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BACKGROUND REJECTION AND SENSITIVITY FOR NEW GENERATION Ge DETECTORS EXPERIMENTS. Héctor Gómez Maluenda University of Zaragoza (SPAIN) hgomez@unizar.es. IDM’10 Montpellier, July 2010. OUTLINE. Motivation. Setup. Geometry & Materials. Simulated Events. Pulse Generation. - PowerPoint PPT Presentation
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BACKGROUND REJECTION AND SENSITIVITY FOR NEW GENERATION Ge
DETECTORS EXPERIMENTS.
Héctor Gómez Maluenda
University of Zaragoza (SPAIN)
hgomez@unizar.es
IDM’10 Montpellier, July 2010.
IDM’10 Montpellier, July 2010.
OUTLINE
Motivation.
Setup.
Geometry & Materials.
Simulated Events.
Pulse Generation.
Pulse Analysis.
Results
Summary & Conclusions.
IDM’10 Montpellier, July 2010.
MOTIVATION
Germanium detectors have been used in several experiments searching for Rare Events:
Detection Efficiency.
Energy Resolution.
Robustness.
…
Some new experiments are based on the operation of new generation Ge detectors:
Dark Matter Edelweiss, CDMS.
Double Beta Decay Gerda, Majorana.
Expected sensitivity of these experiments needs to develop different techniques for background events suppression keeping high detection efficiency levels.
Analysis of the Pulse Shape generated in segmented detectors with 3D resolution, seems to be one of the most powerful tools to identify background events, for further rejection (H. Gómez et al. Astrop. Phys. 28 (2007) 435-447).
IDM’10 Montpellier, July 2010.
SETUP
The goal of this work is try to estimate the background rejection capability of 3D-PSA in a 0 experiment using segmented Ge detectors.
Q ~ 2039 keV (76Ge).
For a <m>~50 meV sensitivity:
b
mt
W
fF
atD 261017.4
ε~75-80 % b ~10-3 c keV-1 kg-1y-1
Simulation of background and signal events.
Pulse generation from these events.
Pulse Shape Analysis (PSA).
IDM’10 Montpellier, July 2010.
GEOMETRY & MATERIALS
Geometry has been defined versatile thinking on the possibility of further changes:
Detector:
Natural Germanium cylinder (D=h).
Mass between 0.1 and 4 kg.
Copper Cryostat:
3 parts (based on IGEX design).
Dimensions dependent on detector size.
Experimental Place:
2 m diameter sphere.
Big enough to increase the setup.
Air inside the sphere.
IDM’10 Montpellier, July 2010.
SIMULATED EVENTS
Apart from signal events, main background contributions in the 2.0-2.1 MeV Region of Interest have been considered:
60Co-68Ge
208Tl
214Bi60Co
0
2
Signal:
0 events (DECAY 0).
Background:
Internal 60Co and 68Ge (GEANT4).
60Co in Cu cryostat (GEANT 4).
External 208Tl and 214Bi (GEANT4).
2 events (DECAY 0).
IDM’10 Montpellier, July 2010.
SIMULATED EVENTS
The background considered represents ~95% of the total background in the RoI.
Several tests have been carried out to validate the generated events.
0
Inte
rnal
60C
o
Ext
ern
al 21
4B
i2
IDM’10 Montpellier, July 2010.
PULSE GENERATION
To have 3D spatial resolution is necessary to study the net signal an the induced ones.
IDM’10 Montpellier, July 2010.
PULSE GENERATION
The pulse is the representation of the charge variation vs time:
/ββ
sat
Wiii
EE
V
rErEμq
dt
dq1
0 1
-50 -40 -30 -20 -10 0 10 20 30 40 50
-50
-40
-30
-20
-10
0
10
20
30
40
50
z (m
m)
r (mm)
-50 -40 -30 -20 -10 0 10 20 30 40 50-50
-40
-30
-20
-10
0
10
20
30
40
50
y (m
m)
x (mm)
V0
Voltage V0 is applied to the outer electrodes of the detector.
IDM’10 Montpellier, July 2010.
PULSE GENERATION
The pulse is the representation of the charge variation vs time:
/ββ
sat
Wiii
EE
V
rErEμq
dt
dq1
0 1
Voltage V0 is applied to the outer electrodes of the detector.
Finite element calculation to obtain E.
IDM’10 Montpellier, July 2010.
PULSE GENERATION
The pulse is the representation of the charge variation vs time:
/ββ
sat
Wiii
EE
V
rErEμq
dt
dq1
0 1
Voltage V0 is applied to the outer electrodes of the detector.
Finite element calculation to obtain E.
Ew (weighting field) is the theoretical existing field when all the electrodes are with V=0 excepting one.
IDM’10 Montpellier, July 2010.
PULSE GENERATION
Net Signal:
2 singular points in the pulse per energy deposit.
Total area proportional to the energy.
Only radial sensitivity.
Time (ns)
IDM’10 Montpellier, July 2010.
PULSE GENERATION
Induced Signal:
No new temporal information.
Null net area.
Absolute area (AA) as representative value.
Signal amplitude and AA lower than net signal.
IDM’10 Montpellier, July 2010.
PULSE ANALYSIS
Net Signal:
A singular point corresponds to a maximum in the pulse derivative.
Analysis is based on maxima identification.
