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simple_detector.md 7.45 KiB 
title: "Simple Detector"

Setup

Note all these commands assume you are in an eic-shell singularity session.

git clone https://eicweb.phy.anl.gov/EIC/tutorials/ip6_tutorial_1.git part1
cd part1

Inspect files

Look around see what's what. The files compact/materials.xml, compact/elements.xml, and gem_tracker.xml are the compact detector description files. The first two files are included in the main gem_tracker.xml file. Note these files are often found in directories named compact (but not always).

Compile the detector library

mkdir build
cmake ../. -DCMAKE_INSTALL_PREFIX=../../local 
make -j4 install

Visualize and Check Geometry

Look at disk tracker

dd_web_display --export gem_tracker.xml

Copy the generated file detector_geometry.root to your local machine. Then open the web viewer, click on the ... and open the local root file.

simple gem tracker

Examine the compact description, specifically this part:

<detector id="2" name="GEMTracker" vis="RedVis" type="my_GEMTracker" readout="GEMTrackerHits" > 
  <layer id="1" z="-100 *cm" inner_r="40*cm"  outer_r="120*cm"   phi0_offset="0.0*deg" />
  <layer id="2" z="-80 *cm"  inner_r="30*cm"  outer_r="90*cm"    phi0_offset="0.0*deg" />
  <layer id="3" z="-60 *cm"  inner_r="20*cm"  outer_r="70*cm"    phi0_offset="0.0*deg" />
  <layer id="4" z="-40 *cm"  inner_r="10*cm"  outer_r="20.0*cm"  phi0_offset="0.0*deg" />
  <layer id="5" z=" 40 *cm"  inner_r="10*cm"  outer_r="20.0*cm" phi0_offset="0.0*deg" />
  <layer id="6" z=" 60 *cm"  inner_r="25*cm"  outer_r="70.0*cm" phi0_offset="0.0*deg" />
  <layer id="7" z=" 80 *cm"  inner_r="30*cm"  outer_r="90.0*cm" phi0_offset="0.0*deg" />
  <layer id="8" z="100 *cm"  inner_r="40*cm"  outer_r="100.0*cm" phi0_offset="0.0*deg" />
</detector>

Check for overlaps

checkOverlaps -t 0.0001 -c gem_tracker.xml
...
Info in <TGeoNodeMatrix::CheckOverlaps>: Checking overlaps for world_volume and daughters within 0.0001
Check overlaps:      [==========]   11 [100.00 %]           00:00           
Info in <TGeoNodeMatrix::CheckOverlaps>: Number of illegal overlaps/extrusions : 0

Look at all the constants

Look at the constants as defined in the compact description (xml files).

npdet_info dump gem_tracker.xml

This should output something like this:

CrossingAngle                  =        0.020 = 0.020*rad
ForwardTrackerPlane_z0         =      400.000 = 400*cm
Place_Center                   =        0.000 = 0*cm
compact_checksum               = 1924675351.000 = 1924675351
tracker_region_rmax            =      200.000 = 2.0*m
tracker_region_zmax            =      400.000 = 4.0*m
world_side                     =     1000.000 = 10*m
world_x                        =     1000.000 = world_side
world_y                        =     1000.000 = world_side
world_z                        =    10000.000 = 10*world_side

Or if there are too many constants to dump, try searching:

npdet_info search world --all gem_tracker.xml
world_side                     =     1000.000 = 10*m
world_x                        =     1000.000 = world_side
world_y                        =     1000.000 = world_side
world_z                        =    10000.000 = 10*world_side

This is very useful with a good subsystem naming convention.

Running Geant4 simulation

npsim is a clone of DD4hep's ddsim but has a different output plugin.

npsim  --runType run  --enableG4GPS \
   --macroFile gps.mac \
   --compactFile ./gem_tracker.xml \
   --outputFile gem_tracker_sim.root

This will run geant4 using the General Particle Source (GPS) tool. The output will be a root file containing the generated events and GEM tracker hits. Look at gps.mac.

Refer to the GPS Documentation for more setup possibilities.

Using detector description in analysis

Preface: data model

First, the geant4 output data model used by npsim is described in a single yaml file. Note that this is purposefully factorized from the larger EIC data model, eicd, which is used at every step post geant4.

https://eic.phy.anl.gov/eicd/

Look at the generated file in results: local hit position

This is the local position in the segmentation (i.e. pixel, strip, readout pad, etc.).

Cell ID (channel) to Position

At the top of the script you can see these lines:

  dd4hep::Detector& detector = dd4hep::Detector::getInstance();
  detector.fromCompact("gem_tracker.xml");
  dd4hep::rec::CellIDPositionConverter cellid_converter(detector);

This

Later on you can see its use:

  auto pos1 = cellid_converter.position(h.cellID);

Cell size

root -b -q scripts/tutorial2_cell_size.cxx+

You will see this:

...
Segmentation-Cell Position : 39,102,-100
 dim 1, 3,

Id Specification

root -b -q scripts/tutorial3_id_spec.cxx+
  fmt::print("--------------------------\n");
  fmt::print("ID specification:\n");
  auto decoder = detector.readout("GEMTrackerHits").idSpec().decoder();
  fmt::print("{}\n", decoder->fieldDescription());
  auto layer_index = decoder->index("layer");
  fmt::print(" \"layer\" index is {}.\n", layer_index);

See BitFieldCoder documentation documentation for more details.

Later in the loop over event hits.

      auto detector_layer = decoder->get(h.cellID, layer_index);
      if ((detector_layer !=4 ) && (detector_layer !=5 )) {
        continue;
      }

Note the layer_index is computed once in the beginning when the decoder is also initialized. This means it will be fast because it only parses the ID specification string once.

ID specification:
system:0:5,barrel:5:3,layer:8:4,module:12:5,r:32:-16,phi:48:-16
 "layer" index is 2.

or

ID specification:
system:0:8,barrel:8:2,layer:10:4,module:14:12,sensor:26:2,x:32:-16,y:48:-16
 "layer" index is 2.

References