Question about the cross-section

B-10 has a large absorption cross-section for neutrons.
If I defined the B-10 as

G4Isotope* isoB10 = new G4Isotope("B10", 5, 10, 10.012937 * g/mole);

and add it to a material,will this material have a large absorption cross-section for neutrons?
Or i have to modify the cross-section data?

You do not have to modify the cross section data. When Geant builds the material it will retrieve the appropriate cross section data for B10 from the installed data files and calculate the effective cross section of the new material. The material will have a large effective absorption cross section if there is enough B10 in it. As a trivial example, if the material has only 1 ppt B10 in it, the effective absorption cross section will not be large.

Thank u for helping!