The Germanium Detector Array for the search of neutrinoless

decays of 76Ge at LNGS(GERDA) LNGS, ITALY

Stefan Schoenert, MPIK HeidelbergInternational School of Nuclear Physics

27th courseNeutrinos in Cosmology, in Astro-, Particle and Nuclear Physics

INFN LNGS, Assergi, ItalyA.Di Vacri, M. Junker, M. Laubenstein, C. Tomei, L. Pandola

JINR Dubna, RussiaV. Brudanin, V. Egorov, K. Gusev, S. Katulina, A. Klimenko, O. Kochetov, I. Nemchenok, V. Sandukovsky, A. Smolnikov, J. Yurkowski, S. Vasiliev,

MPIK, Heidelberg, GermanyC. Bauer, O. Chkvorets, W. Hampel, G. Heusser, W. Hofmann, J. Kiko, K.T. Knöpfle, P. Peiffer, S. Schönert, J. Schreiner, B. Schwingenheuer, H. Simgen, G. Zuzel

Univ. Köln, GermanyJ. Eberth, D. Weisshaar

Jagiellonian University, Krakow, PolandM.Wojcik

Univ. di Milano Bicocca e INFN, Milano, ItalyE. Bellotti, C. Cattadori

IMMR, Geel, BelgiumM. Hult

INR, Moscow, RussiaI. Barabanov, L. Bezrukov, A. Gangapshev, V. Gurentsov, V. Kusminov, E. Yanovich

ITEP Physics, Moscow, RussiaS. Belogurov, V.P. Bolotsky, E. Demidova, I.V. Kirpichnikov, A.A. Vasenko, V.N. Kornoukhov

Kurchatov Institute, Moscow, RussiaA.M. Bakalyarov, S.T. Belyaev, M.V. Chirchenko, G.Y. Grigoriev, L.V. Inzhechik, V.I. Lebedev, A.V. Tikhomirov, S.V. Zhukov

MPP, München, GermanyI. Abt, M. Altmann, C. Büttner. A. Caldwell, K. Kroeninger, X. Liu, B. Majorovits, H.-G. Moser, R.H. Richter

Univ. di Padova e INFN, Padova, ItalyA. Bettini, E. Farnea, C. Rossi Alvarez, C.A. Ur

Univ. Tübingen, GermanyM. Bauer, H. Clement, J. Jochum, S. Scholl, K. Rottler

GERDA Collaboration

Physics goals of GERDA

0: (A,Z) (A,Z+2) + 2e-

d

d

u

u

e-

e-

W-

W- e

e

L=2

Primary Objective:

Effective mass:

Method: Operation of HP Ge-diodes enriched in 76Ge in (optional active) cryogenic fluid shield. Line search at Qββ = 2039 keV

mee = |i Uei ² mi |

(decay generated by (V-A) cc-interaction via exchange of light Majorana neutrinos)

Other Physics: WIMP DM search

Majorana nature

= G(Q,Z) |Mnucl|2 mee2,

GERDA @ Gran Sasso:experimental concept

• HP Ge-diodes (86%76Ge): point-like energy deposition at Q = 2039 keV

• Operation of bare Ge diodes in high-purity LN2 or LAr shield (Heusser, Ann, Rev. Nucl. Part. Sci. 45 (1995) 543); proposals based on this idea: GENIUS (H.V. Klapdor-Kleingrothaus et. al., hep-ph/9910205 (1999)); GEM (Y.G. Zdesenko et al., J. Phys. G27 (2001))

• Baseline: LN2; possible upgrade LAr: =1.4 g/cm3, active anti-coincidence with scintillation light from LAr

• Reduction of backgrounds key to sensitivity : – Half-life limit

• w/o backgrounds: t1/2 (MT)• with backgrounds: t1/2 (MT)1/2

Goal: “background free within planned exposure”!

