Status and Perspective of the GERDA Neutrinoless Double...

Karl Tasso Knöpfle

MPI Kernphysik, Heidelberg

on behalf of the GERDA collaboration

http://www.mpi-hd.mpg.de/GERDA

The 19 th Particles and Nuclei International Conference (PANIC11)

Massachusetts Institute of Technology, Cambridge, July 24th – 29th, 2011

T2ν = 1.74·1021 y ½

T0ν = ? ½

Status and Perspective of the GERDA

Neutrinoless Double Beta Decay Experiment

PANIC11 28jul 2011 GERDA – status & perspectives K.T.Knöpfle

Zürich, June 2011

About 100 members, 19 institutions, 7 countries


introduction

Introduction

Construction and Status

Results from Commissioning Runs

Perspectives and Conclusion


Introduction motivation for Ge-76 experiment

Discovery of neutrinoless double beta decay would imply:

● Neutrino is its own anti-particle, has Majorana mass

● Access to absolute neutrino mass scale

● Lepton number violation ΔL = 2

● Further new physics beyond the standard model

sum of kinetic energies

exp. signature

observed searched

for

Ge-76 : Qββ=2039 keV

2νßß

0νßß

measured deduced

phase space

nuclear matrix element

So far, best limits for neutrinoless double beta decay from

Ge-76 experiments, IGEX and Heidelberg-Moscow (HdM),

T1/2 > 1.9∙1025 y at 90% confidence limit,

as well as claim for evidence by part of HdM collaboration

KKDC, PL B586 (04) 198 ( 71.7 kg∙y, BI ~ 0.11 cts/(keV∙kg∙y)

Reach background index (BI) at Qββ = 2039 keV of 0.01 / 0.001 cts / (keV۰kg۰y)

KKDC

BI=0.01 phase I :

use Ge-76 diodes of HD-Moscow & IGEX

~18 kg

BI ~ 0.01 cts / (keV۰kg۰y)

intrinsic background expected


Introduction GERDA’s goals & sensistivity

Reach background index (BI) at Qββ = 2039 keV of 0.01 / 0.001 cts / (keV۰kg۰y)

KKDC

BI=0.01

BI=0.001

phase II :

add new enriched Ge-76 detectors, 20 kg


► 37.5 kg enriched Ge-76 bought

35 kg ∙ 3 y exposure

phase I :

use Ge-76 diodes of HD-Moscow & IGEX

~18 kg


intrinsic background expected

phase III: depending on results worldwide collaboration for real big experiment

close contacts & MoU with MAJORANA collaboration

~ 0.12eV


Introduction GERDA’s goals & sensistivity

34

00

m w

.e.

GERDA in

Hall A of

LNGS

LNGS: Laboratori Nazionali del Gran Sasso

Gran Sasso

GERDA strategy: underground site to suppress cosmics

improved shield against external radiation

discrimination between single- (0νββ) & multi-site events

f

14.8m

clean room with lock

control rooms

water plant &

radon monitor

water tank, Ø10m, part of muon-veto detector

cryostat, Ø4m,

with internal

Cu shield

muon & cryogenic

infrastructure


LAr

wate

r

wa

ter


Construction

Introduction





Construction delivery of cryostat

6 mar 2008


Construction water tank

5 may 2008

water tank roof

Construction clean room

29 feb 2009



Construction L’Aquila M=6.3 earthquake & aftershocks

14.8m

0.52g peak ground acceleration

LNGS galleria

0.03g

negligible impact in underground lab

6 April 2009


Construction heat exchanger for active cooling & radon shroud

18 jul 2009


Construction muon veto instrumentation in water tank

aug 2009

1 of 66 PMTs

x


Construction nitrogen flushed clean bench and commissioning locks

18 may 2010

1-string c-lock

3-string c-lock


construction status of infrastruture

Cryogenic system

Commissioning lock

Safety systems

Water plant

Maintaining quality of ultra-pure water (resistivity >17MΩ∙cm & TOC


commissioning

Introduction




Runs started in June 2010

using 3 refurbished Genius-TF natGe-diodes (7.61 kg)

