To see a list of the available Geant4 constructors:
gemc -showPhysics
Generator
The default kinematics is a 2 GeV proton generated along the z-axis just before the target.
A beam size of 0.1 cm is used.
This is defined in the YAML file:
The gparticle option allows to control the Geant4 particle gun.
For the complete list of parameters that can be passed to it: gemc help gparticle.
Some of them:
name: Particle name (mandatory), for example “proton”.
multiplicity: How many copies of this particle will be generated in each event.
notice that the copies are not identical if some additional parameters are specified,
for example delta_p, delta_theta.
p: Particle momentum.
delta_p: Particle momentum range, centered on p.
theta: Particle polar angle.
delta_theta: Particle polar angle range, centered on theta. D
phi: Particle azimuthal angle.
For example, to define a particle gun with one electron along z plus 1 proton at theta=30,phi=90 degrees, use
The FluxPlane is associated with the flux digitization (one of the available GEMC pre-built routines)
in simple_flux.py, with identifier flux_plane = 1.
The flux digitization collects all G4Steps into hits by using the track id: all steps within a
sensitive element produced by the same track define a single hit. Different tracks’ steps in the
same sensitive element are collected in separate hits.
This digitization can be used to count and analyze the tracks passing through a volume.
Flux hit definition example: three tracks goes through two sensitive cells.
Track 1 creates two hits: < cell2, hit #1 > and < cell1, hit #1 >.
Track 2 creates two hits: < cell1, hit #2 > and < cell2, hit #2 >.
Track 3 creates one hit: < cell2, hit #3 >.
The DIGITIZED VARIABLES below, resulting from the flux digitization, are stored in the output.
the sensitive element identifier, assigned by the user. For example
sector
layer
hitn: hit number
pid: particle id
tid: track id
E: track energy
time: time of the hit
totEdep: total energy deposited by the track
In this case, the identifier contains one name: flux_plane.
In addition to the digitized variables, the true information is saved on the output stream.
True Information
The true information information is processed by GEMC and in certain instances
averaged (weighted by the energy deposition if present); in others,
like pid, tid or processName, the data come from the first step in the hit.
The complete list of variables are:
the sensitive element identifier, assigned by the user. For example
sector
layer
hitn: hit number
pid: particle id
tid: track id
avgTime: average time
avglx: average local x position
avgly: average local y position
avglz: average local z position
avgx: average global x position
avgy: average global y position
avgz: average global z position
totalEDeposited: total energy deposited
processName: process name creating the first step of this hit
Usage
Building the detector
Use the Python script simple_flux.py to build the detector. By default, the setup is stored in a SQLite file
named gemc.db. Command-line options can define the database type, variations, and run number.
python API
Pass -h for additional command line options:
options:
-h, --help show this help message and exit
-f, --factory FACTORY
ascii, sqlite
-v, --variation VARIATION
Set variation name
-r, --run RUN Set run number
-sql, --dbhost DBHOST
SQLite filename or MYSQL host
-pv, --pyvista Show geometry using pyvista (needs pyvista)
-pvb, --pvb, --pyvista-background
Use PyVista BackgroundPlotter (needs pyqt6 pyvistaqt)
-pvw, --width WIDTH Set plotter width
-pvh, --height HEIGHT
Set plotter height
-pvx, --x X Set plotter x position
-pvy, --y Y Set plotter y position
-axes, --add_axes_at_zero
If you have pyvista (see also install pyvista),
you can use the -pv and -pvb options to display the setup without having to run GEMC
Running gemc
The file simple_flux.yaml can be used to run the setup. Add -gui to run interactively:
gemc simple_flux.yaml -gui
Modify simple_flux.yaml as needed, in particular to add particles, control the number of threads, or change the output.
Output
The gstreamer option is used to select the name and format of the output. Two simultaneous streams are selected,
ROOT and CSV:
Because flux is a per-event digitization, GEMC will produce one output file per thread.
For ROOT files, you can use hadd to merge the files.
gstreamer
The gstreamer option allows select the name and format of the output.
Run gemc help gstreamer to check its documentation:
-gstreamer=<sequence> ......: define a gstreamer output
• filename: name of output file. Default value: NODFLT
• format: format of output file. Default value: NODFLT
• type: type of output fileDefault value: event
Define output formats and filenames.
It can be used to select <events> or <frame> streams.
The file extension is added automatically based on the format.
Supported formats:
- jlabsro
- root
- ascii
- csv
- json
Output types:
- event: write events
- stream: write frame time snapshots
Example that defines two gstreamer outputs:
-gstreamer="[{format: root, filename: out}, {format: csv, filename: out}]"
The produced files structure depends on the accumulation method used:
- event-based digitization (like <flux>) will have one file per
thread, with "_t<thread#>" appended to the filename
- run-based digitization (like <dosimeter>) will have one output file
Plotting with the GEMC Analyzer
Run GEMC with 10,000 events first. The default YAML file writes simple_flux_t0_digitized.csv and simple_flux_t0_true_info.csv.
