Software/Geant4/Tutorials/Basic/Monoenergetic Photon Pencil Beam

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Monoenergetic Photon Pencil Beam

This example shows the dose distribution in water along the incident photon beam. The beam hits the water cube surface and deposits a dose under the surface of the water. The volume of the water cube is divided into slices perpendicular to the incident beam. At each slice the deposited dose and energy is computed.

The slices are created using class G4PVReplica. The energy and dose are scored using classes G4UserSteppingAction and G4UserRunAction. Photons are generated using G4ParticleGun class. There is an option to chose among several EM physics lists.

PhotonPB g4 00 6000e.gif

The image shows the water box divided into slices using class G4PVReplica. Photons are in green, electrons are in red.

Running the Tutorial

All the source files for the tutorial can be found in

Connect to the HEP cluster and create folder PhotonPBFolder in your area

ssh -X's password: type your password here
[username@plus1 ~]$ mkdir PhotonPBFolder 

[username@plus1 ~]$ cd PhotonPBFolder  

Setup your environment

[username@plus1 PhotonPBFolder]$ source /unix/pbt/software/scripts/

Copy the code to your working directory and rename it

[username@plus1 PhotonPBFolder]$ cp -r /unix/pbt/tutorials/basic/PhotonPB .
[username@plus1 PhotonPBFolder]$ mv PhotonPB PhotonPB_source

Inside /home/username/PhotonPBFolder/ create a directory

[username@plus1 PhotonPBFolder]$ mkdir PhotonPB_build  

Compile the code with cmake and make

[username@plus1 PhotonPBFolder]$ cd PhotonPB_build 

[username@plus1 PhotonPB_build]$ cmake -DGeant4_DIR=/unix/pbt/software/dev /home/username/PhotonPBFolder/PhotonPB_source 

[username@plus1 PhotonPB_build]$ make  

Run macro gamma.mac

[username@plus1 PhotonPB_build]$ ./photonPB gamma.mac

Analysing the Data

The macro produces root file Gamma.root with a histogram showing the energy deposition in water box along the beam line. It also produces text files: DoseFile.txt with energy and dose deposited in each slice and PlotDose.txt with dose deposited in each slice.

Text files

This is output from DoseFile.txt with physics process emstandard_opt0 and incident photon energy of 20 MeV. Use your favorite editor pico, vi, emacs etc. to open text files. For example, use editor pico:

[username@plus1 PhotonPB_build]$ pico DoseFile.txt
 Layers :	 x[mm] 	Edep   	     Edep/Ebeam[%] 	Dose 	       Dose/MaxDose[%]
 layer 1: 	5	119.065 MeV	0.0992211	9.53818e-11 Gy	15.1483	
 layer 2: 	10	206.505 MeV	0.172088	1.65429e-10 Gy	26.2731	
 layer 3: 	15	259.584 MeV	0.21632	        2.07949e-10 Gy	33.026	
 layer 4: 	20	311.898 MeV	0.259915	2.49858e-10 Gy	39.6819	
 layer 5: 	25	395.255 MeV	0.329379	3.16634e-10 Gy	50.2871	
 layer 6: 	30	439.052 MeV	0.365876	3.51719e-10 Gy	55.8592	
 layer 7: 	35	503.311 MeV	0.419426	4.03197e-10 Gy	64.0348	
 layer 8: 	40	573.742 MeV	0.478118	4.59618e-10 Gy	72.9954	
 layer 9: 	45	624.707 MeV	0.520589	5.00446e-10 Gy	79.4796	
 layer 10: 	50	678.383 MeV	0.565319	5.43445e-10 Gy	86.3086	
 layer 11: 	55	694.602 MeV	0.578835	5.56437e-10 Gy	88.3721	
 layer 12: 	60	710.771 MeV	0.592309	5.6939e-10 Gy	90.4292	
 layer 13: 	65	744.826 MeV	0.620689	5.96672e-10 Gy	94.762	
 layer 14: 	70	742.436 MeV	0.618697	5.94757e-10 Gy	94.4579	
 layer 15: 	75	771.713 MeV	0.643094	6.1821e-10 Gy	98.1827	
 layer 16: 	80	767.22 MeV	0.63935	        6.14611e-10 Gy	97.611	
 layer 17: 	85	775.608 MeV	0.64634	        6.21331e-10 Gy	98.6783	
 layer 18: 	90	762.779 MeV	0.635649	6.11053e-10 Gy	97.046	
 layer 19: 	95	785.997 MeV	0.654997	6.29653e-10 Gy	100	
 layer 20: 	100	735.186 MeV	0.612655	5.88949e-10 Gy	93.5355	
 layer 21: 	105	761.414 MeV	0.634512	6.0996e-10 Gy	96.8724	
 layer 22: 	110	721.836 MeV	0.60153	        5.78254e-10 Gy	91.837	
 layer 23: 	115	730.726 MeV	0.608939	5.85376e-10 Gy	92.9681	
 layer 24: 	120	728.394 MeV	0.606995	5.83508e-10 Gy	92.6714	
 layer 25: 	125	744.904 MeV	0.620753	5.96734e-10 Gy	94.7719	
 layer 26: 	130	731.53 MeV	0.609608	5.8602e-10 Gy	93.0703	
 layer 27: 	135	702.85 MeV	0.585709	5.63045e-10 Gy	89.4215	
 layer 28: 	140	671.716 MeV	0.559763	5.38104e-10 Gy	85.4604	
 layer 29: 	145	676.297 MeV	0.56358	        5.41773e-10 Gy	86.0432	
 layer 30: 	150	653.849 MeV	0.544875	5.23791e-10 Gy	83.1873	
 layer 31: 	155	674.152 MeV	0.561793	5.40055e-10 Gy	85.7703	
 layer 32: 	160	687.008 MeV	0.572507	5.50354e-10 Gy	87.406	
 layer 33: 	165	706.2 MeV	0.5885	        5.65729e-10 Gy	89.8478	
 layer 34: 	170	700.841 MeV	0.584034	5.61435e-10 Gy	89.1659	
 layer 35: 	175	686.009 MeV	0.571674	5.49553e-10 Gy	87.2788	
 layer 36: 	180	658.891 MeV	0.549076	5.2783e-10 Gy	83.8287	
 layer 37: 	185	644.999 MeV	0.537499	5.16701e-10 Gy	82.0613	
 layer 38: 	190	655.81 MeV	0.546508	5.25362e-10 Gy	83.4367	
 layer 39: 	195	645.691 MeV	0.538075	5.17255e-10 Gy	82.1493	
 layer 40: 	200	0 eV 	        0	        0 Gy	        0	

