M. Wójcik

Instytut Fizyki, Uniwersytet Jagielloński

Instytut Fizyki Doświadczalnej, Uniwersytet Warszawski

Warszawa, 10 Marca 2006

74 physicists13 institutions

5 countries

Location of the GERDA Experiment

Double Beta decay

Double Beta Decay

Motivation for GERDA

Open questions:

• What is the absolute mass-scale for neutrinos?• Which mass hierarchy is realized in nature?• What is the nature of neutrino? Dirac or Majorana

• Neutrinoless double beta decay experiment has the potential to answer all three questions

Absolute mass-scale for neutrinos

Especially sensitive ways to measure the neutrino mass

• 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

Tritium Experiments

Neutrino mas hierarchy <mee> value allow to distinguish between NH, IH, QD

• < mee> (100 – 500) meV – claim of an observation of 0 in 76Ge

suggests 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 parameters

would 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

Heidelberg-Moscow Experiment

Isotope enriched Germanium diodes (86% in 76Ge)

IGEX Experiment

Isotope enriched Ge detectors (86 % in 76Ge)

GERDA Phase I

use existing 76Ge (86 %) detectors of HD-M & IGEX

15 kg existing detectors

• Background, assume 0.01 cts/(keV kg y)

• Energy resolution (FWHM), assume = 3.6 keVNbck 0.5 cts for 15 kg y

– Klapdor-K.: 28.86.9 events in 71.7 kg y

expect 6.01.4 cts above Nbck

For 1 events: signal excluded at 98 % CL

Bare Ge crystals for Phase I

- As small as possible holder mass

- Ultra-pure materials

GERDA Phase II15 kg existing detect. + 20 kg new segmented

detect.

• Verify background index 0.001 cts/(keV kg y)• Statistics 3 y x 35 kg 100 kg y• Assume energy resolution = 3.6 keV

• Nbck 0.36 counts

T1/2 > 2 x 1026 y <mee> < 0.09 – 0.29 eV

Segmented Ge detectors for Phase II

- As small as possible holder mass

- Ultra-pure materials

Hexagonally placed detectors

Nuclear Matrix Elements Calculations

Our Goal: background index of 0.001 cts/(keV kg y) gigantic step in background reduction needed

~ 100

• 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

GERDA

Graded shielding of external backgr.

Shielding layer Tl concentration

• ~ 3 m purified water (700 m3) 208Tl < 1 mBq/kg• ~ 4 cm copper kriostat + 3rd wall 208Tl < 10 mBq/kg• ~ 2 m LN2/LAr (50 m3) Tl ~ 0

Shielding and cooling with LN2/LAr is best solution ‘reduce all impure material close to detectors as much

as possible’

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

Factor ~ 10 smaller ext. bck. for LAr

Background reduction

• Underground experiment (mion shield)• Specific background reduction techniques

- mion veto – water Cerenkov detector

- photon-electron discrimination

- scintillation in kryo-liquid as anticoincidence

Internal Backgrounds

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!

As short as possible exposure to cosmic radiation

• 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 cosmics

Internal backgrounds

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

Background of the Ge detector

Part Source Rate [10-3 keV-1kg-1y-1]

Cristal U-238

Th-232

Co-60

Ge-68

Pb-210 (sf)

Th-232 (sf)

0.25

0.05

0.03

1.53

0.13

0.17

Holder all (copper)

all (teflon)

0.14

0.20

Cable all (copper)

all (kapton)

0.02

~1.5

Sum ~4

Mions and Neutrons at LNGS < 10-4 cts keV-1

kg-1 y-1

Summary GERDA

• GERDA approved by LNGS – location in Hall A

• Phase I: use existing detectors, test Klapdor-K. result in 1 year Background level of 0.01 cts/(keV kg y)

Expected start of data taking 2008

• Phase II: add new segmented detectors

factor 10 in T1/2 sensitivity Challenging background level of 0.001 cts/(keV kg y)

Expected sensitivity <mee> ~ 50 meV

Background suppression is the key to success!

Double beta decay

Double beta experiments