search for neutrino-less double beta decay of 76 ge in the gerda experiment

M. Wojcik, Jagellonian University: GERDA status report

Search for neutrino-less double beta decay of 76Ge in the GERDA experiment

M. WojcikInstytute of Physics, Jagiellonian University


Outline Introduction to the double beta decay

and neutrino mass problem Goals of GERDA GERDA design

Cryostat Water tank and the muon veto Cleanroom and the lock system

Status of selected subprojects Phase I detectors Phase II detectors Front-end electronics

Background in GERDA Internal External Argon purity Liquid Argon scintillation

Summary


Double beta decay


Absolute neutrino mass scale

3H beta-decay, electron energy measurement Mainz/Troisk Experiment: me < 2.2 eV KATRIN

Cosmology, Large Scale Structure WMAP & SDSS: cosmological bounds m < 0.8 eV

Neutrinoless double beta decay evidence/claims?? Majorana mass: <mee> 0.4 eV


Neutrino mass hierarchy

< mee> (100 – 500) meV – claim of an observation of 0 in 76Gesuggests quasi-degenerate spectrum of neutrino masses

< mee> (20 – 55) meV – calculated using atmospheric neutrino oscillation parameters

suggests inverted neutrino mass hierarchy or the normal-hierarchy – very near QD region

< mee> (2 – 5) meV – calculated using solar neutrino oscillation parameterswould suggest normal neutrino mass hierarchy


Neutrino mass hierarchy

quasi-degenerate (QD) mass spectrum

mmin>> (m212)1/2 as well as mmin>>(m32

2)1/2


From T1/2 to the <mee>

QRPA, RQRPA: V.Rodin, A. Faessler, F. Š., P. Vogel, Nucl. Phys. A 793, 213 (2007)Shell model: E. Caurieratal. Rev. Mod. Phys. 77, 427 (2005).


Open questions

What is the nature of neutrino? Dirac or Majorana?

Which mass hierarchy is realized in nature? What is the absolute mass-scale for

neutrinos?

A neutrinoless double beta decay experiment, like GERDA has the potential to answer all three questions


Goals of GERDA

Investigation of neutrino-less double beta decay of 76Ge

Significant reduction of background around Q down to 10-3 cts/(kgkeVy) Use of bare diodes in cryogenic liquid (LAr) of very high

radiopurity Use of segmented detectors Passive/active background suppresion

If KKDC-evidence not confirmed: O(1 ton) experiment in worldwide collaboration (cooperation

with Majorana)


Collaboration

~70 physicists13 institutions 6 countries


Why 76Ge?

Enrichment of 76Ge possible Germanium semiconductor diodes

source = detector excellent energy resolution ultrapure material (monocrystal)

Long experience in low-level Germanium spectrometry


Phases of GERDA

Phase I: Use of existing 76Ge-diodes from Heidelberg-Moscow and

IGEX-experiments 17.9 kg enriched diodes ~15 kg 76Ge Background-free probe of KKDC evidence

Phase II: Adding new segmented diodes (total: ~40 kg 76Ge) Demonstration of bkg-level <10-3 count/(kg·keV·y)

Phase III: If KKDC-evidence not confirmed: O(1 ton) experiment in

worldwide collaboration


Sensitivity

Assumed energy resolution:

E = 4 keV

Background reduction is critical!

phase II

phase I

KKDC claim


GERDA design

Clean room Lock

Cryostat (70 m3 LAr)

Water tank (650 m3 H2O)


Cryostat

Vacuum-insulated double wall stainless steel cryostat

Copper shield


Cryostat

• The cryostat has been constructed• Pressure test for inner and outer vessel have been performed• Evaporation test at SIMIC sucessfully done• First 222Rn emanation test done (~ 30 mBq)• Vessel is ready for transportation to GS• Next steps at GS: - 222Rn emanation measurement - Evaporation test - Copper mounting - 222Rn emanation measurement


Water tank and muon veto Passive shield Filled with ultra-pure water 66 PMTs: Cherenkov detector Plastic scintillator on top Bottom plate has been installed


Cleanroom and the lock system


Phase I diodes

IGEX and HdM diodes were removed from their cryostats

Dimensions were measured Construction of dedicated low-

mass holder for each diode Reprocessing of all diodes at

manufacturer (so far two has been processed)

Storage underground during reprocessing (HADES)

17.9 kg enriched and 15 kg non-enriched crystals (GENIUS-TF) are available


Phase I prototypes

Establishing handling procedures Optimization of thermal cyclings (> 40 cycles

performed) Design and tests of the low-mass detector holder Long-term stability tests Study of leakage current (LC)

