SIMULATION OF BACKGROUND REDUCTION TECHNIQUES FOR Ge DBD DETECTORS Héctor Gómez Maluenda. University of Zaragoza. GERDA/Majorana MC Meeting. München. 15/16 Feb 2007

Motivations. Simulation Features. Obtained Data & Analysis. - Granularity. - Segmentation. - Spatial Resolution & PSA. - 0 ββ Events. Summary & Conclusions. Outlook. OUTLINE

MOTIVATIONS GERDA/Majorana MC Meeting. München. 15/16 Feb 2007  New generation of neutrinoless double beta decay experiments using enriched germanium detectors need to reach a background level of ~10 -3 c/(keV kg y) in the Region of Interest to have the expected sensitivity.  There are several techniques that could reduce the background in order to have the level required.  It is necessary to evaluate the potential of these techniques before the development of the experiment to build the optimal set up. SIMULATION A reliable simulation package can help us to build the experiment and to improve it during its operation.

GERDA/Majorana MC Meeting. München. 15/16 Feb 2007 The first goal of our simulation is to study the optimal configuration for the mass and the distribution of the germanium detectors and to quantify the background reduction that can be reached using different techniques: PULSE SHAPE ANALYSIS SEGMENTATION GRANULARITY MOTIVATIONS

SIMULATION FEATURES GERDA/Majorana MC Meeting. München. 15/16 Feb 2007 Hardware & Software:  HARDWARE: - 6 CPU Intel Xeon 64 bits (3.00 GHz) Cluster - Super Micro X6DA8-G2 BOARD - UPS rack model “Riser Evolution 1500”  SOFTWARE: - Linux SuSe 9.3 ( smp kernel) - Geant: patch01 & patch01 - ROOT: 5.12/00

SIMULATION FEATURES GERDA/Majorana MC Meeting. München. 15/16 Feb 2007 Geometry & Physics: D h E 1, x 1, y 1, z 1, t 1 E 2, x 2, y 2, z 2, t 2 Cylindrical detectors (D=h). Natural Germanium. Masses between 0.1 and 4 kg. Neither cryostat nor shielding. Background sources: - Internal 60 Co, 68 Ge. - External keV photons. Standard GEANT4 models. Position, Energy and Time of each interaction produced in an event have been registered. Possibility of different analysis. BASED ON GEANT4:

GERDA/Majorana MC Meeting. München. 15/16 Feb 2007 OBTAINED DATA & ANALYSIS Granularity: Two main simulations External Contamination: Emitted from an external sphere (R=1m.) Internal Contamination: Homogeneously distributed in whole crystal. NORMALIZATION OF THE DATA DEPENDING ON THE ORIGIN OF THE CONTAMINATION c keV -1 every 10 4 decays per kg c keV -1 kg -1 every 10 4 interacting photons

OBTAINED DATA & ANALYSIS Granularity: GERDA/Majorana MC Meeting. München. 15/16 Feb 2007  Region of Interest: 2.0 – 2.1 MeV.  Comparative study: Need to know specific activity to calculate background levels. c keV -1 kg -1 y -1 OPTIMAL MASS OF THE COMPONENT DETECTORS DEPENDS ON WHAT KIND OF BACKGROUND EVENTS WE WANT TO REDUCE.

OBTAINED DATA & ANALYSIS GERDA/Majorana MC Meeting. München. 15/16 Feb 2007 Segmentation:  After the simulation of the whole crystal, we consider different segmentation schemes offline. Considerations:  Difficulty associated to the reduction of the width of the segment.  Equilibrium Bkg. rejection ↔ DBD evts. detection. TRANSVERSAL SEGMENTS OF 1cm WIDTH: Available in a technical way. Width enough for DBD events detection.

OBTAINED DATA & ANALYSIS Segmentation: GERDA/Majorana MC Meeting. München. 15/16 Feb 2007  Spectrum of the events that leave all the energy in one segment.  Study of the events in the 2.0 – 2.1 MeV Region of Interest. REJECTED EVENT NON REJECTED EVENT

OBTAINED DATA & ANALYSIS Segmentation: GERDA/Majorana MC Meeting. München. 15/16 Feb 2007

OBTAINED DATA & ANALYSIS Segmentation: GERDA/Majorana MC Meeting. München. 15/16 Feb 2007 Percentage of rejected events in MeV region by segmentation and anticoincidence techniques. 2 kg4 kg 7 planes9 planes 7 planes & 6 sectors 9 planes & 6 sectors9 planes11 planes 9 planes & 6 sectors 11 planes & 6 sectors 60 Co Ge keV gammas

