MaGe: a Monte Carlo framework for the GERDA and
Majorana experiments
Luciano PandolaINFN, Laboratori Nazionali del Gran Sasso
for the MaGe development group
Geant4 WorkshopHebden Bridge, UK 13 September 2007
13 September, 2007 Geant4 Workshop – Hebden Bridge
Search for neutrinoless decay of 76Ge
0: (A,Z) (A,Z+2) + 2e-
Neutrinoless 2-decay violates the lepton
number conservation: ΔL=2
76Ge
76Se
Q = 2039 keV
Explore the Dirac/Majorana nature of neutrino and the absolute mass
scaleVery rare process:
T1/2 > 1025 y
New generation experiments require unprecedented low-
background conditions and large masses!
Two experiments with 76Ge, GERDA (Europe-Russia) and Majorana (US-Japan) have been
proposed for next generation (100 kg
scale). They will explore the feasibility of a
world-wide ton-scale 76Ge experiment
13 September, 2007 Geant4 Workshop – Hebden Bridge
Common issues in Monte Carlo’s
GERDA and Majorana have very similar requirements
and issues in terms of Monte Carlo simulations for background and sensitivity
studies
1. To provide a physics simulation package to aid in the optimal design, operation and analysis of data.
2. It must persist over the long lifetime of the experiments.
3. It must be well-maintained, documented, and robust.
4. Maintain record of results.
functionality
OO and abstraction capabilities of C++ and STL for flexibility
1. Energy deposition of particles from radioactive sources, cosmic rays, and signal sources.
2. Pulse-shape formation in crystals, different segmentation schemes, and crystal geometries.
3. Electronics.4. Shielding (neutron absorption and muon
tagging).5. Radioactive decay chains and
emissions.6. Signal: double-beta decay7. Activation in detector material.
physics
Geant4 meets all physics requirements, has OO
structure, well established
MaGe framework
13 September, 2007 Geant4 Workshop – Hebden Bridge
What’s MaGe ?
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 simulation
groupsIdea: to share a common simulation framework with an abstract set of interfaces, while each experiment adds its concrete implementations (geometry, output, etc...).
No constraints to both sides (geometry, physics, etc.) each component can be independently re-
written
The whole package can be configured and tuned by
macros without accessing the code accessible to new users
and non experts of C++
13 September, 2007 Geant4 Workshop – Hebden Bridge
MaGe block structure
Takes care of all common
parts that are not
experiment-specific
MJ output GERDA output
Event generators, description of
physics processes, properties of the
materials, management
Majorana
geometry
GERDA geomet
ryMaGe
The common CVS repository allows easy and parallel development of the code (Geant4 philosophy)
tunable and customizable by macro
Different formats supported (AIDA interfaces, ROOT, ASCII-
based)
13 September, 2007 Geant4 Workshop – Hebden Bridge
Why to develop MaGe ?
Basic documentation available, paper in preparation
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)
allows a more extensive validation of the simulation with experimental data coming from both experiments also Geant4 validation
Why MaGe?
can be run by script and is flexible for experiment-specific implementation of geometry and output
is suitable for the distributed development
13 September, 2007 Geant4 Workshop – Hebden Bridge
Some common tools in MaGe
Generators
Radioactive isotopes and 2
Cosmic ray muons
Neutrons
beam
Pencil beams
Selectable by macro
General-purpose samplersRandom sampling of the
primary position uniformly within an arbitrary volume or
surface (even of complex shape)
13 September, 2007 Geant4 Workshop – Hebden Bridge
A MaGe/GERDA applications
Top muon veto
Neck
Cryostat
Water
Water tank
Detector array
GERDA geometry in MaGe
MaGe widely used for background and
sensitivity studies in GERDA, and for
design optimization
cosmic ray muons
maximum tolerable radioactivity for detector parts
efficiency of multiplicity cuts
neutrons
NIM A 570 (2007) 149
several GERDA notes
NIM A 570 (2007) 479
in preparation
13 September, 2007 Geant4 Workshop – Hebden Bridge
MaGe/Geant4 validation for -rays - I
MaGe results compared with test-stand
experimental data
18-fold segmented n-type detector (Canberra-France)
mass: 1.6 kg
height: 70 mm
radii: 10 and 70
mm
6 segments in
3 segments in z
Max-Planck-Institut für Physik, Munich
Detector irradiated with
radioactive sources
13 September, 2007 Geant4 Workshop – Hebden Bridge
MaGe/Geant4 validation for -rays - II
60Co source: data, MC, background
Core electrode energy spectrum Occupancy of each segment
substructure
average deviation ~5%
Substructure effect is reproduced in MaGe using an effective model for drift anisotropy. DAQ efficiency also
included nucl-ex/0701005v1
13 September, 2007 Geant4 Workshop – Hebden Bridge
MaGe/Geant4 validation for -rays - III
SFL ratio between all events in a given peak and the single-
