Search for double electron capture in 106Cd using HPGe detectors and Si pixel detectors

Ivan Štekl for TGV collaborationInstitute of Experimental and Applied Physics

Czech Technical University in Prague

TGV experiment – TGV II detector description, results of Phases I and

II

Present status - utilization of Si pixel detectors (MC simulations,

results of background measurements)

Future plans

JINR Dubna, RussiaJINR Dubna, RussiaIEAP CTU in Prague, Czech IEAP CTU in Prague, Czech

RepublicRepublicCSNSM Orsay, FranceCSNSM Orsay, France

Since 2000, focus on 2EC/EC decay of 106Cd

),(22 10646

10648 raysXPdCde e

keVEkeVkeV XECEC 2319:ROI,2778Q /

106Cd

106Ag

106Pd

0+

0+

0+

2+

1133.8

511.9

511.9

622

Q(EC/EC) = 2770 keV 7.2

1.25%

1+

++ +/EC EC/EC

2νEC/EC 2KX Pd (~21 keV) (+ for e.s.) Main background: Cd KX-rays (~23 keV)

+/EC KXPd + 2 511 (+ for e.s.)

2741.04+

0νEC/EC KXPd + LXPd + 2741 (2229 + 512)

2229

2741

++ 4 511 (+ for e.s.)

1557.7

2717.6

3+

1046

1160

0νEC/EC 2KXPd + (1160 + 1046 + 512)

Decay modes + signatures

32 HPGe planar detectors 60 mm x 6 mm

with sensitive volume: 20.4 cm2 x 6 mm

Total sensitive volume: ~400 cm3

Total mass of detectors: ~3 kg

Total area of samples : 330 cm2

Total mass of sample(s) : 10 25 g

Total efficiency : 50 70 %

E-resolution : 3 4 keV @ 60Co

LE-threshold : 5 6 keV

16 samples (~ 50 µm ) of 106Cd (enrich.75%)

13.6 g ~ 5.79 x 1022 atoms of 106Cd

HPGe

HPGe

Cd

Detectors: 32 HPGe Ø 60 mm x 6 mm

Sensitive volume 20.4 cm2 x 6 mm

Weight of cryostat ~2500 g (Al, Cu, …)

Al ~ 1200 g (including holders ~ 360 g)

Cu ~ 1300 g

PASSIVE SHIELDINGPASSIVE SHIELDING (TGV is located in LSM) (TGV is located in LSM)

Copper> 20 cm

Airtight box

Lead> 10 cm

Polyethylene

filled withboron 16

cm

Schedule of TGV II experiment:

(in Laboratoire Souterrain de Modane, France)

Phase I ~ 10 g (12 samples) of 106Cd (75%) T= 8687h (Feb.2005 – Feb.2006)

Phase II ~ 13.6 g (16 samples) of 106Cd (75%) T ~ 12900h (Dec.2007 – July 2009)

Background I no samples (Aug.2009 – Mar.2010)

Background II 16 samples of Cd.-nat (Apr.2009 – …2010)

Phase I final result

%)90(100.3)( 20106/22/1 yCdT ECEC

• acquisition with 10g of 106Cd after 8687 hours: additional analysis (2D, different energy windows)

T1/2 = 1.0 · 1020 y (Suhonen)

T1/2 = 4.4 · 1021 y (Simkovic)

T1/2 = 8.7 · 1020 y (Hirsch)

T1/2 > 3.0 · 1020 y (TGV-2)

4

8

12

16

20

24

28

32

048121620

-line 238 keV

Counts / day /detector4

8

12

16

20

24

28

32

0.00.51.01.52.02.53.03.54.0

-line 352 keV

Counts / day /detector

Comparision of background in Phase I and Phase II

KXPd

KXPd

KXCd

KXCdROI

ROI

%)90(102.4)( 20106/22/1 yCdT ECEC

ROI

ROI

Phase II, 13.6g of 106Cd, T=12900h

How it compares with calculations

> > 4.2 10 4.2 1020 20 p.w.p.w.

approaching

closed

TGV-2 limits on double beta decay of 106Cd

Phase I Phase II

(10g of106Cd,8687h) (13.6g of 106Cd,12900h)

EC/EC T1/2 ≥ …(90%CL) T1/2 ≥ … (90%CL) (0+→0+,g.s.) 3.0 x 1020 yr 4.2 x 1020 yr (0+→2+

1,512) 4.2 x 1019 yr 1.2 x 1020 yr (0+→0+

1,1334) 3.1 x 1019 yr 1.0 x 1020 yr 0 res.(0+→4+,2741) - 1.7 x 1020 yr 0 res.(0+→ 2718) - 1.6 x 1020 yrβ+/EC (0+→0+,g.s.) 5.9 x 1019 yr 1.1 x 1020 yr(0+→2+

1,512) 5.9 x 1019 yr 1.1 x 1020 yr (0+→0+

1,1334) - 1.6 x 1020 yr β+ β+

(0+→0+,g.s.) 6.0 x 1019 yr 1.4 x 1020 yr(0+→2+

1,512) 5.7 x 1019 yr 1.7 x 1020 yr

2β+ β+ (0+→0+,g.s.) - 1.3 x 1020 yr

Advantages:

better efficiency comparing with TGV II (factor 2)

information about energy + position of registered

X-ray

particle recognition (background vs. signal)

much less material needed (lower background)

measurement at room temperature (easy access)

SiSi

Control unit

Cd foil

TGV-III (SPT) ideaThe potential of the TGVThe potential of the TGV II II setup (based on HPGe detectors) setup (based on HPGe detectors) can be can be extended using more enriched material, bigger weight and longerextended using more enriched material, bigger weight and longer acquisition acquisition timetime

Considering to build a new generation of the setup based on Si pixel Considering to build a new generation of the setup based on Si pixel detectors Timepixdetectors Timepix

SPTSPT - Silicon Pixel TelescopeSilicon Pixel Telescope

Timepix detector

• Portable tracking detector• Room temperature &

noiseless operationChipboard + USB readout interface

Vacuum operation, no cooling

USB readout interface (developed in IEAP CTU in Prague), frame-rate up to 5 fps (USB 1.1)

Compact size, Plug&Play, hot swap

Fully USB powered

Integrated source of variable detector bias voltage (5 – 100V)

Pixelman software package + plugin for particle identificationPixelman software package (developed in IEAP CTU in Prague)

Response examples • Particle type identification• Clusters selected according to size, roundness, linearity,...

