One of the important things about these scattering experiments is the thickness of your target object. If the target is thin the interaction can be treated as individual scatterings on the atoms of your target object. If the object is thick the interaction at the front of the object effect the interaction at the back, so your scattering is no longer the sum of individual scatterings.

Knowing the crossection of the experiment you can calculate a distance that the particles of the oncoming beam can freely travel in your target. Which means that a collision isn't guaranteed. A thin target would let a lot of the particles through without interaction but some of them would end up colliding and those can be treated as independent collisions. If the target is thicker than that distance almost all particles of the beam would collide statistically even those that scattered at the front of the object scattering again in the target.

So you want your target to be as thin as possible and gold foil can be really thin like a few atoms thin. Which is perfect for scattering experiments but of course there are other options too.