Keep in mind that the rock had to be solid when the polonium first entered it. And they are missing. It could not enter and leave fast enough. 4 - A polonium 218 halo could have been in that rock in the very beginning. That could happen, if the rock was suddenly created solid, with those specks of polonium inside it when it was made.
Gentry decides to find secondary origin halos.
Gentry was patient and thorough. The next step was to try and find secondary halos, and see what they looked like. He had written many articles, published in scientific journals, and the angry readers either replied that it was a mystery which would eventually be solved or that, somehow, the Po-218 halos must be secondary. Gentry answered this in two ways. Second, he went out and located actual secondary halos to see what they looked like.