 The run consists of 6000 gamma of 20 MeV through 20 cm  of Water (density: 1 g/cm3 ) 
 divided into 40 slices.

 Edep is the deposited energy in every slice.
 Total incident energy(Ebeam)= 120 GeV
 Total energy deposit= 24.8344 GeV
 Dose is the deposited dose in every slice.
 MaxDose is the highest dose value from all slices.

This is PlotDose.txt. These values can be analyzed with MATLAB and ROOT.

5	15.1483	
10	26.2731	
15	33.026	
20	39.6819	
25	50.2871	
30	55.8592	
35	64.0348	
40	72.9954	
45	79.4796	
50	86.3086	
55	88.3721	
60	90.4292	
65	94.762	
70	94.4579	
75	98.1827	
80	97.611	
85	98.6783	
90	97.046	
95	100	
100	93.5355	
105	96.8724	
110	91.837	
115	92.9681	
120	92.6714	
125	94.7719	
130	93.0703	
135	89.4215	
140	85.4604	
145	86.0432	
150	83.1873	
155	85.7703	
160	87.406	
165	89.8478	
170	89.1659	
175	87.2788	
180	83.8287	
185	82.0613	
190	83.4367	
195	82.1493	
200	0	

Root file

Open Gamma.root file in the following way:

[username@plus1 PhotonPB_build]$ root -l Gamma.root

root [1] new TBrowser

Select ROOT files and Gamma.root

The histogram inside Gamma.root shows the energy deposition in water box:

PhotonPB Edep PhotonB1.png

To exit the ROOT terminal type


You can plot the dose deposition along the depth of the absorber (PlotDose.txt) using script PlotSimulation.C from folder PhotonPB_source. Copy this script to your current PhotonPB_build directory:

[username@plus1 PhotonPB_build]$ cp /home/username/PhotonPBFolder/PhotonPB_source/PlotSimulation.C .

Then run the script:

[username@plus1 PhotonPB_build]$ root -l 

root [1] .x PlotSimulation.C

This will create Simulation.root file. Open the root file:

[username@plus1 PhotonPB_build]$ root -l Simulation.root

root [1] new TBrowser

Select ROOT files and Gamma.root

This is the result:

PhotoPB DoseDeposition1.png

You can also plot the file PlotDose.txt using MATLAB. This software is installed on the UCL HEP Linux cluster but using it via ssh is not recommended because it is slow. If you have MATLAB installed on your computer you can import the PlotDose.txt file and plot it. You can also use the MATLAB installed on the computers at the Science Library. Before proceeding with MATLAB you need to copy the text files from the cluster to your computer. In the terminal at your computer write:

scp .

The file PlotDose.txt will be copied in your current directory. Then, open MATLAB and follow the procedure:

  • Import the file: Chose 'HOME' tab and 'Import Data'.
  • In the 'Import Data' window select the 'PlotDose.txt' file choosing the right path.
  • In the opened window select the data points in the 'IMPORT' tab. If you like, you can change the name of the variables. For example, 'x' instead of 'VarName1' and 'Dose' instead of 'VarName2'. Then, press 'Import Selection'/'Import Data'.
  • Close the Import Window and in the Command Window type plot(x,Dose). Press Enter.

