It's strange that the count increase too much near the wall.
If you have a 1/r^2 decreasement from a point source, the if you have an infinite wall it becomes a constant value that does not decrease with the distance.
Clearly the wall is not infinite, but when the distance from the wall is much smaller (1/5?) than the distance to the border of the wall, the approximation is good enough. The wall is like 8 tiles tall , so I expect the radioactivity at 1/2 tile (15cm, 5in) to be almost equal to the radioactivity when the counter is touching the wall.
If you have a 1/r^2 decreasement from a point source, the if you have an infinite wall it becomes a constant value that does not decrease with the distance.
Clearly the wall is not infinite, but when the distance from the wall is much smaller (1/5?) than the distance to the border of the wall, the approximation is good enough. The wall is like 8 tiles tall , so I expect the radioactivity at 1/2 tile (15cm, 5in) to be almost equal to the radioactivity when the counter is touching the wall.