Geant 4: Hadronic Shower Development Simulation

The simulation of the interaction of the incident particles with the detector matter was performed with Geant4 (cf. subsection 3.6.2).

Geometry

For that a detailed model of the CALICE W-AHCal geometry as it was at the testbeam was put into Geant4. The dimensions and the material budget for a single layer of

the CALICE W-AHCal and the T3B experiment are shown in Figure 4.13. These plots show the used materials and their corresponding width plotted against their nuclear interaction length (full line) and their radiation length (dashed line). A single CALICE W-AHCal layer sums up to a total of 24.5 mm, which corresponds to 0.13λI or 2.97 X0.

Thus the entire detector of 38 layers has a depth of 931 mm, 4.9λI or 112.9 X0. The

material budget of the T3B layer on the other hand is negligible with a depth of 13 mm and a corresponding 0.02λI and 0.06 X0. Between the CALICE W-AHCal and the

W-absorber or W-AHCal cassette) and the missing absorber of the T3B layer. At the testbeam a steel/scintillator tailcatcher was installed downstream of the T3B layer. Albeit very seldom, it can cause neutrons to back scatter to the T3B layer. Thus in the simulation a steel block with a depth of 50 cm was installed after an air gap of 9 cm.

The geometry alongside with an example event originating from the interaction of a 60 GeV pion is shown in Figure 4.14. The pion is incident from the left, first interacting with the 38 sandwich layers of the CALICE W-AHCal (absorber in gray, scintillator in cyan) and forming a hadronic shower before passing through the T3B layer (blue) and finally the tailcatcher steel block (orange). The tracks of the individual particles are shown as well, with the pions shown in red, the protons in magenta and the electromagnetic component (photons, electrons) in blue. The neutrons tracks are drawn in green. One can see that the first hard interaction starts about 10 layers in the W-AHCal, which corresponds to 1.3λI or 1.1λπ. The resulting low energetic neutrons

perform a random walk and do in fact also back scatter from the tailcatcher towards the T3B layer.

Shower Start Definition

The shower starting point is defined as the first hard interaction of the incident particle with the detector material. As we have full information on all steps, it is defined as the position where the particle is destroyed and converted into any kind of daughter particles.

This definition provides a more accurate shower starting position than the one from the Primary Track Finder in the CALICE W-AHCal (cf. subsection 3.5.6). However, as the former one is not available for data for obvious reasons, the different accuracies in the shower starting point identification can lead to undesired systematic errors in the analysis when comparing data with simulation. The obvious solution would be to use the Primary Track Finder both for simulation and for data. However, for technical reasons this was not possible. Instead the resolution of the Primary Track Finder was successfully reproduced by smearing the exact shower starting position with a Gaussian of σ= 2.0 layers.

Physics lists

As it was already discussed in subsection 3.6.2, there are different models describing the possible physics processes responsible for the shower development. However, in

Geant4 version 9.4p03, which was used for the simulation, only two physics lists

provide the high precision neutron tracking (HP). This high precision neutron tracking is expected to be of importance for the proper description of the late neutron component in tungsten absorber. The two physics lists that include it areQBBC and QGSP BERT HP.

QGSP BERT is a version of the latter without the high precision neutron tracking. It was used for a long time for the mass production at the LHC experiments CMS and ATLAS. It, too, was used for the six benchmark processes of the CLIC Conceptional Design Report, one of which is presented in chapter 5. ThusQGSP BERT is included here to see the effect of the high precision neutron tracking code.

0 , 43.1 mX I λ 0.5 mm, 2.9 m Steel Support 0 , 2564.1 mX I λ 10.0 mm, 92.5 m W Absorber 0 , 0.0 mX I λ 1.2 mm, 0.0 m Air Gap 0 , 172.4 mX I λ 2.0 mm, 11.8 m Steel Cassette 0 , 5.7 mX I λ 1.0 mm, 2.1 m PCB 0 , 0.7 mX I λ 1.5 mm, 0.2 m Cable-Fibre-Mix 0 , 12.1 mX I λ 5.0 mm, 7.3 m Scintillator 0 , 172.4 mX I λ 2.0 mm, 11.8 m Steel Cassette 0 , 0.0 mX I λ 1.2 mm, 0.0 m Air Gap 24.50 mm Length [mm] 0 2.971 X I λ 0.129

Radiation / Nucl. Interaction Length

Material Budget: CALICE Layer

I

λ

Nucl. Interaction Length 0 Radiation Length X (a) CALICE 0 , 11.2 mX I λ 1.0 mm, 2.5 m Al Cassette 0 , 0.0 mX I λ 2.3 mm, 0.0 m Air Gap 0 , 12.1 mX I λ 5.0 mm, 7.3 m Scintillator 0 , 0.0 mX I λ 1.0 mm, 0.0 m Air Gap 0 , 9.7 mX I λ 1.7 mm, 3.5 m PCB 0 , 22.5 mX I λ 2.0 mm, 5.0 m Al Cassette 13.00 mm Length [mm] 0 0.056 X I λ 0.018

Radiation / Nucl. Interaction Length

Material Budget: T3B

I

λ

Nucl. Interaction Length

0

Radiation Length X

(b) T3B

Figure 4.13: The material budget of a single layer of the CALICE W-AHCal and the T3B layer as implemented into the Geant4 simulation.

Figure 4.14: An example of a simulated 60 GeV pion event. The particle is incident from the left. The geometry consists of first the 38 CALICE W-AHCal layers, of which the cassette which includes the scintillator is shown in cyan and the tungsten absorber is shown in grey. After that the single T3B layer is shown in blue before the steel block simulating the tailcatcher is shown in orange. Particle tracks are shown in red (pion), magenta (proton), green (neutron) and blue (electron, photon).

Simulation Detail and Storage

Geant4 simulates the interaction of particles with matter as a series of single steps,

each of which is a simulation of a certain physical process. The time and the amount of deposited energy are available for each these steps. In the present case this information was saved into a ROOT tree for all steps that occurred within the T3B layer. Conversion into photons or other detector and electronic specific effects are not considered in this phase. However, Birks’ Law [71], which describes the non linearity in photon conversion in scintillators, is already included at this point by artificially modifying the amplitude of the deposited energy of the step.