background rejection and sensitivity for new generation ge detectors experiments

BACKGROUND REJECTION AND SENSITIVITY FOR NEW GENERATION Ge

DETECTORS EXPERIMENTS.

Héctor Gómez Maluenda

University of Zaragoza (SPAIN)

[email protected]

IDM’10 Montpellier, July 2010.


OUTLINE

Motivation.

Setup.

Geometry & Materials.

Simulated Events.

Pulse Generation.

Pulse Analysis.

Results

Summary & Conclusions.


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).


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).


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.


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).


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


PULSE GENERATION

To have 3D spatial resolution is necessary to study the net signal an the induced ones.


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.


PULSE GENERATION


/ββ

sat

Wiii

EE

V

rErEμq

dt

dq1

0 1


Finite element calculation to obtain E.


PULSE GENERATION


/ββ

sat

Wiii

EE

V

rErEμq

dt

dq1

0 1


Finite element calculation to obtain E.

Ew (weighting field) is the theoretical existing field when all the electrodes are with V=0 excepting one.


PULSE GENERATION

Net Signal:

2 singular points in the pulse per energy deposit.

Total area proportional to the energy.

Only radial sensitivity.

Time (ns)


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.


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


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


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).


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


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)


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


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


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


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


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


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)

[email protected]


background rejection and sensitivity for new generation ge detectors experiments

Documents

pulse generation

signal events

pulse analysis

simulated events

events decay

background events suppression

generated events

rare events