Phases and physics reach of GERDA

M·T

(M·T)1/2

10-3 / (keV·kg·y)

10-1 / (keV·kg·y)

Phase-IHdM & IGEX

~20 kg

3·1025 (90 % CL)

Phase-IIHdM & IGEX +new diodes

~40 kg

2·1026 (90 % CL)<mee> < 0.09 – 0.29 eV

KK

2008 2010

Phases and Physics reach of GERDA world-wide collaboration for Phase-III; coop. with MAJORANA started

Phase-II

Phase-III

Phase-I

10-1/(keV·kg·y)

10-2/(keV·kg·y)

10-3/(keV·kg·y) 6·1027

Phases and Physics reach of GERDA

| m

ee|

in e

V

Lightest neutrino (m1) in eV

F.F

eruglio, A. S

trumia, F

. Vissani, N

PB

659

Phase I:

Phase II:

Phase III:

…how to reach <10-3/(keV·kg·y)?

Borexino Collab. PLB 563 (2003)

10-3/(keV·kg·y)

Q

BOREXINO Counting Test Facility (CTF)(‘world record’)

Liquid scintillator target

BOREXINO ~10-5/(keV·kg·y)

External background: graded shielding Activity of Tl-208 (Bq/kg)

Rock/concrete 3 ·106

Cu (NOSV/OFO1),Pb <20Water <5LN, Ar ~0

GERDA: Baseline design

Clean roomlock

Vacuum insulated copper vessel

Water tank / buffer/ muon veto

Liquid N/Ar

Ge Array

Under discussion: ‘third wall’

copper cryogenic vessel

Detector support & contacts

Cu: 81 gPTFE: 6.4 gSi: 4.2 g

MC: <2 ·10-3 (keV kg y) -1

Phase I: HdM & IGEX (p-type coaxial)

PrototypeSpring

Phase II:•true-coaxial •3x6 segmented•n-type Ge diode

Support: ~ 40 g

New detectors for Phase II:Procurement of enriched Ge

natGe sample received March 7, 2005

1) procurement of 15 kg of natural Ge (‘test run’)

2) enrichment of 37.5 kg of Ge-76 completed !

Specially designed protective steel container reduces activation by cosmic rays by factor 20

Backgrounds in GERDA

Source B [10-3 cts/(keV kg y)]

Ext. from 208Tl (232Th) <1

Ext. neutrons <0.05

Ext. muons (veto) <0.03

Int. 68Ge (t1/2= 270 d) 12

Int. 60Co (t1/2= 5.27 y) 2.5

222Rn in LN/LAr <0.2

208Tl, 238U in holder <1

Surface contam. <0.6

180 days exposure after enrichment + 180 days underground storage

30 days exposure after crystal growing

phase I : B 10-2 cts/(keV kg y)phase II: B 10-3 cts/(keV kg y) additional bgd. reduction

derived from measurements and MC simulations

Muon veto

Background reduction techniques

• Muon Veto

• Anti-coincidence between detectors

• Segmentation of readout electrodes (Phase II)

• Pulse shape analysis (Phase I+II)

• Coincidence in decay chain (Ge-68)

• Scintillation light detection (LArGe)

Background reduction techniques

• Muon veto

• Anti-coincidence between detectors

• Segmentation of readout electrodes (Phase II)

• Pulse shape analysis (Phase I+II)

• Coincidence in decay chain (Ge-68)

• Scintillation light detection (LArGe)

K. Kroeniger

Background simulations with MaGe

(common Majorana–Gerda Geant4 MC framework)

Description of the Gerda setup including shielding

(water tank, Cu tank, liquid Nitrogen), crystals array and

kapton cables

top -vetowater tank

lead shieldingcryo

vessel

neck

Ge array

MaGe simulation of muon induced backgrounds

Flux at Gran Sasso: 1.1 /m2 h (270 GeV)

Input energy spectru

m

from Lipari and Stanev, Phys.

Rev. D 44 (1991) 3543

Energy (keV)

Input angular spectru

m

cos

Teramo side

MaGe: cosmic ray muons – Ge signalPhase I: 9 Ge crystals for a total mass of 19 kg; threshold: 50 keV

Energy (MeV)

5.5 yearsSum spectrum without and with anticoincidence

Energy (MeV)

(1.5 2.5 MeV): 3.3·10-3 counts/keV kg y

annihilation peak

(~4·10-3 counts/keV kg y in H-M simul.) C. Doerr, NIM A 513 (2003) 596

1.5 MeV 2.5 MeVanticoincidence between 9 crystals reduces background index by factor 3

1.0 · 10-3 cts/keV kg y

MaGe: cosmic ray muons - muon veto

Energy (MeV)Threshold 120 MeV all events cut but two 120MeV in water (~60 cm) 30,000 ph. 40 p.e. (0.5% coverage) 80-90 PMTs