Detector handling procedure Irradiation with -sources and LEDs Problems with increasing LC seems to be under

control

The same performance in LAr and LN2 observed


Phase II detectors 37.5 kg enrGe produced

~87% 76Ge enrichment in form of GeO2

Chemical purity: 99.95 % (not yet sufficient) Underground storage Investigation of different options for crystal

pulling – IKZ Berlin most probable 18-fold n-type diodes preferred:

Segmentation easier Thin outside dead layer

little loss of active mass


Phase II detectors: testsSuppression of events from external 60Co and 228Th source

(10 cm distance)


Front-end electronics

Requirements: Low noise, low radioactivity, low power consumption,

operational at 87 K Candidates:

F-CSA104 (fully integrated, under tests) PZ-0/PZ-1 (not yet ready for tests) IPA4 (under tests) CSA-77 (partly cold, under tests)

Tests in different configurations, with different cables etc.


Background in GERDA

• External background- from U, Th decay chain, especially 2.615 MeV from 208Tl in concrete, rock, steel...

- neutrons from (,n) reaction and fission in concrete, rock and from induced reactions

external background will be reduced by passive and active shield

• Internal background- cosmogenic isotopes produced in spallation reactions at the surface, 68Ge and 60Co with half lifetimes ~year(s)

- surface and bulk Ge contamination internal background will be reduced by anticoincidence between

segments and puls shape discrimination


Internal background reduction

Cosmogenic 68Ge product. in 76Ge at surface: ~1 68Ge/(kg·d)

(Avinione et al., Nucl. Phys B (Proc. Suppl) 28A (1992) 280)

68Ge 68Ga 68Zn T1/2: 271 d 68 min stable

Decay EC +(90%) EC(10%) Radiation X – 10,3 keV – 2,9 MeV

After 6 months exposure at surface and 6 months storage underground

58 decays/(kg·y) in 1st year Bck. index = 0.012 cts/(keV·kg·y) = 12 x goal!


Internal background reduction• Cosmogenic 60Co production in natural Ge at sea level:

6.5 60Co/(kg·d) Baudis PhD4.7 60Co/(kg·d) Avinione et al.,

60Co 60Ni

T1/2: 5.27 y

Decay: -

Radiation: (Emax = 2824 keV), (1172 keV, 1332 keV)

After 30 days of exposure at sea level 15 decays/(kg·y) Bck. index = 0.0025 cts/(keV·kg·y) = 2.5 x goal!

As short as possible exposure to cosmic rays!!!


Internal background reductionPhoton – Electron discrimination

• Signal: local energy deposition – single site event• Gamma background: compton scattering – multi site

event

Anti-coincidence between segments suppr. factor ~10

Puls shape analysis suppr. factor ~2


External background reduction

Shielding layer Tl concentration

• ~ 3 m purified water (650 m3) 208Tl < 1 mBq/kg• ~ 4 cm SS cryostat + 2 walls 208Tl < 10 mBq/kg• ~ 3 cm Copper shield 208Tl ~ 10 µBq/kg• ~ 2 m LAr (70 m3) Tl ~ 0

Shielding and cooling with LAr is the best solution ‘reduce all impure material close to detectors as much

as possible’

external / n / background < 0.001 cts/(keV·kg·y) for LAr will be reached

Graded shielding


External background reduction

Segmentation(phase II): 3·10-4

Energy (keV)

Counts

/kg

/keV

/y

goal

no cut: 10-2

No -veto!

Anticoincidence(phase I): 10-

3

75% effective muon-veto is sufficient to achieve 10-4 counts/kg/keV/y

Muon-induced background: Prompt background


External background reductionMuon-induced background: Delayed background

77Ge produced from 76Ge by n-capture. Reduction by delayed coincidence cut (muon, -rays,

β-decay).

Bcg Index for LAr [cts/(kg·keV·y)]77,77mGe 1.1 · 10-4

Others 5 · 10-5


Liquid Argon purification

Same principle as N2 purification Initial 222Rn conc. in Ar higher than in N2

In gas phase achieved:

Even sufficient for GERDA phase III Purification works also in liquid phase

(efficiency lower more activated carbon needed)

222Rn in Ar: <1 atom/4m3 (STP)


Liquid Argon scintillation

LArGe (~1 m3 LAr) in GDL

MC example: Background suppression for contaminations located in detector support


Summary

Main goal: background reduction by a factor of 102 – 103 comparing to previous Ge experiments

The cryostat has been contructed, transportation to Gran Sasso very soon

Construction of the water tank has started (bottom plate is ready) Reprocessing of existing diodes is ongoing at Canberra Handling of bare diodes under control (LC problem) 76Ge for Phase II detectors is available, crystal pulling is under

discussion Various background reduction techniques are under

investigations Start of data taking: 2009

search for neutrino-less double beta decay of 76 ge in the gerda experiment

Documents

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gerda status reportfrom

normal neutrino mass

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