M. Wójcik Instytut Fizyki, Uniwersytet Jagielloński Instytut Fizyki Doświadczalnej, Uniwersytet Warszawski Warszawa, 10 Marca 2006
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