OBTAINED DATA & ANALYSIS Spatial Resolution & PSA: GERDA/Majorana MC Meeting. München. 15/16 Feb 2007  Most powerful tool for background rejection.  Differentiation between “multisite” background events and “monosite” DBD events even inside the same segment.  Current techniques can locate an energy deposit with an accuracy between 2.5 and 5.4 mm. (Kröll, Bazzaco. NIM A 565 (2006) ) SEGMENTATION OF THE CRYSTAL IS NECCESSARY TO LOCATE THE DEPOSITS

OBTAINED DATA & ANALYSIS Spatial Resolution & PSA: GERDA/Majorana MC Meeting. München. 15/16 Feb 2007  GEANT4 provides much more accuracy than 2.5 mm.  Need to group the single hits of the simulation in “Super Hits”.  A SUPER HIT contains all the single hits of an event with interdistances smaller than d lim. d < d lim d > d lim

OBTAINED DATA & ANALYSIS Spatial Resolution & PSA: GERDA/Majorana MC Meeting. München. 15/16 Feb 2007 MULTISITE EVENT: 2 or more Super Hits. 3 mm: Best resolution reachable. 5 mm: Conservative option. REJECTABLE BY PSA.

OBTAINED DATA & ANALYSIS Spatial Resolution & PSA: GERDA/Majorana MC Meeting. München. 15/16 Feb 2007 Percentage of “monosite” events in MeV region assuming different spatial resolutions. 2 kg4 kg 3 mm resolution5 mm resolution3 mm resolution5 mm resolution 60 Co Ge keV gammas

OBTAINED DATA & ANALYSIS 0 ββ Events: GERDA/Majorana MC Meeting. München. 15/16 Feb 2007 Input of a 0 ββ event: Initial position (x 0, y 0, z 0 ) Energy of the electrons (E 1, E 2 ) Angle between electron momentum (  ) E 1 = E 2 = 1020 keV.  = Random  = 0º  = 180º Simulation of 0 ββ with  = Random provides a good mean value for detection efficiency estimations.

OBTAINED DATA & ANALYSIS GERDA/Majorana MC Meeting. München. 15/16 Feb ββ Events: … e e e e e e e e e e e e e e … (Track ID, Parent ID)  Further analysis

OBTAINED DATA & ANALYSIS 0 ββ Events: GERDA/Majorana MC Meeting. München. 15/16 Feb 2007 Energy configurations: keV keV keV keV (not a real case) ANALYSIS: Same as Background events. - Full Crystal. - Different segmentation schemes. - Study of the maximum interdistances. - PSA.

OBTAINED DATA & ANALYSIS 0 ββ Events: GERDA/Majorana MC Meeting. München. 15/16 Feb 2007

OBTAINED DATA & ANALYSIS 0 ββ Events: GERDA/Majorana MC Meeting. München. 15/16 Feb ββ 2 energy deposits closer than 3 mm. MONOSITE EVENT But these “border events” deposit the energy in two adjacent segments. It is possible to identify it to improve our efficiency? NEED TO STUDY THE ELECTRIC FIELD INSIDE THE DETECTOR

SUMMARY & CONCLUSIONS GERDA/Majorana MC Meeting. München. 15/16 Feb 2007  A simulation package for DBD with Ge detectors was developed.  Preliminary studies about background rejection techniques applied to the Ge crystals were made.  These works indicates that it is possible to reduce the background level in a factor 20 after application of some techniques.  Detection efficiency for 0 ββ events was also studied.  It is necessary to complete the code in order to obtain more reliable conclusions and to make more tests about other topics.

GERDA/Majorana MC Meeting. München. 15/16 Feb 2007 OUTLOOK  Simulation of other background sources: Bi. - Neutrons. - 2 ββ Events. …  Complete the experimental set up to make more simulations: - Cryostat  60 Co contamination inside it.  Simulation of Pulses in order to develop a PSA system. - True coaxial detector  Electric Field.

SIMULATION OF BACKGROUND REDUCTION TECHNIQUES FOR Ge DBD DETECTORS Héctor Gómez Maluenda. University of Zaragoza. GERDA/Majorana MC Meeting. München. 15/16 Feb 2007