Am I doing what I intend to do with GPS?

Hello everyone,
Using the MIRD human phantom I am trying to simulate iodine 131 decay in the stomach so I tried the GPS but the results that I got led me to believe that my approach is wrong;


/gps/pos/type Volume #my iodine is contained in a capsule
/gps/pos/shape Cylinder #shape of capsule
/gps/pos/centre 8 -4 0 cm #center of emission is to be center of the stomach in the phantom
/gps/pos/radius 3.36 mm #radius of the capsule
/gps/pos/halfz 9.7 mm #half length of the capsule
/gps/ang/type iso #I don’t really know the angular distribution that I should use
/gps/ene/type Mono #I don’t know which energy type is compatible also
/gps/particle ion
/gps/ion 53 131 53 0.
/gps/energy 957. keV
/gps/number 1602 #Number of particles in 30mCi of 131I


I have some questions about my work:

1/Am I in the right to consider the coordinates 8 -4 0 cm as the center of the stomach in the human phantom? If yes why there’s no energy deposition in the stomach but only in left leg or trunk?

2/Was I right to consider the capsule as a cylinder that contains iodine or should I consider a point beam instead?

3/Is there any documentation to angular distribution and energy type that would help me choose which suits me best?

4/The Excitation energy of iodine should I set to 0 or is that part of the problem?

5/Is there a way to enter the activity as input in mCi or Bq rather than calculating Number of particles?

And in the end thank you all for taking the time to read and respond to my topic.

what you are doing is to simulate the decay of I-131. Please remove:
/gps/ene/type Mono
/gps/energy 957. keV

Make sure you have the Radioactive Decay set appropriately in the simulation. It is correct to use the “iso” angular distribution. Let me know how it goes.

PS. I suggest to do some tests, which will help to understand. Set the source as a point shaped source in water and look if you obtain the correct Dose Point Kernel. Once you have done that, simulate a spherical source and look at the dose distribution in 3D. Do you expect that? Then change the source from a sphere to a cylinder and check, try to understand the results again. It is very important to understand what is done in simulation and often it is better to proceed step by step, refining the simulation at each step and understanding the results at each step. I hope this helps you,