (H 2 O +L Ar) 228 Th • HP Cu shield (25 µBq/kg; 10 -15 cm thick)+LAr (<10 m. Bq/kg) in Cryostat (SS) µ induced prompt signals µ induced delayed signals n+76 Ge 77 m. Ge 77 As (t 1/2= 53 s) Internal to crystals Cosmogenic 60 Co (t 1/2= 5. 27 y) Best estimate 3. 5 10– 5 y. DRU (detector production) Internal to crystals Cosmogenic 68 Ge (t 1/2= 270 d) Best estim. (2008) <0. 5 10– 3 y. DRU (crystal and detector productions) Front-end electronics 20/02/2021 • 1400 m rock overburden • Anticoincidence between crystals&segments • µ-vetos: top (plastic scint. ) +Water Cherenkov • Low-Z shields • Tag g’s from 77 m. Ge decay • Minimize time above ground after crystal growing • Diode & segments antic. , PSA • Minimize time above ground after enrichment; shielded transport container Diode & segments, PSA • ASIC @ LAr temperature (high pass band needed) A. Bettini. INFN 12

Status and perspectives • All existing detectors in hand, tested, refurbishment started • Water tank: order signed; construction to begin soon • Cryostat: baseline Cu solution discarded for increased costs; SS solution evaluated and decided. Tender closed; offers due within September 22 • (new) safety review to be completed in September • Progress in suspension mechanics, readout electronics, DAQ, lock, clean room and infrastructures • 37. 5 kg of Ge already enriched, stored underground in form of Ge. O 2 • Possible crystal growers and detector manufacturers being investigated • Natural Ge 18 -fold segmented detector prototype in hands since three months. Tests and measurements showing very good performance • Lo. I signed with Majorana Collaboration for next phase 1 t scale experiment • 2007. Installation and commissioning 20/02/2021 A. Bettini. INFN 13

GERDA Sensitivity “Discovery”= Bayesian probability to observe only background < 10– 4 PHASE 1 use existing HM+IGEX detectors b=10– 2 y. DRU Exposure= 15 kg yr Check KK evidence Deduced with Rodin et al ME Measured 10 [NP A 766 (2006) 107] RU D y DRU y 10 – 3 10– 2 y. DR U KK claim PHASE 2 additional ca 20 kg of enr. Ge segmented detectors b=10– 3 y. DRU Exposure= 100 kg yr Sensitivity 200 me. V 2 orders of magnitude better than now 20/02/2021 KK claim A. Bettini. INFN 14

The New Detectors Enrichment of 37. 5 kg 76 Ge completed by Sep. 05 Transportation to Europe for further processing in a specially designed container: reduce activation by factor 20 20/02/2021 A. Bettini. INFN 15

Background Discrimination - Geometry Multi Site Events (MSE) (e. g. Compton bgd. ) vs Single Site Events (SSE) (e. g. 0 ) Phase 2 True coaxial, n-type, segmented Prototype: 6 f-3 z segmentation Single-segment vs. multiple-segment events Qbb Measured with 3 x 6 segmented, truecoaxial n-type prototype for phase 2 20/02/2021 A. Bettini. INFN 16

Background Discrimination - PSA Multi Site Events (MSE) (e. g. Compton bgd. ) vs Single Site Events (SSE) (e. g. 0 ) Multiple-site event Single-site event Original signal First difference + smoothing Second difference + smoothing 20/02/2021 A. Bettini. INFN 17

Background Discrimination - LAr scintillation Discrimination of Multi Site Events (MSE) (e. g. Compton bgd. ) from Single Site Events (SSE) (e. g. 0 ) by: liquid argon scintillation anti-coincidence (LAr. Ge) 232 Th (208 Tl) Liquid Argon Q GERDA data Q (40, 000 scintillation photons/Me. V) • 20 cm diameter test setup • Suppression factor (~20) limited by escape from setup 20/02/2021 source w/o anticonicidence with LAr anticonicidence A. Bettini. INFN 18

Conclusions GERDA will confirm or rule out the KK et al. evidence Phase 2, if background level 10– 3 y. DRU is reached, will have sensitivity of about 200 me. V WE STILL NEED SOME TIME Adapted from Elliot & Vogel 20 me. V sensitivity requires ca 1 t 76 Ge and b=10– 4 y. DRU Will need a global scale effort E. Majorana 1938: Forse non è “ancora possibile chiedere all’esperienza una decisione fra questa nuova teoria e quella consistente nella semplice estensione delle equazioni di Dirac alle particelle neutre” “it is (probably) not yet possible to ask to the experiment a decision between this new theory and the one consisting in a simple extension of the Dirac equations to neutral particles” 20/02/2021 A. Bettini. INFN 19

Additional Pages 20/02/2021 A. Bettini. INFN 20

Matrix Elements from Faessler Rodin, Faessler, Simkovic, Vogel; Nucl. Phys. A 766 (2006) 107 20/02/2021 A. Bettini. INFN 21

Existing Detectors. GERDA phase 1 Detectors from Heidelberg Moscow (5) and IGEX (3) experiments Total mass approx 18 kg As will be assembled in GERDA 20/02/2021 A. Bettini. INFN 22