Simulation program VENOM

The full energy peak (FEP) or total efficiency (compare Section 2.3.1) for radiation emitted by a sample can often only be measured at great expenses. This holds especially for complex germanium detector systems like the DLB (described in Chapter 3). Due to the complexity of the physical processes of interaction of the different types of radiation with matter it is as well not possible to calculate these efficiencies analytically. Therefore, often computer programs are used that de-scribe the different interactions by models and choose the types of interaction occurring at a certain point in the geometry by the parametrised probabilities for these.

The simulation program used in this work is called VENOM, since its develop-ment started in 2005 by scientists working at the COBRA experidevelop-ment. It is based on the Geant4 MC framework [AAA⁺03] for the propagation of different types of particles through matter. Similar to the analysis software GeAna described in Section 4.4, which uses the classes and methods provided by ROOT, VENOM makes use of the many particles, geometry interfaces, interaction models, algo-rithms, etc. provided by Geant4. Geant4 was initially designed for high energy physics simulations. VENOM therefore uses the low energy extensions and pro-vides many interfaces to generate different particles or decays at a certain loca-tion within the implemented geometries. Furthermore it handles the informaloca-tion produced during the tracking and interactions of particles by Geant4, extracts relevant data and allows to store these to structured ROOT files. By the different detector types provided by VENOM, for example only basic information like the energy deposited in a HPGe or CdZnTe crystal or more complex information like the hit voxel of a depth sensing pixel detector can be stored to the output files.

At the begin of this work VENOM was still designed for an old Geant4 version (~9.0), which did not support the usage of GDML geometries (see also Chapter 5).

Together with T. Köttig, VENOM was adapted to the new methods and physics

models provided by the newer Geant4 versions. VENOM was first adapted to function with Geant4 9.3 and during the time also upgraded to work with the releases 9.4 and 9.5. Geant4 does only provide methods for the import and ex-port of GDML files but does not provide any methods to influence the data read-out from certain volumes of the geometry during the propagation of particles through it. T. Köttig extended the functionalities of VENOM and added many sophisticated methods for the adaptation of the system to many different simula-tion purposes. He implemented methods to assign a sensitivity (the Geant4 term for marking the information of interactions as relevant) to different volumes of an imported GDML geometry. As geometries of a detector set-up can be quite large, the precise tracking of particles in volumes far away from a sensitive volume (for example the detector) are usually only of limited relevance. Since the simulation of charged particles needs a lot of computing time, the reduction of the simula-tion mainly to regions close to the detector is desirable. In Geant4 it is possible to assign so called UserCuts in form of a remaining range or energy of a tracked particle to the different volumes of the geometry. T. Köttig therefore implemented several commands to set these values on a per volume basis and apply these to different selectable particles. These huge efforts, which are in detail described by him in [Köt12], made the simulation studies in Chapter 5 possible in the first place.

In case of complex geometries it can be inconvenient to set tracking cuts for ev-ery single volume. Therefore, an option was added to the already implemented commands to be able to apply a certain cut value to all volumes of the geometry.

The cut value can then afterwards be lowered for important areas again. In addi-tion, a command was added to set cuts for all volumes containing one material, which is useful to apply a certain cut value for example to the lead shielding of a detector but not to other components of the shield.

The simulation studies conducted by the users of VENOM cover a wide range of different processes of interest. For example in some cases the production of nuclides by cosmic particles or interaction of muons with the COBRA detectors are of interest, in others the appropriate shielding of neutrons and in others again the detection efficiency for double β decays is examined by MC studies. This wide field requires VENOM to handle many different tasks and leads to a certain size of the software package. Especially the database files of Geant4 describing the properties of many different particles, interactions and materials can have a remarkable size. The data files needed for all physics processes defined in the so called PhysicsList in VENOM are loaded to the memory of the executing PC.

For the propagation of neutrons VENOM uses the high precision neutron data (NeutronHPElasticData, etc.). The amount of available neutron data did increase with the release of Geant4 9.5 tremendously so that the required memory rose by a factor of approximately 3. For analysis and simulation tasks the working group Physik EIV runs a small computing cluster managed by the Hadoop® framework

for distributed computing and load balancing. Since the single computing nodes fetch the required Geant4 libraries and data from the distributed files system AFS each time a job is started, this slows down the upstart of VENOM significantly. In addition, some computing notes even ran out of memory with the more precise data so that simulation jobs failed. Since the propagation of neutrons is usually not necessary in simulations for the DLB, a compiler option was implemented in VENOM to switch off physics processes related to neutrons. If the option VENOM_NO_NEUTRONS=trueis specified during the compilation of the VENOM ap-plication, the described problems can be circumvented and the single simulation jobs accelerated. The ‘no neutrons’ status is written to the log contained in the simulation output so that it is possible to track this setting also in the data.

VENOM uses the PhysicsList of the dark matter experiment (DMX) example of Geant4, which uses by default the Livermore physics models for processes like Compton scattering. Since some discrepancies were found in the simulation studies described in Chapter 5, an additional compiler option was implemented to be able to use alternative physics models from the PENELOPE [SFVS11] sim-ulation package, also provided by Geant4, instead. Although it is in principle possible to implement an interface in VENOM to change the used physics mod-els at runtime, this is of limited value, since a new simulation output file should be created anyway to separate the different results. The models used by default can be replaced by the PENELOPE models by specifying VENOM_PENELOPE=true during the compilation of VENOM. This is as well logged to the output files to be able to reconstruct the settings used for the production of a simulation data set.

5. Monte Carlo simulation for full energy