GEMC Hit and Time Window

Hit Definition

The definition of a hit is tied up to the detector electronics, in particular its Time Window (TW), defined by the user (notice the TW is fixed and never changes during the simulation).

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