BACKGROUND REJECTION AND SENSITIVITY FOR NEW GENERATION Ge DETECTORS EXPERIMENTS. Héctor Gómez Maluenda University of Zaragoza (SPAIN) IDM’10 Montpellier, July 2010.

OUTLINE Motivation. Setup. Geometry & Materials. Simulated Events. Pulse Generation. Pulse Analysis. Results Summary & Conclusions.

IDM’10 Montpellier, July 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) ).

IDM’10 Montpellier, July 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 ( 76 Ge). For a ~50 meV sensitivity: ε ~75-80 % b ~10 -3 c keV -1 kg -1 y -1 Simulation of background and signal events. Pulse generation from these events. Pulse Shape Analysis (PSA).

IDM’10 Montpellier, July 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 SIMULATED EVENTS Apart from signal events, main background contributions in the MeV Region of Interest have been considered: 60 Co- 68 Ge 208 Tl 214 Bi 60 Co 0  2  Signal: 0  events (DECAY 0). Background: Internal 60 Co and 68 Ge (GEANT4). 60 Co in Cu cryostat (GEANT 4). External 208 Tl and 214 Bi (GEANT4). 2  events (DECAY 0).

IDM’10 Montpellier, July 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  Internal 60 Co External 214 Bi 2 

IDM’10 Montpellier, July PULSE GENERATION To have 3D spatial resolution is necessary to study the net signal an the induced ones.

IDM’10 Montpellier, July PULSE GENERATION The pulse is the representation of the charge variation vs time: Voltage V 0 is applied to the outer electrodes of the detector.

IDM’10 Montpellier, July PULSE GENERATION The pulse is the representation of the charge variation vs time: Voltage V 0 is applied to the outer electrodes of the detector. Finite element calculation to obtain E.

IDM’10 Montpellier, July PULSE GENERATION The pulse is the representation of the charge variation vs time: Voltage V 0 is applied to the outer electrodes of the detector. Finite element calculation to obtain E. E w (weighting field) is the theoretical existing field when all the electrodes are with V=0 excepting one.

IDM’10 Montpellier, July 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 PULSE GENERATION Induced Signal: No new temporal information. Null net area. Absolute area (A A ) as representative value. Signal amplitude and A A lower than net signal.

IDM’10 Montpellier, July 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 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

IDM’10 Montpellier, July 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 PULSE ANALYSIS Induced Signal: Absolute Area (A A ) is the most representative feature of induced signals A A value is independent of Characteristic Time. Analysis is based on A A comparison with the corresponding CoE event. MONOSITEMULTISITE P z & P φ ≤ 1  Monosite P z or P φ > 1  Multisite

IDM’10 Montpellier, July 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 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 RESULTS Background level for a 2-kg detector (10 -3 c/keV/kg/y) with 6x9 segmentation Background source Activity RawNet Signal (40ns)Induced Signal Internal 60 Co 5 kg -1 d -1 ; 30d exp; 0d cool.* < 0.01 Internal 68 Ge 1kg -1 d -1 ; 180d exp; 180d cool.* External 208 Tl  0.1  cm -2 s -1 * External 214 Bi  0.38  cm -2 s -1 * Internal 232 Th in lead 1  Bq/kg* Co from Cu criostat 1mBq/kg  kg -1 y TOTAL  detection ε *Values from H. Gómez et al, Astroparticle Physics 28 (2007)

IDM’10 Montpellier, July RESULTS Background level for a 4-kg detector (10 -3 c/keV/kg/y) with 6x11 segmentation Background source Activity RawNet Signal (40ns)Induced Signal Internal 60 Co 5 kg -1 d -1 ; 30d exp; 0d cool.* < 0.01 Internal 68 Ge 1kg -1 d -1 ; 180d exp; 180d cool.* External 208 Tl  0.1  cm -2 s -1 * External 214 Bi  0.38  cm -2 s -1 * Internal 232 Th in lead 1  Bq/kg* Co from Cu criostat 1mBq/kg  kg -1 y TOTAL  detection ε *Values from H. Gómez et al, Astroparticle Physics 28 (2007)

IDM’10 Montpellier, July RESULTS Estimation of the sensitivity from the ε and b values obtained 2 kg 4 kg

IDM’10 Montpellier, July RESULTS Estimation of the sensitivity from the ε and b values obtained MT min (kg·y)MT med (kg·y) (meV) for 1000 kg·y 2 kg Theoretical PSA (3mm) D PSA Radial PSA x9 Segmentation Full Crystal> kg Theoretical PSA (3mm) D PSA Radial PSA x11 Segmentation Full Crystal>

IDM’10 Montpellier, July 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 76 Ge 0  experiment. Obtained results show that ~ 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) IDM’10 Montpellier, July 2010.