These cannot be altered by alloying, etc. Nor can any achievable EM field repel such particles as cosmic ray primaries. Boron is strong and a good neutron sink. Other elements have other nuclear absorption properties that vary with the energy of the incoming particle. As far as absorbing X-rays, the important property seems to be in high atomic number, such as lead or bismuth. Tungsten is OK, and very strong.

Frankly, I think that a rather large mass of shielding may be required for interplanetary voyages. This is another reason to stick with robot spacecraft for a long time. I know that such an idea is not very popular here, but it is very practical.

David F Mayer