Activity report of TG10

(simulations and background studies)

L. Pandola (LNGS) for the TG10 group Gerda Collaboration Meeting, February 3-5, 2005

Goals:

The Task Group 10

evaluation of the background index optimization of Gerda detector and data analysis sensitivity to 0 n 2 b signal

Simulation of signal and backgrounds in the Gerda detector Geant4

-based MaGe framework in collaboration with Majorana

including Validation and cross-check Pulse shape, segmentation, mirror charges, etc.

With TG9: definition of data format

Who: LNGS, Munich, Russian groups, MPIK

http://wwwgerda.mppmu.mpg.de/MC/monte_carlo.html

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,

(kindly supplied by the MC group)

evaluated and

found suitable

for Gerda needs and for joint development

Report: wwwgerda.mppmu.mpg.de/MC/gerda_monte_pic/gerda.pdf

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 Present situation: Common CVS repository hosted at Munich Discussion forum hosted at Berkeley

The MaGe structure

Each group has its own geometry setup and corresponding output, everything else can be shared.

mjgeometry gerdageometry Generator, physics processes, material, management, etc.

mjio gerdaio 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 0 n 2 b signal according to several theoretical models  Random sampling (generic) volume of points uniformly from a specified   Generator for cosmic ray muons Access to the trajectories of all the secondaries All this work would have been duplicated ...

Activity for the Gerda-specific part

Gerda geometry

top m -veto neck water tank cryo vessel lead shielding Ge array

Description of the Gerda

setup including shielding (water tank, Cu tank, liquid Nitrogen), crystals array and kapton cables

Gerda MC Geometry

New

OO structure

of geometry classes

Flexible executable

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

Activity for the Gerda-specific part

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.) ready to be interfaced with software for the simulation of

pulse shape



Munich

Generic AIDA interface for other analysis tools (e.g. HBOOK) Physics studies in progress: background induced by cosmic ray muons and neutrons g background in electronics and support segmentation effect for background and 0 n 2 b signal external g background and shielding requirements

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 /m 2 h (270 GeV) Small flux, small Ge volume:

59 events/kg y

~ 60 – 70 events/kg y in H-M Further reduced by

anti-coincidence

with other Ge crystals and with top (or Cerenkov) m -veto Input energy spectrum from Lipari and Stanev, Phys. Rev. D 44 (1991) 3543 Input angular spectrum uniform in   1 cos  in  first approximation Energy (keV)

Cosmic ray muons (Phase I)

9 Ge crystals for a total mass of 19 kg; threshold: 50 keV annihilation peak 3.93 years (1.5  2.5 MeV): 2·10

-3 counts/keV kg y

(~4·10 -3 counts/keV kg y in H-M simul.) 1.5 MeV C. Doerr, NIM A 513 (2003) 596 2.5 MeV

Sum spectrum

149 counts in 1500  2500 keV 21 counts in 2000  2100 keV Energy (MeV) single-Ge

Number of hit detectors

multi-hit: 35.2% below threshold Energy (MeV)

Cosmic ray muons (Phase I)

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

Cosmic ray muons (Phase I)

No cuts Ge anti-coincidence Ge anti-coincidence Top m -veto (100% eff.) Ge anti-coincidence Top m -veto (98% eff.) Ge anti-coincidence Top m -veto (95% eff.) Cerenkov m -veto (thr = 5 MeV, 100% eff.) Counts in 1.5

 2.5 MeV (3.93 years) 149 46 6 8 9 0 Counts in 2.0

 2.1 MeV (3.93 years) 21 (H-M=34) 6 1 1 1 0 Background index (cts/keV kg y) ~ 2-3 · 10 -3 ~ 6 · 10 -4 < 1.6 · 10 -4 (95% CL)

< 1.9 · 10 -4 (95% CL)

< 2.1 · 10 -4 (95% CL)

< 0.4 · 10 -4 (95% CL)

Background

substantially lower

than previously estimated Instrumentation of water as a Cerenkov m -veto is an

open issue for the Collaboration

(  redundancy)

Cosmic ray muons (Phase I)

Correlated issue: production of short-lived radioactive isotopes induced by the muon showers

delayed energy deposition

Most dangerous isotopes ( g above Q bb ): Isotope 15 C 13 B 16 N 14 O Life time 2.44 s 17.4 ms 7.13 s 70.6 s Gammas 5.2 MeV 3.68 MeV 6.1, 7.1 MeV 2.31 MeV where Water Water Water Water rate 1.8 c/year 0.6 c/year

3.5 c/day

6.1 c/y Production in dangerous isotopes in nitrogen is much smaller Background index not evaluated yet  probably

negligible Cross-check

of isotope production with independent codes (e.g. FLUKA) would be very welcome

Neutrons (Phase I)

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/0401032 Negligible in Gerda: < 3.8 · 10 -5 with Ge-anticoincidence cts/keV kg y (95% CL)

Neutrons from fission and (

a

,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

Probably not an issue

In H-M: 3 · 10 -3 cts/keV kg y . g from n+p shielded by LN 2 (without water shielding) !

