Creating a Radial Radiation Source

I created a Spherical Surface source. I want to have the radiation converging towards the center of the sphere where I’ll be having a Human Phantom. If I use /gps/direction all the particles follow that direction. I can imagine using a lot of Particle Guns but is there a better way to do this?

Once I create a GPS, how can I visualize the GPS on the screen, like a wireframe or any other way? /gps/source/list lists out the information about the GPS but doesn’t show it.

Also, is there any resource where I can read about the isotropic and cosine angular distributions?

A) If you want isotropic radiation within spherical space the GPS option is “iso” - or is it “cos”. (I did this once and could dig out the G4 commands. To normalise to a specific radiation field, there is a formula.)

B) There are /vis/ commands to draw the GPS source. “help vis” or just “ls” to see commands with guidance. I think it is “/vis/gps”.

Sorry not to be more specific but I’m away from my laptop at the moment.


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Thank you for the answer John!

A) “cos” seems to be similar to what I was looking for, thank you! Can you direct me to resources where I can learn to normalise to a specific radiation field using the formula?
B) I’ve looked through “help vis” and didn’t see anything related to the GPS sources. I saw a few commands that toggle the visibility of other volumes. “/vis/gps” doesn’t exist for some reason.
“/vis/scene/add/gps” does the trick

Hi Sriram

This worked for me:

Spherical cavity

/gps/pos/type Surface
/gps/pos/shape Sphere
/gps/pos/radius 1 m # or whatever
/gps/ang/type cos

This creates an isotropic flux of particles inside the sphere. For and isotropic fluence F per unit area per steradian, F/4 particles pass through unit area in a particular direction. So if you generate N particles inside a sphere of radius R (area A = 4*pi*R^2), F = 4*N/A = N/(pi*R^2). Think that’s right - please check.

Hi Sriram, John,

the GPS commands are indeed the same I use. I always add
/gps/ang/maxtheta 90 deg
to be sure I only shoot towards the inside of the sphere.

For the normalisation, I believe a factor 4pi is missing, if F is per steradian? Myself I always use:
F 4 pi^2 R^2 / N
(with N the number of particles shot from the surface of a sphere of radius R, with cos-bias (and max theta pi/2)

You can verify the whole chain by putting a smaller spherical “detector” or radius r inside the GPS sphere, count the particle reaching the detector, divide by pi r^2 (cross section of your detector), use the normalisation factor above, and see if you get back F (in a vacuum)


Yes, you are right, Giovanni. My expression, N/(pi*R^2), is for the omnidirectional fluence, which is 4pi times the per-steradian fluence for isotropic radiation. Apologies.