MaGe framework for Monte Carlo simulations

MaGe is a Geant4-based Monte Carlo simulation package dedicated to experiments searching for 02 decay of 76Ge

(and low-background experiments in general). It is developed jointly by the Majorana and GERDA Monte Carlo

groups 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)

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

arXiv:0802.0860

MaGe/GERDA applications

Top muon veto

Neck

Cryostat

Water

Water tank

Detector array

GERDA geometry in MaGe

MaGe used for background and

sensitivity studies in GERDA and for design

optimization

Description of the Gerda setup including shielding

(water tank, stainless steel cryostat, copper lining,

cryogenic liquid), crystals array and suspension

system

MaGe includes the whole simulation chain

(generator, physics processes, material, management, etc.)

MaGe/GERDA applications

MaGe used for the simulation of the main GERDA setup and of many GERDA-related test stands

Siegfried 2

GDL test stand

Gerda array

Phase I detector

Muon-induced background

e± and -rays from electromagnetic showers, -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

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

Prompt background:

Delayed background:

10 m

(Reduction by PS discrimination not

considered!)

Prompt muon-induced backgroundEnergy spectrum in the

detectors

Anti-coincidence: factor from 15 (Phase I) to 25

(Phase II)

Segmentation: extra factor of two (Phase II)

Cherenkov muon veto required !

With 70 8” PMTs and VM2000 foils a veto efficiency >95% for “dangerous” muons (those that can possibly

give a fake signal) can be achieved ~3·10-5

counts/(keV kg y)

crystal anticoincidence

segmentationcounts

/(k

eV

·kg·y

)

goal

Energy (keV)

Qno cut

(EM showers)

Isotopes production rate

Isotope

nucl/(kg·y) cts/(keV·kg·y)(no cuts)

74Ga/75Ga/76Ga < 0.1 < 4·10-5

68Ge 0.08 5·10-6

69Ge 1.8 5·10-6

77Ge/77mGe 0.51 1.1·10-4

38Cl 46 day-1 3.3·10-5

40Cl 2.7 day-1 4·10-6

Actual background depends on:

production rate, location and decay scheme. Isotopes

produced in water and cryostat < 10-6

cts/(keV·kg·y)38Cl and 40Cl reduced

below 10-5

cts/(keV·kg·y) by segmentation cuts

Main contribution from 77mGe (thermal neutron capture, Q=2.862 MeV, T1/2=53 s) up to 10-4 cts/(keV·kg·y)

Background can be reduced by at least a factor of two by a dedicate time cut (taking into account: primary , prompt -

rays produced in capture and delayed decay of 77mGe)