Energy deposition in water Cherenkov detector in coincidence with Ge detectors (Ethr>50 keV)

5.5 years

No cuts 3.3 · 10-3 (cts/keV kg y)

Ge anti-coincidence 1.0 · 10-3

Ge anti-coinc.+ Top -veto (plastic scint.) 4.4 · 10-4

Cerenkov -veto < 3 · 10-5 (95% CL)

2: 1.332 MeV: Emax = 318 keV

1: 1.173 MeV

• T0 : crystal growing• 0.017 Bq/kg per day exposure• Test: detector production in 7.4 days • Assume 30 days 2.5 ·10-3 / (keV·kg·y)

• T0 : crystal growing• 0.017 Bq/kg per day exposure• Test: detector production in 7.4 days • Assume 30 days 2.5 ·10-3 / (keV·kg·y)

Internal backgrounds: example 60Co

2 kg

60Co background spectrum

1 2

1+ 2

Q

MC simulation

2 kg

60Co: suppression by segmentation

QMC simulation

2 kg

60Co: suppression by segmentation

~10 (7 seg.)Nseg = 1

QMC simulation

Nhit = 3

illustration: Simple 7-fold segmentation

MaGe: 60Co suppression by segmentation and anti-coincidence

6-3z segmentation;Prototype under construction

N crystals = 1

N segments = 1

Number of crystals

Number of segments

probability per decay to deposit energy within Q ROI per 1 keV energy bin after combined cuts:(18-fold segm.)

P = 4.7·10-6/keV

(factor ~35 reduction w/rto single unseg. detector)

60Co: suppression by LAr Ge-anticoinc.

~100LAr anticoinc.

Liquid Argon

QMC simulation

2 kg

60Co: segmentation and LAr Ge-anticoinc. are orthogonal suppression methods

~1000

Liquid Argon

Nseg = 1AND

LAr anticoinc

Q

2 kg

MC simulation

Experiment: LAr Ge-anticoincidence

54Mn-source

Liquid Argon

20 cm

0.18 kg

Experiment: LAr Ge-anticoincidence

Liquid Argon

20 cm

Experimental Data

~5

0.18 kg

LAr Ge-anticoincidence

Liquid Argon

20 cm

MC Simulation

Suppression factor limitedby escape events!

~5

LAr Ge-anticoincidence

Liquid Argon

100 cm

MC Simulation

~60

Suppression factor limitedby dead layer0.18 kg

90 cm

Status - Outlook2004

• Feb Letter of Intent to LNGS, hep-ex/0404039• Sep formation of collaboration• Oct funding requests approved by MPG• Oct Proposal to LNGS, www.mpi-hd.mpg.de/GERDA/proposal.pdf

2005• Jan `prior information notice’ about cryostat tendering in SIMAP• Feb GERDA approved by LNGS, location in Hall A in front of LVD• Mar Technical Proposal to LNGS, 30 kg of enriched Ge-76 ordered• May funding requests approved by INFN• Jun funding request approved by BMBF• Jul Ge-76 order enlarged to 37.5 kg• Jul FMECA & HAZOP safety study for cryostat submitted to LNGS(‘safe if designed & fabricated according to the rules’)• Sep safety iteration with LNGS: “ALARP - third wall?”• subsequently: tendering for water tank, first orders for cryostat

2006• Summer start of construction of water tank in Hall A• ... installation of cryostat, clean room & lock, μ veto, lab rooms

2007• commissioning, start of physics run

Summary

• approved by LNGS with location in Hall A,

• substantially funded by BMBF, INFN, MPG, and Russia in kind

• construction to start in LNGS Hall A in summer 2006

• parallel and fast R&D for phase II

• start of data taking in 2007

goal: phase I : background 0.01 cts / (kg٠keV٠y)

► scrutinize KKDC result within 1 year

phase II : background 0.001 cts / (kg٠keV٠y)

► T1/2 > 2٠1026 y , <mee> < 0.09 – 0.29 eV

phase III : world wide collaboration

► T1/2 > ~1028 y , <mee> ~10 meV