CLAS12 Low-Threshold Cherenkov Counter Example

This example shows the CLAS12 Low-Threshold Cherenkov Counter (LTCC) geometry from clas12-systems. It combines native GEMC mirror optics with CAD meshes loaded through the gcad feature. Both systems are stored in one SQLite database.

Upcoming in the next release: the LTCC geometry and digitization are currently available from the clas12-systems development branch.

Quickstart

From the clas12-systems repository, build the geometry database and run a short simulation:

cd $GEMC_HOME/../clas12-systems/geometry_src/ltcc
./ltcc.py
gemc ltcc.yaml -n=1

Geometry

The geometry, shown below, is defined in geometry_src/ltcc/geometry.py and orchestrated by geometry_src/ltcc/ltcc.py. The native builders port the original clas12Tags perl scripts (mirror shapes, PMTs, and iron shields), while the reflective Winston cones and the mechanical frame are imported as CAD meshes. The native rows use the ltcc SQLite system and the mesh rows use the separate ltcc_cad CAD system. Both share the same database, variation, and run, so one gemc.db loads the complete detector.

The world (a box named root) contains, for each of the six sectors:

a gas mother wedge named ltccS<sector> holding that sector’s optics
cylindrical, hyperbolic, and elliptical mirror segments recorded with the ltcc_AlMgF2 optical surface
photomultiplier tubes named pmt_s<sector><side>_<n>, read out on the left and right of each segment
5 mm iron shields covering the outer face of the Winston cones
CAD Winston cones (WC_S, WC_M, WC_L) and frame meshes reused as copyOf placements

Interactive viewer:

Physics List

The LTCC is a Cherenkov detector, so optical photons must be produced and tracked. The YAML file selects FTFP_BERT + G4OpticalPhysics with phys_list: FTFP_BERT + G4OpticalPhysics.

phys_list

The physics list can be selected using the option
gemc -phys_list <value>
where <value> can be a combination of the Geant4 physics constructors separated by the + sign. For example
gemc -phys_list="FTFP_BERT + G4NeutronCrossSectionXS"
To see a list of the available Geant4 constructors:
gemc -showPhysics

Generator

The particle kinematics are defined in the YAML file:

gparticle:
  - name: e-
    p: 5000*MeV
    delta_p: 1*GeV
    theta: 25*deg
    delta_theta: 15*deg
    randomThetaModel: cosine
    phi: 60*deg
    delta_phi: 180*deg

See also the Internal Generator Documentation for more information.

Digitization

The photomultiplier tubes use the CLAS12-specific ltcc digitization plugin. The identifiers record the sector, the PMT side, and the mirror segment the tube reads out:

pmt.digitization = "ltcc"
pmt.set_identifier("sector", sector, "side", side_id, "segment", n)

Usage

Building the detector

Use geometry_src/ltcc/ltcc.py to build the detector. The script defines the materials and mirror surfaces, publishes the native ltcc volumes, uploads the CAD definitions for the ltcc_cad system from stls/cad__default.yaml, and adds the CAD copyOf placements. By default, both systems are stored in a SQLite file named gemc.db.

See also the Building Geometry for more information.

Running GEMC

The file ltcc.yaml can be used to run the setup. Add -gui to run interactively:

gemc ltcc.yaml -gui

Modify ltcc.yaml as needed, in particular to add particles, control the number of threads, or change the output.

Running Events

CLAS12 LTCC simulation: generated electrons crossing the low-threshold Cherenkov counter geometry.

Output

The gstreamer option selects the output filenames and formats:

gstreamer:
  - format: csv
    filename: ltcc
  - format: hipo
    filename: ltcc

See also the Output Documentation for more information.

Plotting with the GEMC Analyzer

Run GEMC with 2,000 events first. The default YAML file writes the analyzer CSV streams.

gemc ltcc.yaml -n=2000 -no_field=all -plugin_path=/opt/projects/gemc/clas12-systems/build

Plot the total energy deposited per hit:

gemc-analyzer ltcc_t0_true_info.csv totalEDeposited --kind csv --data true_info

ltcc total energy deposited per hit

Plot the y vs x hit positions:

gemc-analyzer ltcc_t0_true_info.csv --kind csv --data true_info --plot yvsx --bins 80

ltcc y vs x hit positions