Search for double electron capture in 106 Cd using HPGe detectors and Si pixel detectors Ivan Štekl for TGV collaboration Institute 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, Russia IEAP CTU in Prague, Czech Republic CSNSM Orsay, France Since 2000, focus on 2 EC/EC decay of 106 Cd

106 Cd 106 Ag 106 Pd   622 Q(EC/EC) = 2770 keV  % 1+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.) νEC/EC KXPd + LXPd +  2741 (   512)  2229  2741  +  + 4  511 (+  for e.s.)  1046  νEC/EC 2KXPd + (    512) Decay modes + signatures

32 HPGe planar detectors  60 mm x 6 mm with sensitive volume: 20.4 cm 2 x 6 mm Total sensitive volume: ~400 cm 3 Total mass of detectors: ~3 kg Total area of samples : 330 cm 2 Total mass of sample(s) : 10  25 g Total efficiency : 50  70 % E-resolution : 3  4 60 Co LE-threshold : 5  6 keV 16 samples (~ 50 µm ) of 106 Cd (enrich.75%) 13.6 g ~ 5.79 x atoms of 106 Cd HPGe Cd

Detectors: 32 HPGe Ø 60 mm x 6 mm Sensitive volume 20.4 cm 2 x 6 mm Weight of cryostat ~2500 g (Al, Cu, …) Al ~ 1200 g (including holders ~ 360 g) Cu ~ 1300 g

PASSIVE SHIELDING (TGV is located in LSM) Copper > 20 cm Airtight box Lead > 10 cm Polyethylene filled with boron 16 cm

Schedule of TGV II experiment: (in Laboratoire Souterrain de Modane, France) Phase I ~ 10 g (12 samples) of 106 Cd (75%) T= 8687h (Feb.2005 – Feb.2006) Phase II ~ 13.6 g (16 samples) of 106 Cd (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 acquisition with 10g of 106 Cd after 8687 hours: additional analysis (2D, different energy windows) T 1/2 = 1.0 · y (Suhonen) T 1/2 = 4.4 · y (Simkovic) T 1/2 = 8.7 · y (Hirsch) T 1/2 > 3.0 · y (TGV-2)

 -line 238 keV Counts / day /detector  -line 352 keV Counts / day /detector

Comparision of background in Phase I and Phase II

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

How it compares with calculations > p.w. approaching closed

TGV-2 limits on double beta decay of 106 Cd Phase I Phase II (10g of 106 Cd,8687h) (13.6g of 106 Cd,12900h) EC/EC T 1/2 ≥ …(90%CL) T 1/2 ≥ … (90%CL) (0 + →0 +,g.s.) 3.0 x yr 4.2 x yr (0 + →2 + 1,512) 4.2 x yr 1.2 x yr (0 + →0 + 1,1334) 3.1 x yr 1.0 x yr 0 res.(0 + →4 +,2741) x yr 0 res.(0 + → 2718) x yr β + /EC (0 + →0 +,g.s.) 5.9 x yr 1.1 x yr (0 + →2 + 1,512) 5.9 x yr 1.1 x yr (0 + →0 + 1,1334) x yr β + (0 + →0 +,g.s.) 6.0 x yr 1.4 x yr (0 + →2 + 1,512) 5.7 x yr 1.7 x yr 2 β + β +  (0 + →0 +,g.s.) x 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) Si Control unit Cd foil TGV-III (SPT) idea The potential of the TGV II setup (based on HPGe detectors) can be extended using more enriched material, bigger weight and longer acquisition time Considering to build a new generation of the setup based on Si pixel detectors Timepix SPTSilicon Pixel Telescope SPT - Silicon Pixel Telescope

Timepix detector Portable tracking detector Room temperature & noiseless operation Chipboard + 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 identification Pixelman software package (developed in IEAP CTU in Prague)

Response examples Particle type identification Clusters selected according to size, roundness, linearity,... May 5, 2010Pavel Cermak AlphasElectronsMuons Muons +  electrons 214 Bi  214 Po  210 Pb  

K1K1 K2K2 Single-side-events Double-side-event K1K1 K2K2 a) Distance face to face in detector pair: 2 mm (detector - source = 1 mm) b) Detector dimensions: cm x cm x 2 mm number 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 ( 106 Pd 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 detector 2) double-side events (DSE) – pixels fired in both detectors 3) Total number of good KK-events  SSE+DSE K2K2 K1K1 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: % - double-side events (DSE): % - single-side events (SSE): % - single-side events in adjacent pixels: % 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: % - double-side events (DSE): % - single-side events (SSE): % - 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 34.5% of registered KK events D (0.7 – 4) mm => 68% of registered KK events Mean distance = 3.2 mm KK events, source-detector distance = 0 mm D 84.7% of registered KK events D 98.7% of registered KK events Mean 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 Chipboard with Si detector Measured by low-background setup in Modane lab HPGe planar detector, 150cm 3, range 20keV – 1.5MeV Bonding (In+Sn)Readout chipEmpty PCBSi module 228 Th< < ± 8187 ± Th< < ± ± K< < 6.2< ± 28 Contributions per unit (comparison of samples and Si module) [mBq/unit]:

Background signal measurement E1.vs.E2 SSE Entries 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 proposal Estimation of limit for EC/EC decay of 106 Cd for 1 pair of Timepix quads: If background = 0 : T 1/2 > (e. t. N at. ln2) / ln (1-CL) = 1,95 × years 90% CL  ln (1-CL) = 2.3 e full efficiency (for SPT = 8,54 %) t time of measurement [years], expected 4 years N at... number of 106 Cd atoms in foil, 98% of enrichment  N at = 1.89 × atoms To reach limit of 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 2 EC/EC of 106 Cd 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 106 Cd) 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 years.