MaGe framework for Monte Carlo simulations

1
MaGe framework for Monte Carlo simulations
MaGe is a Geant4-based Monte Carlo simulation
package dedicated to experiments searching for
0n2b decay of 76Ge (and low-background
experiments in general). It is developed jointly
by the Majorana and GERDA Monte Carlo groups

• can be run by script and is flexible for
  experiment-specific implementation of geometry
  and output
• allows a more extensive validation of the
  simulation with experimental data coming from
  both experiments ? also Geant4 validation

• avoids duplication of the work for the common
  parts of the simulations (generators, physics,
  materials, management)
• can provide the complete simulation chain
  (including pulse shape)

arXiv0802.0860
2
MaGe/GERDA applications
GERDA geometry in MaGe
Top muon veto
MaGe includes the whole simulation chain
(generator, physics processes, material,
management, etc.)
Neck
Water
Description of the Gerda setup including
shielding (water tank, stainless steel cryostat,
copper lining, cryogenic liquid), crystals array
and suspension system
Water tank
Cryostat
MaGe used for background and sensitivity studies
in GERDA and for design optimization
Detector array
3
MaGe/GERDA applications
MaGe used for the simulation of the main GERDA
setup and of many GERDA-related test stands
GDL test stand
Gerda array
Phase I detector
4
Muon-induced background
Prompt background
10 m
e and g-rays from electromagnetic showers,
g-rays from neutron inelastic interactions or
captures Reduced by anticoincidence or
segmentation (Phase II) and muon veto.
Background reduction depends essentially on the
veto efficiency only
Delayed background
Production of long-lived unstable isotopes in the
crystals or in the surrounding material ? veto
not effective Reduced by multiplicity or
segmentation. Delayed coincidence cuts
m
5
Prompt muon-induced background
Energy spectrum in the detectors
Anti-coincidence factor from 15 (Phase I) to 25
(Phase II) Segmentation extra factor of two
(Phase II)
(EM showers)
(Reduction by PS discrimination not considered!)
Cherenkov muon veto required !
With 70 8 PMTs and VM2000 foils a veto
efficiency gt95 for dangerous muons (those that
can possibly give a fake signal) can be achieved
? 310-5 counts/(keV kg y)
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Isotopes production rate
Actual background depends on production rate,
location and decay scheme. Isotopes produced in
water and cryostat lt 10-6 cts/(keVkgy)
Isotope
Isotope nucl/(kgy) cts/(keVkgy) (no cuts)
74Ga/75Ga/76Ga lt 0.1 lt 410-5
68Ge 0.08 510-6
69Ge 1.8 510-6
77Ge/77mGe 0.51 1.110-4
38Cl 46 day-1 3.310-5
40Cl 2.7 day-1 410-6
38Cl and 40Cl reduced below 10-5 cts/(keVkgy)
by segmentation cuts
Main contribution from 77mGe (thermal neutron
capture, Qb2.862 MeV, T1/253 s) ? up to 10-4
cts/(keVkgy) Background can be reduced by at
least a factor of two by a dedicate time cut
(taking into account primary m, prompt g-rays
produced in capture and delayed b decay of 77mGe)