# How define my geometry as the gamma Source

Hi Dear Geant4 expert member !
I want to collect the total dose deposited in human phantom situated in center of house with dimension 453 m and wall thick is 20 cm, where the walls and floor are our gamma source. The ceiling does not participate to the gamma emission.
firstly I use gps to define my source. I confine the geometry to define as a source, but with this method, the ceiling contribute also as a gamma emitter.
Someone can give an idea how to exclude the ceiling as the source emitter ?
Thk

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You could build the house as two volumes â€“ the walls/floor, and the ceiling separately.

Thank to you for your answer. Please can you help to better understand, how can I build it as two volumes ? In my work, I build the house as a paralepiped volume, where the top part of my geometry is seen as the ceiling. How can I separate it with the others parts of the volume ?
Thk

here is my geometry

So you have two nested volumes, the green one and the blue? Is the blue just a simple daughter volume? Or is the green a â€śhollowâ€ť solid with the blue at the same placement level (i.e., theyâ€™re both placed in the World)? If the green is hollow, then you can make it a hollow â€śbucketâ€ť instead of a closed box, and place a second flat volume on top of the blue.

The blue is just the simple daughter volume. Now I have to construct the hollow box volume.
Thank to you.

I define the green volume as the hollow solid, but my problem is

1. how can I define it as the hollow bucket ( with closed buttom part and open top part ) ?
2. how ca I define the inner volume material is the Air ?

You can create box with dimensions equal to your room without the ceiling thickness and than place the inner box (which material is set to air) inside this outer box so that the top side of both boxes is the same. Then you create ceiling as another box with an adequate position.