In case anyone else needs it, I found this webpage which solved my problem...

http://geant4.lngs.infn.it/courseBelfast2013/exercises/task2/task2a/index.html

Was going to say that if you happen to be using the python environment a pencil beam can be generated like this:

http://nbviewer.ipython.org/github/SimonBiggs/ipython-geant4-linac/blob/master/main.ipynb#Generating-the-beam

https://github.com/SimonBiggs/ipython-geant4-linac/blob/master/README.md

Dear Simon,

Thank you very much! I do not use python but that's still helpful. 

Regards,

Oznur

Glad to know it's helpful :)

Dear Oznur:

I;m very new on Geant4 and I;m trying to do exactly whad you did:

Send electrons with defined energy and direction on to the given geometry via command line by using /gun and /run commands.

How did you do that?

thank in advance

Jilberto Zamora-Saá you can use the general particle source (GPS). With the GPS you setup a generalized source interface that you can control through macro files. In this interface there are commands that allow you to create (possibly) any source that you are going to need.

If you want to use it, replace your PrimaryGeneratorAction.hh and. cc with the PrimaryGeneratorAction.hh and .cc in the /extended/eventgenerator/exgps/include and /extended/eventgenerator/exgps/src directories respectively from the Geant4 examples directory.

Then make a macro file with the instructions to create your own source. Or you can add them in your existing macro file. That way you do not have to recompile your code every time you want to make a change.

You can create a circular parallel beam or give a Gaussian spatial distribution to your source. Of course you can setup a Gaussian spread in energy, if you need so.

You can find many examples here:

http://hurel.hanyang.ac.kr/Geant4/Geant4_GPS/reat.space.qinetiq.com/gps/examples/examples.html

Hope that helps.

Marios Sotiropoulos Thank a lot for your answer!!!!!!

Dear Marios Sotiropoulos

I'm trying to modify the beam in the example B1, so that means that I already have the propers PrimaryGeneratorAction.hh and. cc. However, I do not understand the following step:

Then make a macro file with the instructions to create your own source. Or you can add them in your existing macro file. That way you do not have to recompile your code every time you want to make a change.

How can I recognize a macro file?

sorry for my ignorance, but I'm very new in GEANT4.

thanks in advance,

Jilberto

Hi Jilberto,

I think a more simple solution would be to make the changes directly in your B1PrimaryGeneratorAction, sampling from the distribution you want. This method is not as general as the previous, but might be more simple. The only changes you need are made inside the B1PrimaryGeneratorAction.

The initial X and Y position of the source are given in (line 141 and 142):

G4double x0 = size * envSizeXY * (G4UniformRand()-0.5);

G4double y0 = size * envSizeXY * (G4UniformRand()-0.5);

You can change the previous two sentences in the code, and sample for the distribution you want.

For example, for a circular source replace the previous two sentences with:

G4double x0;

G4double y0;

G4double r = 50*mm; // this is the radius of the circular area; notice the units

do

{

x0 = r*(2*G4UniformRand()-1);

y0 = r*(2*G4UniformRand()-1);

}while( x0*x0+y0*y0 > r*r);

*In case you want to use my previous suggestion:

1. You will need to modify your PrimaryGeneratorAction.hh and .cc to include the GPS. The easiest way to do it is to copy from the example mentioned before. You will probably need to modify the ActionInitialization so that you have the PrimaryGeneratorAction rather than B1PrimaryGeneratorAction, and remove a section in RunAction named as ''Run conditions''. Do not remove the 'G4String runCondition;' as needed later.

In case you are using the gun/ commands, after the change of the PrimaryGeneratorAction it won't be available anymore.

2. As Geant4 is a toolkit that allows to create your simulation from a C++ library, you have to recompile your code every time you make a change. However, in most of the cases, we need to run the same simulation for a set of parameters, for instance the primary particle energy. For this purpose Geant4 has an interface that allows to control some functionality through a macro file. It also offers the tools to be able to create a simulation where basic parameters are given from a macro file, although this might be a bit advance at the moment. In B1 example there are two macro files, run1.mac, run2.mac, vis.mac, and init_vis.mac. The first two are to be used in batch mode, and the later two are to setup the visualization in interactive mode (look comment below for what the modes are). Please note that usually macro files have the extension .mac.

3. B1 example can be run in two modes, batch and interactive:

-Batch mode: the simulation runs without any user interface, ie graphics. All the information are given in a macro file. To run in batch mode use the name of the application followed by the macro file. In the case of example B1 is './exampleB1 run2.mac'.

-Interactive mode: you can run the simulation command by command, or you can run a macro file which contains a set of commands. You can start an interactive mode by writing './exampleB1'.

Now the important part, the following commands will create a square 2x2 cm2 source on the X-Y plane with the particles traveling towards +Z. I have 6 MeV gamma (i.e. photons).

# anything starting with a hash (#) is considered a comment

# if you want to run this macro file in batch mode

# uncomment (ie remove the #) the following command

# /run/initialize

# SOURCE DEFINITION

# select particle type

/gps/particle gamma

# select energy

/gps/energy 6 MeV

# select source type /gps/pos/type Plane /gps/pos/shape Square

# center of the source /gps/pos/centre 0. 0. 0. cm /gps/pos/halfx 1. cm /gps/pos/halfy 1. cm

# RUN SIMULATION

# this will run the simulation with 1 primary

/run/beamOn 1

# end of macro file

I think that now you can change the parameters of the GPS and create your own source, see the site with the examples from the previous post.