C. Doerr, NIM A 513 (2003) 596 y To do next: validation and cross-check of the simulation with data with independent codes

CNGS muons

Flux at Gran Sasso:

0.86

m

/m 2 d

( ~ 15 GeV) LVD Collaboration, hep-ex/0304018 30 times smaller than cosmic ray flux and softer spectrum Top m -veto uneffective: only Ge-anticoin. and water m -veto LVD Collaboration, hep-ex/0304018 Not evaluated yet in detail

Rough estimate (15-GeV

m

):

No cuts: < 1.2· 10 -4 cts/keV kg y (95%) Ge-anticoincidence: < 8 · 10 -5 cts/keV kg y (95%) Ge and Cerenkov m -veto: < 4 · 10 -5 cts/keV kg y (95%)

Not a critical issue

Signal and background studies

Example: 60 Co Photons carry energy to more than one crystal/segment

(multiple-site)

~19% ~6% Cut on the number of hit crystals or segments reduces 60 Co events to 19% (6%) Hit crystals Hit segments Kevin Kröninger MPI München

Signal and background studies

Background suppression efficiency:

Source Signal 60 Co (crystal) 60 Co (cable) 208 Tl (crystal) 208 Tl (cable) 68 Ge (crystal) 210 Pb (crystal) 1 crystal 0.96

0.19

0.28

0.18

0.24

0.22

1 1 crystal AND signal window 0.92

3.0 · 10 -4 1.7 · 10 -4 2.4 · 10 -4 2.2 · 10 -4 9.8 · 10 -4 0 1 segment 0.89

0.06

0.14

0.06

0.12

0.05

9.9 · 10 -3 1 segment AND signal window 0.86

2.6 · 10 -5 9.6 · 10 -6 5 · 10 -5 8 · 10 -5 1.2 · 10 -4 0 Number of events 100k 1 M 1 M 1 M 1 M 1 M 10k Segmentation: 6 (phi) x 3 (z) Threshold: 10 keV; Energy window: Q bb ± 5 keV Pulse shape analysis and pattern recognition not included Kevin Kröninger - MPI München

MPI Munich MC activities

 Maintenance of a common CVS server for MaGe  Background and signal studies /background suppression Segmentation studies  Update of geometry: crystals and support structure

Future tasks:

Pulse shape analysis (incl. MC) Test facility for Ge-crystals (incl. MC) Kevin Kröninger - MPI München

Other background calculations

Background from

inner tank envelope

: direct simulation of g transportation signal window: 1800  2300 keV Cu: 25 · 10 -6 Bq/kg of 232 Th Fe: 20 · 10 -3 Bq/kg of 232 Th (c/keV kg y) Center 50 cm below center Cu 10 -4 1.2 · 10 -4 Fe (neck) 1.1 · 10 -4 2 · 10 -5

10 -3 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: 0.0625 cm -2 s -1 Water shielding: 300 cm in the cylindrical part 200 cm above and below A. Klimenko – INR, ITEP, Dubna, MPIK 6.6 · 10 -6 c/keV kg y 1-2 · 10 -4 c/keV kg y

2

Other background calculations

1 10 1 0,1 0,01 1E-3 1E-4 1E-5 1E-6 upper part cylindrical part lower part 3 Cylindrical part Upper spherical part Bottom flat part Open neck Neck with 10cm Pb Neck with 15cm Pb Cts/keV kg y 6.6 · 10 -6 1.1 · 10 -4 2.0 · 10 -4

1.1 · 10 -2

1.1 · 10 -4 1.1 · 10 -5 0 50 100 150 200 250 300 S,water thickness, cm To go 1 – Cylindrical part 2 - Upper spherical part

lower than 10 -5 c/keV kg y

: bottom part: 7 cm of Pb 3 – Bottom part upper part: 6 cm of Pb cylindrical part: no further shielding needed neck : 15 cm of Pb Cu tank: LAr is required A. Klimenko – INR, ITEP, Dubna, MPIK

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 Preliminary results of m -induced and n background Top m -veto enough for background of a few ·10 -4 c/kg keV y Neutrons, CNGS and isotopes production presumably not critical  First results of signal and bck in crystals & cables  Estimation of external g 10 -4 background and shielding c/kg keV achievable with present shielding, 10 -5 needs LAr 3-month activity and still a lot of work to do in the future...

...Well begun is half done !