May 5, 2010 Pavel Cermak

Alphas Electrons Muons

Muons + electrons

214Bi 214Po 210Pb

K1 K2

Single-side-events

Double-side-event

K1

K2

a) Distance face to face in detector pair: 2 mm (detector - source = 1 mm)

b) Detector dimensions: 1.408 cm x 1.408 cm x 2 mmnumber of pixels: 256 x 256 (pixel size: 55 x 55 microns)

detector material: Si backside contact thickness: 1800 angstroms

backside contact material: Aluminum

c) Source foil: 1.2 cm × 1.2 cm × 50 mm

Vertex and physics: Two 21 keV gamma (106Pd X-rays) emitted from single point chosen in bulk of source in full isotropic solid angle

Tagged patterns:Each X-ray is fully detected in the single pixel either:1) single-side events (SSE) – pixels fired in the same detector2) double-side events (DSE) – pixels fired in both detectors3) Total number of good KK-events SSE+DSE

K2K1

SPT MC simulations (1)

Setup with 2 mm thick Si Timepix detector, source distance = 1 mm:Number of good KK-events registered in two pixels: 12.71 %

- double-side events (DSE): 44.11 % - single-side events (SSE): 55.89 %

- single-side events in adjacent pixels: 0.009 %

Number of events with energy deposit in foil: 76.0 %

Setup with 2 mm thick Si Timepix detector, source distance = 1 m:Number of good KK-events registered in two pixels: 16.66 % - double-side events (DSE): 42.32 % - single-side events (SSE): 57.68 % - single-side events in adjacent pixels: 0.16 %

Number of events with energy deposit in foil: 76.2 %

SPT efficiency of registration (compare with 5.5% for TGV-II):

SPT MC simulations (2)

a) MC results: KK events, source-detector distance = 1 mm

D < 2 mm => 34.5% of registered KK eventsD (0.7 – 4) mm => 68% of registered KK eventsMean distance = 3.2 mm

KK events, source-detector distance = 0 mmD < 2 mm => 84.7% of registered KK eventsD < 4 mm => 98.7% of registered KK eventsMean distance = 1.2 mm

b) For background measurement:D (0.7 – 4) mm => 25% of registered bg events

Using distance cut D (0.7-4) mm

Results of MC:

Improving S/B ratio by factor of 2.6

Distance of pixels hit (comparison of MC and meas.)

Bg measurement from LSM:

Intrinsic background

0

5

10

15

20

25

30

35

40

0 200 400 600 800 1000 1200 1400

E, keV

Co

un

ts /

ho

ur

Chipboard with Si detector

Measured by low-background setup in Modane lab

HPGe planar detector, 150cm3, range 20keV – 1.5MeV

Bonding (In+Sn) Readout chip

Empty PCB Si module

228Th < 10-8 < 0.2 263 ± 8 187 ± 11234Th < 10-6 < 0.9 168 ± 11 123 ± 10

40K < 10-7 < 6.2 < 25 117 ± 28

Contributions per unit (comparison of samples and Si module) [mBq/unit]:

Background signal measurement

E1.vs.E2 SSE

Entries 15

20 days of experimental data from LSM, shielded 5cm of Pb, recording 1s

frames

ROI

Energy spectrum (up to 300keV)

Looking for SSE candidates

(2 clusters in the frame)

Only random coincidences, 0

events in the ROI (19-23 keV)

SPT setup proposalEstimation of limit for EC/EC decay of Estimation of limit for EC/EC decay of 106106Cd for 1 pair of Timepix quads:Cd for 1 pair of Timepix quads:

If background = 0 :

T1/2 > (e . t . Nat . ln2) / ln (1-CL) = 1,95 × 1020 years

90% CL ln (1-CL) = 2.3

e ...... full efficiency (for SPT = 8,54 %)

t ...... time of measurement [years], expected 4 years

Nat ... number of 106Cd atoms in foil, 98% of enrichment

Nat = 1.89 × 1021 atoms

To reach limit of 10To reach limit of 102121 years: years:

We would need 5 quad Timepix pairs

Next step – using stack of Timepix detectors

• To build a prototype based on the four-fold Timepix stack

Developed slightly modified boards allowing face-to-face configuration

Further optimization of the chip carrier board (material of PCB, minimizing amount of material close to the chip)

Summary and future plans• Study of 2EC/EC of 106Cd

• TGV-2 setup (based on HPGe detector telescope) has provided result comparable with theoretical predictions – plan new start with more enriched foils (98.4% of 106Cd)

• Developing next generation setup SPT (Silicon Pixel Telescope) based on Si pixel detectors TimePix – measurement and tuning of intrinsic background, a prototype based on the four-fold Timepix stack.

Not tuned for low-bg applications, working on

detection module design optimization, material

selection (teflon PCB, capton).

Studying background signals – Si TimePix detector located in Modane

underground lab, 3 months of data (3 weeks processed).

Dreaming of 5 quad pairs setup running for couple of years to reach region

of 1021 years.