How to change the electron beam density distribution?

[CQUswl]

Hi，

When I run the xtube CM in the BEAMnrc GUI，can I change the electron beam density distribution in source 13？ i.e., I want to make the rectangular electron beam denser on both sides and less dense in the middle, how do I achieve this? Can I get the results I want by finding the initial code of source 13 and modifying some of its parameters regarding density? Where can I find the initial code for source 13?