segment ones
only single-segment (=localized) events are
background for neutrinoless decay
Different -energy from radioactive
sources taken into account good agreement with
MaGe
nucl-ex/0701005v1
13 September, 2007 Geant4 Workshop – Hebden Bridge
MaGe/Geant4 validation for -rays - IV137Cs : single line at 662
keV
w/o LAr veto
w/ LAr veto
Data taken at MPIK-
Heidelberg: Ge crystal
immersed in liquid argon
Very good agreement
with MaGe for 137Cs
(spectrum and absolute
rates)
13 September, 2007 Geant4 Workshop – Hebden Bridge
Simulation of low-energy neutrons - I
AmBe source
neutron
Paraffin collimatorDetectorGERDA and
Majorana have irradiated test Ge
detectors with neutron sources. -rays produced by inelastic scattering
and radiative capture
Majorana setup
4.4 MeV from
Am-Be source
p(n,d)
13 September, 2007 Geant4 Workshop – Hebden Bridge
Simulation of low-energy neutrons - II
The general spectral shape is reproduced fairly
well
Additional problem: proper description of
the primary AmBe spectrum (neutrons and
-rays)
DataMaGe + backgroundbackground
175 keV71mGe
140 keV75mGe
198 keV71mGe
The simulation has spurious and missing
peaks (mainly related to
metastable Ge states)
Data
MaGe
13 September, 2007 Geant4 Workshop – Hebden Bridge
Radioactive contaminations - I
Cold Plate
1.1 kg Crystal
ThermalShroud
Vacuum jacket
Cold Finger
Bottom Closure
MaGe has been used for the finalization of the Majorana reference design: estimate of
background from different sources and radiopurity
requirements on detector components
Several radioactive isotopes and detector components have been
considered (2 TB of data produced)
Gross and Net Rates for Important Isotopes
Total Est. Background
(per t-y)
Background Source
Counts in ROI per t-y Counts in ROI 68Ge 60Co Germanium Gross 2.54 1.22 (100 day exp) Net 0.01 0.02 0.03
208Tl 214Bi 60Co Gross 0.12 0.03 0.26
Inner Mount Net 0.01 0.00 0.00 0.01
Gross 0.77 0.16 0.58 Cryostat
Net 0.22 0.04 0.00 0.26 Gross 2.28 0.30 0.02 Copper
Shield Net 0.64 0.06 0.00 0.70 Gross 0.18 0.04 0.34
Small Parts Net 0.02 0.01 0.00 0.03
muons cosmic activity
( ,n)
Gross 0.03 1.33 0.003
External Sources (6000 mwe)
Net 0.003 0.18 0.003 0.18
2 -decay < 0.01
TOTAL SUM 1.21
13 September, 2007 Geant4 Workshop – Hebden Bridge
Radioactive contamination - II
Experiment
MaGe
Crystal1x84x8
Cou
nts
/ ke
V /
106
deca
ys
Different segmentation schemes tested for different radioactive
sources
Segmentation successfully rejects background. Good agreement data
vs. MC60Co
13 September, 2007 Geant4 Workshop – Hebden Bridge
Surface contaminations
MaGe used to estimate background due to surface contamination in the crystals from natural
radioactivity
Example spectrum in Majorana Reference Design (222Rn to 206Pb)
13 September, 2007 Geant4 Workshop – Hebden Bridge
Ongoing development: pulse shape
A (modular) software for the simulation of pulse shapes is under joint development. It can be used in conjunction with MaGe, providing the whole chain from event generation, propagation to pulse simulation
Advantages of running with MaGe is the flexibility and existing software
infrastructure (e.g. geometry, I/O). PS simulation can be
interfaced with any other MC. Different PS implementations
are possible
Gen
erat
or
MaGe
x, y
, z, t
, dE
Pulse shape
simulation
x, y, z, t, dEAny other MC
Common interface
defined (MGDO)
13 September, 2007 Geant4 Workshop – Hebden Bridge
Conclusions
Two experiments, Majorana and GERDA, will look for 0 decay of 76Ge, with different designs
They have similar issues and requirements for MC simulation The MC groups are jointly developing since 2004 a
Geant4-based and OO Monte Carlo framework called MaGe Geant4 well established in physics, has flexible OO interfaces avoids duplication of work, easy to develop and mantain can be validated and tested more deeply and precisely includes general-purpose tools (generators, samplers) easy to use by macro (also for non-experts)
Used for several applications and background studies Simulation results compared with test stand data (
MaGe and Geant4 validation) -rays and low-energy neutrons
Interface with pulse shape generators in progress
13 September, 2007 Geant4 Workshop – Hebden Bridge
Two examples of macros
/MG/geometry/detector MJRDBasicShield
/MG/geometry/idealCoax/setDefaults
/MG/geometry/idealCoax/deadLayerOn true
/MG/geometry/idealCoax/outerDeadLayer 1
micrometer
/MG/generator/select cosmicrays
/MG/eventaction/rootschema MCEvent
/MG/geometry/detector GerdaArray
/MG/geometry/database 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 single coaxial crystal in the Majorana setup. Crystal parameters are customized
Generates events uniformly in the volume of one Ge crystal in the GERDA array. Kinematics
read from an external file
Geometry, tracking cuts, generator and output pattern selectable and tunable via macros (same executable)
No need to recompile, easy to use for non-expert people