M. Wójcik Instytut Fizyki, Uniwersytet Jagielloński Instytut Fizyki Doświadczalnej, Uniwersytet...
-
Upload
charity-joseph -
Category
Documents
-
view
223 -
download
0
Transcript of M. Wójcik Instytut Fizyki, Uniwersytet Jagielloński Instytut Fizyki Doświadczalnej, Uniwersytet...
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