# GEMC Hit¶

## Hit Definition¶

The definition of a hit is tied up to the detector electronics, in particular its Time Window (TW), defined by the user.

All geant4 steps [1] in a detector element [2] within the TW constitute a hit.

## Example¶

Shown in Fig. 1 is a schematic of two tracks and secondaries hitting two detector elements to better illustrate the hit definition. In particular:

• Track 1 has 3 (blue) steps in the first red cell and one (purple) step in the second yellow cell.
• Track 1 also has two secondaries; the first one has one step in the red cell and the second one has two steps, one in each cell.
• Track 2 has 2 steps in each cell, within the TW of the previous steps. So its steps do not create new hits, but add to the previous hits’ steps.
• All the blue steps happens within the detector TW: they constitute one hit.
• All the purple steps happens within the detector TW: they constitute one hit.
• In total, we have two hits. Notice that if the second track was out of the TW, it would generate two additional hits, with two steps each in each cell.

Figure 1: the first track generate two hits. The second track is within the TW of the first one, so it does not generate additional hits and its steps add to the previous hits’ steps.

Notice that lowering a production threshold may not affect the number of hits: there may be more secondaries produced, but these steps may all happen within the same timewindow, thus collecting in the same hit.

## True Information¶

The true information for any system systemname can be written in the output by using the INTEGRATEDRAW option. For example, to record true information for the system dc:

<option name="INTEGRATEDRAW" value="dc"/>


By default INTEGRATEDRAW is disabled.

The option SAVE_ALL_MOTHERS can be used to enable the storing of mother particle information [3] .

The complete list of all the variable is given in the table below. For quantities that can be integrated, the variable is either an average or the weighted sum of the quantity. For quantities such as pid where the integration does not make sense, the value refers to the the first particle entering the sensitive volume, or FP in the table.

 Variable Name ID Description pid 1 ID of the FP [4] mpid 2 ID of the mother of the FP tid 3 Track ID of the FP mtid 4 Track ID of the mother of the FP (if enabled) otid 5 Track ID of the ancestor that generated the FP (if enabled) trackE 6 Total energy of the FP totEdep 7 Total energy deposited (in MeV) avg_x 8 Average X position in the global reference system (in mm) avg_y 9 Average Y position in the global reference system avg_z 10 Average Z position in the global reference system avg_lx 11 Average X position in the local reference system avg_ly 12 Average Y position in the local reference system avg_lz 13 Average Z position in the local reference system px 14 x component of momentum of the FP (in MeV) py 15 y component of momentum of the FP pz 16 z component of momentum of the FP vx 17 x component of the FP’s point of origin (in mm) vy 18 y component of the FP’s point of origin vz 19 z component of the FP’s point of origin mvx 20 x component of the FP mother’s point of origin (if enabled) mvy 21 y component of the FP mother’s point of origin mvz 22 z component of the FP mother’s point of origin avg_t 23 Average time nsteps 24 Number of geant4 steps procID 25 Process that created the FP. See section below. hitn 99 Hit ID

## FLUX Detector¶

A FLUX detector is a special case of sensitive detector. The hit definition for FLUX is different than the one above:

• different tracks will produce different hits, independently of their time.

In the same detector element, all steps of the same truck will form one “integrated hit”.

In the FLUX detector, each particle produced will then produce a separate hit (thus the name FLUX), while in normal sensitive detectors all particles within the same electronic timewindow will collect in one hit.

### Setting FLUX detectors in GEMC¶

In GEMC we can have an arbitrary number of FLUX detector. Their ID is set as follows:

\$detector{"identifiers"}  = "id manual 3";


The “id” variable is part of the true information in the output.

## Process Names / ID Table¶

The link between process name and the procID variable stored in the true information bank is given in the table below.

 Process Name ID (int) e ionization 1 compton scattering 2 e bremsstrahlung 3 Photo Electric Effect 4 Gamma Conversion 5 e+ Annihilation 6 photon-Nuclear 7 electron-Nuclear 8 positron-Nuclear 9 Coulomb Scattering 10 Cherenkov 11 Hadronic Elastic 20 Hadronic bremsstrahlung 21 Hadron ionization 22 Hadron pair production 23 Proton Inelastic 30 Neutron Inelastic 31 Neutron capture 32 pi- Inelastic 40 pi+ Inelastic 41 Decay 50 Decay With Spin 51 muon ionization 60 muon pair production 61 muon bremsstrahlung 62 muon nuclear 63 kaon- Inelastic 70 kaon+ Inelastic 71 kaon0 Inelastic 72 kaon0L Inelastic 73 kaon0S Inelastic 74 alpha Inelastic 80 lambda Inelastic 90 sigma- Inelastic 100 deuteron Inelastic 110 ion ionization 120 triton Inelastic 130 na 90

Footnotes

 [1] Geant4 determines the size of a step, and the energy deposited, based on particle type, momentum, material type and density, and physics processes cross sections. In GEMC users can limit the maximum step for a given detector.
 [2] A detector element is its smallest readout. For example, a wire in a drift chamber, or a strip in a silicon tracker.
 [3] Note: SAVE_ALL_MOTHERS will force a loop on all the existing tracks. Do not use if you have hundreds of tracks in one event as it will slow down the simulation.
 [4] FP: the first particle entering the sensitive volume