A Geant4 [1] simulation of experiments in the Upstream Target Room (UTR) at the High Intensity γ-ray Source (HIγS) [2] of the Triangle Universities Nuclear Laboratory.

1. Description

2. Build

   2.1 Prerequisites

   2.2 Build Variables

3. Development

   3.1 Code Formatting

   3.2 Docker

4. License

5. Acknowledgments

6. References

The main program, user actions, and visualization macros of nutr are based on example B2a of Geant4 ($G4_INSTALL_DIR/share/Geant4-10.6.1/examples/basic/B2/B2a). An analysis manager was added, which is based on the examples AnaEx* of Geant4 ($G4_INSTALL_DIR/share/Geant4-10.6.1/examples/extended/analysis/AnaEx*). Some code has been adapted from a previous simulation of the UTR (utr [3]). nutr can employ the alpaca library to generate gamma-ray cascades with realistic direction-direction and polarization-direction correlations.

• Geant4 11.0 and its prerequisites. It is assumed that Geant4 is installed in G4_INSTALL_DIR. If the low-energy nuclear data (LEND) for Geant4 have been downloaded and the G4LENDDATA environment variable has been set, then nutr can automatically use them.
• A compiler that supports the C++20 standard (nutr uses 'designated initializers' to initialize detector properties in a transparent way, and that is a C++20 feature. See ${NUTR_SOURCE_DIR}/include/detectors/HPGe_Collection.hh, for example).
• boost.program_options from the boost C++ libraries
• ROOT 6 is optional, but since the output is written in the ROOT format by default, it is highly recommended.
• Doxygen and its requirements for typesetting LaTeX formulas (optional)
• alpaca (version >= 0.9.0) to use the angcorr primary generator. Installed automatically if not found.

In the following, the directory where the top-level CMakeLists.txt is located in a downloaded version of nutr is denoted as NUTR_SOURCE_DIR. In a build directory NUTR_BUILD_DIR, execute:

$ cmake $NUTR_SOURCE_DIR
$ cmake --build .

This creates an executable nutr_GEOMETRY in NUTR_BUILD_DIR/GEOMETRY for each detector geometry GEOMETRY that was selected via CMake build variable (see 2.2 Build Variables). By default, executables for all geometries will be built.

If nutr_GEOMETRY is called without any command-line options, it will try to launch the interactive visualization

In most cases, a user will want to supply a macro file MACRO to the simulation, which is done by typing:

$ nutr_GEOMETRY --macro MACRO

For an overview of all available command-line options, execute

$ nutr_GEOMETRY --help

Alternatively, it is also possible to supply commands using a pipe:

$ cat MACRO | nutr_GEOMETRY

After the first build step, several CMake build variables will be available for a customization of the build. Besides the usual Geant4 build variables, nutr provides the following options:

• BUILD_DOCUMENTATION: Create the code documentation using Doxygen (default: OFF).
• PRIMARY_GENERATOR_DIR: Select directory in $NUTR_SOURCE_DIR/src/fundamentals/primary_generator that contains the desired primary generator Possible choices: gps (default), angcorr.
• PRODUCTION_CUT_LOW_KEV: Set the lower energy limit of the production cut for gammas, electrons/positrons and protons in keV (default: "0.99", i.e. use default production cut of G4EmLivermorePolarizedPhysics). A straightforward way to view the current production cuts is the /run/particle/dumpCutValues macro command.
• SENSITIVE_DETECTOR_DIR: Select directory in $NUTR_SOURCE_DIR/src/sensitive_detector that contains the desired sensitive detector. Possible choices: edep, event (default), flux, tracker.
• UPDATE_FREQUENCY: Determine the number of events since the last update after which a new update about the progress of the simulation is printed on the command line (default: 10000).
• USE_HADRON_PHYSICS: Include hadron physics lists (default: ON). Excluding hadron physics can speed up the startup of the simulation. This is useful, for example, when a user only wants to visualize the geometry. It might speed up the actual simulation as well, but, of course, sometimes hadron interactions cannot be neglected.
• WITH_GEANT4_UIVIS: Build nutr with Geant4 UI and Vis drivers (default: ON).

In addition, there is an option

• GEOMETRY_*: Build the given geometry (default: ON).

for each implemented geometry.

All code (all .hh and .cc files) of nutr is formatted using the tool clang-format. The standard "LLVM" format option is used without any modifications for simplicity. This style is defined in NUTR_SOURCE_DIR/.clang-format.

In order to simplify the code formatting, a pre-commit hook has been defined in NUTR_SOURCE_DIR/.pre-commit-config.yaml. The pre-commit formatting can be applied manually by typing

$ pre-commit run --all-files

in the source directory NUTR_SOURCE_DIR.

A docker can be built from the source code using NUTR_SOURCE_DIR/Dockerfile. At the moment, the build process in the Dockerfile includes downloading Geant4 from its public git repository and building it from scratch. This is by far the most time-consuming part of the containerization. The docker build can be triggered by typing

$ docker build .

in NUTR_SOURCE_DIR (i.e. in the directory where the Dockerfile is). Besides the system files of the ubuntu operating system on which the docker image is based, it contains the following directories:

• /opt: Geant4 source code (/opt/geant4) and build directory (/opt/geant4/build).
• /work: nutr source code (/work is equivalent to NUTR_SOURCE_DIR) and build directory (/work/build)
• /output: Directory for output of the nutr simulation, intended to be mounted on the local machine using the -v or --mount flags of docker. See below for instructions on how to mount it.

In order to run an executable nutr_GEOMETRY with a macro file MACRO (at the moment, macro file must be in the docker image) from an image IMAGE and store the output in a local directory LOCAL_OUTPUT_DIR (if no local output directory is mounted, the simulation results will be lost on exit), execute

$ docker run -v LOCAL_OUTPUT_DIR:/output IMAGE /work/build/GEOMETRY/nutr_GEOMETRY --macro MACRO

This program is free software: you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation, either version 3 of the License, or (at your option) any later version.

This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.

You should have received a copy of the GNU General Public License along with this program. If not, see https://www.gnu.org/licenses/.

This product includes software developed by Members of the Geant4 Collaboration http://cern.ch/geant4.

Copyright (C) 2020-2022 Udo Friman-Gayer ([email protected]) and Oliver Papst ([email protected])

The authors would like to thank Ethan McConnell for help with implementing geometry and Jörn Kleemann for helpful discussions.

[1] S. Agostinelli et al., “GEANT4 - a simulation toolkit”, Nucl. Inst. Meth. A 506, 250-303 (2003). doi:10.1016/S0168-9002(03)01368-8; J. Allison et al., “GEANT4 developments and applications”, IEEE T. Nucl. Sci., 53, 270-278 (2006). doi:10.1109/TNS.2006.869826; J. Allison et al., “Recent developments in GEANT4”, Nucl. Inst. Meth. A 835, 186-225 (2016). doi:10.1016/j.nima.2016.06.125

[2] H. R. Weller et al., “Research opportunities at the upgraded HIγS facility”, Prog. Part. Nucl. Phys. 62, 257-303 (2009). doi:10.1016/j.ppnp.2008.07.001

[3] U. Friman-Gayer, J. Kleemann, and O. Papst, “GEANT4 simulation of the Upstream Target Room (UTR) at the HIγS facility“ (2019) doi:10.5281/zenodo.3430154

See also NUTR_SOURCE_DIR/bibliography.bib.