Transcript Prezentacja programu PowerPoint

M. Wójcik for the GERDA Collaboration
Institute of Physics, Jagellonian University
Epiphany 2006, Kraków, Poland, 6-7 January 2006
74 physicists
13 institutions
5 countries
Location of the GERDA Experiment
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: me < 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 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 normalhierarchy – 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
mmin>> (m212)1/2 as well as mmin>>(m322)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 keV
Nbck  0.5 cts for 15 kg y
– Klapdor-K.: 28.86.9 events in 71.7 kg y
expect 6.01.4 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 II
15 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 anticincidence between
segments and puls shape discrimination
GERDA
Graded shielding of external  backgr.
Shielding layer
• ~ 3 m purified water (700 m3)
• ~ 4 cm copper kriostat + 3rd wall
• ~ 2 m LN2/LAr (50 m3)
Tl concentration
208Tl
< 1 mBq/kg
208Tl < 10 mBq/kg
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

T1/2
271 d
Decay
EC
Radiation X – 10,3 keV
68Ga
68 min
+(90%) EC(10%)
 – 2,9 MeV

68Zn
stable
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
Internal backgrounds
• Cosmogenic
60Co
production in natural Ge at sea level :
6.5 60Co/(kg d) Baudis PhD
4.7 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 background reduction
Photon – 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!