NB ► p-type / coaxial ► ‘low background’ diodes

calibration: spectra with Th-228 radioactive source

preamps

(88K)

2.33kg

2.32kg

2.96kg


commissioning calibration & resolution

run 12 (0.564 kg∙y)

27 days run time

energy / keV


commissioning measured background spectra

GTF45

GTF32

GTF112


commissioning low energy spectral region

‘understood’

Fit compatible with 1Bq(39Ar)/kg

39Ar

run 12 (0.564 kg∙y)

energy / keV


commissioning measured background spectra – discrete lines

GTF45

GTF32

GTF112

Rather weak discrete lines from Th/U background, Th / U / 40K lines suppressed by factors of ~4 / ~4 / ~20 wrt HdM.

1525 keV line much stronger than expected,

Bi-

214

609

Tl-

208

2616

1525

K-4

0

39Ar

39Ar

39Ar

42Ar 18

32.9 y O+

= 600

15

25

GERDA proposal assumed : 42Ar / natAr < 3∙10-21


~18%

~82%

commissioning Ar-42 / K-42 decay schemes


commissioning study of Ar-42/K-42 via 1525 keV line

run 1-3

outer shroud only

+3kV

0V

Count rate of 1525 peak factor ~14 larger than expected.

OS (30μm Cu)

Ø 80 cm

IS



run 1-3

run 4

with inner shroud

IS/OS: OV/-400V

run 1-3

outer shroud only

+3kV

0V

OS (30μm Cu)

Ø 80 cm

Ø 11cm


Additional inner shroud (IS) reduces counts by factor of 4-5.

Still larger than expected! IS shields E- field and convection.

NB: Similar observations at Qββ !

IS



run 5

IS/OS -200V/+500V

run 4

with inner shroud

IS/OS: OV/-400V

+3kV

0V


Biasing IS/OS such that positive ions are attracted by IS:

Count rate increases to about previous value.

Additional inner shroud (IS) reduces counts by factor of 4-5.

Still larger than expected! IS shields E- field and convection.

NB: Similar observations at Qββ !

► Clear evidence that 42K-ions drift in electric field.

Potentially severe consequences for background at Qββ!

Field-free configurations desirable!

OS (30μm Cu)

Ø 80 cm

Ar-42 specific activity deduced from almost field-free runs 10-12

Published limit :

42Ar 18

32.9 y O+

= 600

15

25

high energy spectral part

dependent on dead layer

thickness

15

25

for homogeneous distribution

BI ~0.007 expected – has big

uncertainties.

However, if 42K ions are collected

at detector surface, BI increases.


commissioning K-42 spectrum simulation

no dead layer

0.6mm

1mm

~18%

~82%


commissioning background in Qββ-region

Background rate significantly lower than

in previous experiments (HdM, IGEX) -

but still higher than Phase I goal of 0.01.

Background most probably not only due to 42Ar/42K; could (partially) be due also to distant

Th-232 source. ► Background at Qββ not yet understood - better statistics needed!

runs 10-12 summed

(76 days run time)

Deduced background index

in interval (Qββ ± 200) keV:

0.06 ± 0.01 cts/(keV∙kg∙y)

Tl-

20

8 2

61

5

Bi-

21

4 1

76

5

Qββ ± 200 keV

commissioning spectrum of enriched Ge-76 diodes, and MC 2ν2β & K-42 contributions

absolutely normalized MC prediction for

2ν2β decay spectrum with T1/2=1.74∙1021y,

and 42K contribution normalized to 1525 line.

data

39Ar

Most background between 39Ar endpoint & 1525 keV line accounted for by 2ν2β decays.

(Basic background studies done better with non-enriched diodes:)

2ν2β

2ν2β + 42K

15

25

Data obtained with string of 3 enriched 76Ge diodes



perspectives

Introduction




perspectives measured pulse shapes


time / ns

cu

rre

nt

a.u

.

+3kV

0V

For Phase I detectors reduction of BI

by a factor of up to 2 might be feasible.

Blind analysis indispensible.

