Activity report of TG10 L. Pandola (LNGS) for the TG10 group Gerda Collaboration Meeting, February 3-5, 2005 (simulations and background studies)

The Task Group 10 Goals: evaluation of the background index Simulation of signal and backgrounds in the Gerda detector Geant4-based MaGe framework in collaboration with Majorana Who: LNGS, Munich, Russian groups, MPIK Validation and cross-check Pulse shape, segmentation, mirror charges, etc. With TG9: definition of data format including optimization of Gerda detector and data analysis sensitivity to 0 2  signal

The MaGe framework Mid-October 2004: Gerda & Majorana joint MC workshop Idea: collaboration of the two MC groups for the development of a common framework based on Geant4 abstract set of interfaces: each experiment has its own concrete implementation avoid the work duplication for the common parts (generators, physics, materials, management) provide the complete simulation chain more extensive validation with experimental data runnable by script; flexible for experiment-specific implementation of geometry and output; suitable for the distributed development

The MaGe framework Majorana already had a working framework, evaluated and found suitable for Gerda needs and for joint development Warning: To have a common framework simply means sharing the same generic interfaces. No contraints to the Gerda side (geometry, physics, etc.)  each component can be independently re-written (kindly supplied by the MC group) Report: wwwgerda.mppmu.mpg.de/MC/gerda_monte_pic/gerda.pdf Present situation: Common CVS repository hosted at Munich Discussion forum hosted at Berkeley

The MaGe structure Generator, physics processes, material, management, etc. mjgeometry mjio gerdaio gerdageometry Each group has its own geometry setup and corresponding output, everything else can be shared. To run a new simulation: write only your geometry and your output register them in the management classes Can be downloaded from the CVS repository in Munich   setup instructions at: wwwgerda.mppmu.mpg.de/MC/monte_carlo_pic/setup.ps

Activity for the common part Development of generic (not Gerda-specific) tools Optimization and modularization of the framework Interface to the decay0 generator by V.I. Tretyak Generator for cosmic ray muons Random sampling of points uniformly from a specified (generic) volume 0 2  signal according to several theoretical models All this work would have been duplicated... Access to the trajectories of all the secondaries

Activity for the Gerda-specific part Description of the Gerda setup including shielding (water tank, Cu tank, liquid Nitrogen), crystals array and kapton cables Gerda geometry top  -veto water tank lead shielding cryo vessel neck Ge array

New OO structure of geometry classes Gerda MC Geometry Flexible executable: set of commands to configure geometry Number of columns and orientation, segmentation of crystals, support structure/shielding on/off, etc. Kevin Kröninger - MPI München segmented crystals (6x3) 10 columns standard geometry

Output: Class to create a ROOT TTree with all the interesting information (energy deposition and position of hits in Ge, Liquid N 2, water, etc.) Activity for the Gerda-specific part Physics studies in progress: background induced by cosmic ray muons and neutrons  background in electronics and support segmentation effect for background and 0 2  signal ready to be interfaced with software for the simulation of pulse shape  Munich external  background and shielding requirements Generic AIDA interface for other analysis tools (e.g. HBOOK)

Two examples of macros /MG/geometry/detector GerdaArray /MG/geometry/database false /MG/geometry/detector/crystal/truecoaxial false /MG/geometry/detector/general/numcol 3 /MG/geometry/detector/general/crypercol 3 /MG/geometry/detector/crystal/height 8.5 cm /MG/generator/select cosmicrays /MG/eventaction/rootschema GerdaArray /MG/geometry/detector GerdaArray /MG/geometry/database false /MG/geometry/general/constructshield false /MG/generator/select decay0 /MG/eventaction/rootschema GerdaArray /MG/generator/confine volume /MG/generator/volume Ge_det_0 /MG/generator/decay0/filename myfile.dat Generates cosmic ray events in a 3x3 array of non-coaxial crystals in the Gerda shielding Generates events uniformly in the volume of a Ge crystal (without shielding). Kinematic read from a decay0 file Geometry, tracking cuts, generator and output pattern  selectable and tunable via macros No need to recompile, easy to use for non-expert people

Cosmic ray muons (Phase I) Flux at Gran Sasso: 1.1  /m 2 h (270 GeV) Small flux, small Ge volume: 59 events/kg y Further reduced by anti-coincidence with other Ge- crystals and with top (or Cerenkov)  -veto Input energy spectrum from Lipari and Stanev, Phys. Rev. D 44 (1991) 3543 Energy (keV) Input angular spectrum uniform in  in  first approximation ~ 60 – 70 events/kg y in H-M

Cosmic ray muons (Phase I) 9 Ge crystals for a total mass of 19 kg; threshold: 50 keV Energy (MeV) 3.93 years Sum spectrum Energy (MeV) Number of hit detectors multi-hit: 35.2% 149 counts in 1500  2500 keV 21 counts in 2000  2100 keV (1.5  2.5 MeV): 2·10 -3 counts/keV kg y annihilation peak below threshold single-Ge (~4·10 -3 counts/keV kg y in H-M simul.) C. Doerr, NIM A 513 (2003) MeV2.5 MeV

Cosmic ray muons (Phase I) Threshold for plastic scintillator (top  -veto): 1 MeV 3.93 years Sum spectrum Ge anti-coincidence Energy (MeV) (suppression factor: ~2) Ge and top  -veto anti-coincidence (suppression factor: ~20) ~ 4 events/kg y