This plot will be created with added axis labels and a legend:

PhotonPB matlab0photon.png

Run with different settings

You can change the physics process, incident photon energy, phantom material, number of slices etc. by modifying the macro gamma.mac. Open the macro with editor pico:

[username@plus1 PhotonPB_build]$ pico gamma.mac

This is the content of the macro:

# gamma.mac
/control/verbose 2
/run/verbose 2
/tracking/verbose 0
/run/particle/verbose 1
# set geometry and material
/photonPB/det/setMat Water
#/photonPB/det/setMat Lead
/photonPB/det/setSizeX  20 cm
/photonPB/det/setSizeYZ 20 cm
/photonPB/det/setSliceSizeYZ 20 cm
/photonPB/det/sliceNumber 40 
# set physics process
/photonPB/phys/addPhysics emstandard_opt0
#/photonPB/phys/addPhysics emlivermore
#/photonPB/phys/addPhysics empenelope
# production tresholds (range cut off-
# not bigger than 10% of slice thickness)
/photonPB/phys/setCuts 1 mm
#/photonPB/phys/setGCut 1 um
#/photonPB/phys/setECut 1 um
#/photonPB/phys/setPCut 1 um
# initialize
# visualisation
#/control/execute visualisation.mac

# particle gun properties (type of 
#particle and energy)
/gun/particle gamma
#/gun/particle e-
/gun/energy 20 MeV
# beam size
#/photonPB/gun/rndm 3 mm
# step limit (not bigger than 5% of 
# slice thickness)
/photonPB/stepMax 0.5 mm
/photonPB/event/printModulo 50
# output root file
/analysis/setFileName Gamma
# number of events
/run/beamOn 6000

Change the physics process

The default physics process is emstandard_opt0. This package is used in high energy experiments. In gamma.mac change

/photonPB/phys/addPhysics emstandard_opt0


/photonPB/phys/addPhysics emlivermore

The process emlivermore is used in low energy physics experiments.

Change the incident particle energy

The default energy is 20 MeV. The typical therapeutic beam energy used in radiotherapy is in the interval 0.3 to 20 MeV. In gamma.mac you can change the value of 20 MeV:

/gun/energy 20 MeV

to, for example, 1.25 MeV:

/gun/energy 1.25 MeV

This is the energy of the photon beams from cobalt-60 therapy machines.

Keep in mind that the primary particle generation is done at /PhotonPB_source/src/ This is part of the

fParticleGun  = new G4ParticleGun(1);
G4ParticleDefinition* particle = G4ParticleTable::GetParticleTable()->FindParticle("proton");

G4double x0 = -0.5*(fDetector->GetAbsorSizeX());
G4double y0 = 0.*cm, z0 = 0.*cm;



The line

fParticleGun  = new G4ParticleGun(1);

means that only one particle is generated, incident from (x0,y0,z0). The default number of events is set to 6000. Therefore, 6000 particles are incident to the water box.

Change the diameter of the beam

You can set the diameter of the beam with the command:

/photonPB/gun/rndm 3 mm

Change the material

In this example we compute the energy deposition of photons in water box. However, there is an option to change the box material from water to lead.

In gamma.mac change:

/photonPB/det/setMat Water


/photonPB/det/setMat Lead

Change the type of incident particle

In gamma.mac change the photon

/gun/particle gamma

to electron

/gun/particle e-

Change the number of slices

You can change the number of slices. The default number is 40. Keep in mind that if you want to have bigger number of slices you need to modify the file DetectorConstruction.hh in /PhotonPB_source/include/.

In DetectorConstruction.hh set MaxLayer to a value which is bigger then the number of your slices. The default number is MaxLayer=50. For example, if you want your box to be divided to 60 slices you need to set the value of MaxLayer to, for example, 65 then in gamma.mac you need to change the number of slices:

/photonPB/det/sliceNumber 60 

Every time you modify files in directory PhotonPB_source you need to compile your code. In directory PhotonPB_build do:


then run the macro

[username@plus1 PhotonPB_build]$ ./photonPB gamma.mac


If you want to use visualisation, in macro gamma.mac uncomment the line /control/execute visualisation.mac. This will run macro visualisation.mac with a specific visualisation setup. In this example, we use DAWN event display. Before running the visualisation look at the DAWN tutorial.

To run the visualisation, uncomment line /control/execute visualisation.mac and run the gamma.mac macro:

[username@plus1 PhotonPB_build]$ ./photonPB gamma.mac

In addition to the text files the code will create two .prim files, g4_00.prim and g4_01.prim. Both files contain detector geometry and particle interactions. While running the gamma.mac you will be asked to open g4_00.prim in DAWN. In the opened window press OK. This will create your first visualisation:

PhotonPB g4 00 6000e.gif

The second file g4_01.prim will not open automatically. It will be created after the running is finished. Open the file in the following way:

[username@plus1 PhotonPB_build]$ dawn g4_01.prim

In the DAWN display change the polar and azimuthal angles to 0 and 90 degrees, then press OK. This will create the image:

PhotonPB g4 01 6000e.gif

The color in the images indicates the type of the particle. Photons are in green, electrons are in red and positrons are in cyan.


List of files for Mononenergetic Photon Pencil Beam simulation.

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