2 maxima Mono Site Event
3 or more Multi Site Event
IDM’10 Montpellier, July 2010.
PULSE ANALYSIS
Net Signal: Characteristic Time
Electronics could distort pulses decreasing the maxima identification capability.
This effect has been taking into account by convoluting pulses with a transfer function.
RC=20 ns
RC=40 ns
RC=20 ns
RC=40 ns
RC
t
eRC
h(t)dt)h(τio(t)
1
IDM’10 Montpellier, July 2010.
PULSE ANALYSIS
Induced Signal:
Net signal provides information about energy and r coordinate of the event.
z and φ coordinates could be defined form induced signals.
For multisite events these coordinates are for Center of Energy point (CoE).
IDM’10 Montpellier, July 2010.
PULSE ANALYSIS
Induced Signal:
Absolute Area (AA) is the most representative feature of induced signals
AA value is independent of Characteristic Time.
Analysis is based on AA comparison with the corresponding CoE event.
MONOSITE
MULTISITE
)()()()(
;CDErCDEl
rl
CDEdCDEu
duz AA
AAP
AA
AAP
Pz & Pφ ≤ 1 Monosite
Pz or Pφ > 1 Multisite
IDM’10 Montpellier, July 2010.
RESULTS
Pulse generation and analysis has been carried out in 2 and 4 kg Ge crystals.
First step: Net Signal Analysis (after anticoincidence between segments).
2 kg
4 kg
40 ns seems to be the best value for RC
RC (ns)
IDM’10 Montpellier, July 2010.
RESULTS
Second Step: Induced signals analysis (only for non rejected events).
/(b)1/2 after induced signal analysis
2 kg; RC = 40 ns 4 kg; RC = 40 ns
IDM’10 Montpellier, July 2010.
RESULTS
Background level for a 2-kg detector (10-3 c/keV/kg/y) with 6x9 segmentation
Background source
ActivityRaw Net Signal (40ns) Induced Signal
Internal 60Co
5 kg-1 d-1; 30d exp; 0d cool.*2.90 0.01 < 0.01
Internal 68Ge
1kg-1 d-1; 180d exp; 180d cool.*12.50 0.26 0.24
External 208Tl
0.1 cm-2 s-1*0.38 0.16 0.16
External 214Bi
0.38 cm-2 s-1*0.17 0.08 0.08
Internal 232Th in lead
1Bq/kg*2.82 1.23 1.21
60Co from Cu criostat
1mBq/kg28.47 0.15 0.14
21.21 10-4 kg-1 y-1
0.09 0.08 0.08
TOTAL 47.33 1.97 1.91
0 detection ε 76.66 76.28
*Values from H. Gómez et al, Astroparticle Physics 28 (2007) 435-447
IDM’10 Montpellier, July 2010.
RESULTS
Background level for a 4-kg detector (10-3 c/keV/kg/y) with 6x11 segmentation
Background source
ActivityRaw Net Signal (40ns) Induced Signal
Internal 60Co
5 kg-1 d-1; 30d exp; 0d cool.*3.73 0.01 < 0.01
Internal 68Ge
1kg-1 d-1; 180d exp; 180d cool.*38.66 0.23 0.20
External 208Tl
0.1 cm-2 s-1*0.30 0.11 0.11
External 214Bi
0.38 cm-2 s-1*0.14 0.07 0.07
Internal 232Th in lead
1Bq/kg*2.25 0.88 0.86
60Co from Cu criostat
1mBq/kg33.67 0.27 0.24
21.21 10-4 kg-1 y-1
0.09 0.08 0.08
TOTAL 78.84 1.65 1.56
0 detection ε 76.74 75.59
*Values from H. Gómez et al, Astroparticle Physics 28 (2007) 435-447
IDM’10 Montpellier, July 2010.
RESULTS
bΓ
MTε
W
fx.; F
FF
mm
atD
ND
eν
2610174
Estimation of the sensitivity from the ε and b values obtained
2 kg
4 kg
IDM’10 Montpellier, July 2010.
RESULTS
Estimation of the sensitivity from the ε and b values obtained
MTmin (kg·y) MTmed (kg·y) <m> (meV) for 1000 kg·y
2 kg
Theoretical PSA (3mm) 268 468 36-61
3D PSA 369 664 39-56
Radial PSA 377 677 39-56
6x9 Segmentation 537 966 43-61
Full Crystal >1000 >1000 79-112
4 kg
Theoretical PSA (3mm) 189 339 33-47
3D PSA 303 500 37-53
Radial PSA 314 564 38-54
6x11 Segmentation 550 991 43-61
Full Crystal >1000 >1000 81-116
IDM’10 Montpellier, July 2010.
SUMMARY & CONCLUSIONS
Germanium detectors are one of the best options for experiments searching for Rare Events.
3D PSA in segmented detectors seems to be one of the most powerful background rejection techniques.
A setup for pulse generation and analysis from simulated events has been developed to study this technique in 76Ge 0 experiment.
Obtained results show that <m> ~ 50meV could be reachable with this technique.
It is necessary to make new studies focused on Dark Matter.
BACKGROUND REJECTION AND SENSITIVITY FOR NEW GENERATION Ge
DETECTORS EXPERIMENTS.
Héctor Gómez Maluenda
University of Zaragoza (SPAIN)
hgomez@unizar.es
IDM’10 Montpellier, July 2010.
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