Cosmic ray muons (Phase I) Counts in 1.5  2.5 MeV (3.93 years) Counts in 2.0  2.1 MeV (3.93 years) Background index (cts/keV kg y) No cuts14921 (H-M=34) ~ 2-3 · Ge anti-coincidence466~ 6 · Ge anti-coincidence Top  -veto (100% eff.) 61< 1.6 · (95% CL) Ge anti-coincidence Top  -veto (98% eff.) 81< 1.9 · (95% CL) Ge anti-coincidence Top  -veto (95% eff.) 91< 2.1 · (95% CL) Cerenkov  -veto (thr = 5 MeV, 100% eff.) 00< 0.4 · (95% CL) Instrumentation of water as a Cerenkov  -veto is an open issue for the Collaboration (  redundancy) Background substantially lower than previously estimated

Cosmic ray muons (Phase I) Cross-check of isotope production with independent codes (e.g. FLUKA) would be very welcome Correlated issue: production of short-lived radioactive isotopes induced by the muon showers delayed energy deposition Most dangerous isotopes (  above Q  ): IsotopeLife timeGammaswhererate 15 C2.44 s5.2 MeVWater1.8 c/year 13 B17.4 ms3.68 MeVWater0.6 c/year 16 N7.13 s6.1, 7.1 MeVWater3.5 c/day 14 O70.6 s2.31 MeVWater6.1 c/y Production in dangerous isotopes in nitrogen is much smaller Background index not evaluated yet  probably negligible

Neutrons (Phase I) To do next: validation of the simulation with data and cross-check with independent codes Cosmogenic neutrons (muon interaction in the rock) small flux (200 n/m 2 y), hard energy spectrum (up to tens of GeV) Energy and angular spectrum from H. Wulandari et al. hep-ex/ Negligible in Gerda: < 3.8 · cts/keV kg y (95% CL) with Ge-anticoincidence Neutrons from fission and ( ,n) soft energy spectrum (up to 8 MeV), higher flux (20 n/m 2 h) Work in progress. Difficult to simulate because CPU-intensive 0.05% of the events deposit energy the nitrogen volume  90 ev/m 2 y Probably not an issue.  from n+p shielded by LN 2 In H-M: 3 · cts/keV kg y (without water shielding) ! C. Doerr, NIM A 513 (2003) 596

CNGS muons Flux at Gran Sasso: 0.86  /m 2 d ( ~ 15 GeV) 30 times smaller than cosmic ray flux and softer spectrum LVD Collaboration, hep-ex/ Top  -veto uneffective: only Ge-anticoin. and water  -veto Not evaluated yet in detail Rough estimate (15-GeV  ): LVD Collaboration, hep-ex/ No cuts: < 1.2· cts/keV kg y (95%) Ge-anticoincidence: < 8 · cts/keV kg y (95%) Ge and Cerenkov  -veto: < 4 · cts/keV kg y (95%) Not a critical issue

Signal and background studies Photons carry energy to more than one crystal/segment (multiple-site) Example: 60 Co Hit crystals Hit segments ~19%~6% Cut on the number of hit crystals or segments reduces 60 Co events to 19% (6%) Kevin Kröninger - MPI München

Signal and background studies Background suppression efficiency: Segmentation: 6 (phi) x 3 (z) Threshold: 10 keV; Energy window: Q  ± 5 keV Pulse shape analysis and pattern recognition not included Source1 crystal1 crystal AND signal window 1 segment1 segment AND signal window Number of events Signal k 60 Co (crystal) · · M 60 Co (cable) · · M 208 Tl (crystal) · · M 208 Tl (cable) · · M 68 Ge (crystal) · · M 210 Pb (crystal)109.9 · k Kevin Kröninger - MPI München

MPI Munich MC activities Pulse shape analysis (incl. MC) Test facility for Ge-crystals (incl. MC) Future tasks: Maintenance of a common CVS server for MaGe Update of geometry: crystals and support structure Background and signal studies/background suppression Segmentation studies Kevin Kröninger - MPI München

Other background calculations Background from inner tank envelope: direct simulation of  transportation signal window: 1800  2300 keV Cu: 25 · Bq/kg of 232 Th Fe: 20 · Bq/kg of 232 Th (c/keV kg y)CuFe (neck) Center · cm below center1.2 · · c/kg keV y guaranteed With 50-cm-below position, Fe negligible Background from external gammas: detector placed 50 cm below center intensity of 2.6 MeV: cm -2 s -1 Water shielding: 300 cm in the cylindrical part 200 cm above and below 6.6 · c/keV kg y 1-2 · c/keV kg y A. Klimenko – INR, ITEP, Dubna, MPIK

Cts/keV kg y Cylindrical part6.6 · Upper spherical part1.1 · Bottom flat part2.0 · Open neck1.1 · Neck with 10cm Pb1.1 · Neck with 15cm Pb1.1 · upper part cylindrical part lower part To go lower than c/keV kg y: bottom part: 7 cm of Pb cylindrical part: no further shielding needed neck: 15 cm of Pb upper part: 6 cm of Pb Cu tank: LAr is required A. Klimenko – INR, ITEP, Dubna, MPIK Other background calculations

Conclusions MC package MaGe ready for Gerda & Majorana groups Downloadable from CVS, flexible and runnable by macro Structure complete and ready for physics studies Backgrounds, segmentation, pulse shape (via interface) Precise description of Gerda setup and shielding First results of signal and bck in crystals & cables 3-month activity and still a lot of work to do in the future......Well begun is half done ! Preliminary results of  -induced and n background Top  -veto enough for background of a few ·10 -4 c/kg keV y Neutrons, CNGS and isotopes production presumably not critical Estimation of external  background and shielding c/kg keV achievable with present shielding, needs LAr