• Exploitation of pulse shape analysis

perspectives


8 refurbished and tested diodes (HdM, IGEX) available

● 86% isotopically enriched in Ge-76

● 17.66 kg total mass

● 3-string lock under commissioning

● soon ready for start of Phase I physics run


• Deployment of full array of enriched diodes

perspectives




• Order of phase II detectors

35452.9g of 6N enr Ge metal

BEGe (point-contact) type of diode selected for

Phase II – superb pulse shape discrimination!

DEP: 90%

0-like

-bgd:

11%

D. B

ud

jas e

t a

l., JIN

ST

4 P

10

00

7 (

20

09

)

Full production cycle tested with depleted

material left over from enrichment process.

Enriched material reduced and purified.

Negotiations for Phase II detector order

in progress.

perspectives


R&D for LAr

instrumentation

LArGe

Suppression factor at Qββ of ~5000 measured with Th-228 calibration source.

Alternative to scintillation light readout by PMTs: fibers coupled to SiPMs.

9x8” PMTs

M.Heisel, PhD thesis, Uni HD, 2011



• Order of phase II detectors

• Instrumentation of LAr for scintillation veto

* PR C81(10)028502

• Construction completed.

• All subsystems commissioned and running smoothly. • All phase I detectors (8 pcs ,~18 kg) refurbished & ready.

• Commissioning with natural and enriched Ge diodes in progress.

► 42Ar isotopic abundance found to be about factor of 4

larger than 90% limit reported previously – serious

background for phase II. ► Best background index at Qββ of all Ge experiments so far,

but still factor 6 above phase I goal - not yet fully under-

stood why. LAr instrumentation might be needed already

to reach goal of phase I.

• Start of phase I physics run with enriched detectors soon. • Full production chain tested for phase II BEGe detectors. • Phase II detectors (~20 kg) to be ordered this year.

goals: phase I : background 0.01 cts / (keV۰kg۰y)

► scrutinize KKDC result within ~1 year

phase II : background 0.001 cts / (keV۰kg۰y)

► T1/2 > 1.5۰1026 y , 0.09< < 0.15 eV *


concluding remarks

Backup Slides



Introduction sensitivity for 0νββ decay experiment

72 kg y

background index (BI) [cts/(keV∙kg∙y)]

instrumental spectral width

exposure [kg y] sensitivity*

molecular weight

of source

detection efficiency ( =1 if source=detector)

3.3 keV

0.1

86%

achieved with 76Ge

*RevModPhys 80(08)481

Qββ

?

Bi-214 Bi-214

KKDC: 71.7 kg∙y: T1/2 = 1.2 (0.7-4.2)·1025

= 0.44 (0.24 – 0.58) eV (3σ)

Heidelberg-Moskau Experiment.

BI~0.12

Ø 10 m

Ø 4 m

H 1

0 m

H 2

.5 m

Ø 2.1 m

water: γ & n shield,

Cherenkov medium

for μ veto

LAr

stainless

steel cryostat

w Cu shield,

Rn tight

10 cm electroformed Cu

30 cm PE,

active μ veto,

Rn tight box

*

* LAr can be also active shield!

GERDA: low Z shield, underground

bare Ge diodes in high-purity LAr MAJORANA: high Z, deep underground

Ge diodes housed in vacuum cryostat,

ultra-high-purity electroformed Cu shield

α(LAr) = 0.050/cm

α(H2O) = 0.043/cm

α(Cu) = 0.34/cm

α(Pb) = 0.48/cm

45 cm lead


Introduction generic external background shields

GERDA

storage tanks

LAr for cool down and filling

taken from storage tank. dep

loyed

LA

r / t

on

s


Status cool down and filling of cryostat

date

calibration: deviations from linearity

preamps

(88K)

2.33kg

2.32kg

2.96kg


started in June 2010



commissioning calibration: linearity

muon induced rate ~ 0.01 cts/(keV∙kg∙year)

veto efficiency >94% (system still incomplete)

preamps

(88K)

2.33kg

2.32kg

2.96kg

started in June 2010



commissioning muon veto


Status and Perspective of the GERDA Neutrinoless Double...

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