That claim was debunked last century - we've had at least
one thread where it was covered.
"Gentry's polonium halo hypothesis for a young Earth fails, or is inconclusive for, all tests. Gentry's entire thesis is built on a compounded set of assumptions. He is unable to demonstrate that concentric haloes in mica are caused uniquely by alpha particles resulting from the decay of polonium isotopes. His samples are not from "primordial" pieces of the Earth's original crust, but from rocks which have been extensively reworked. Finally, his hypothesis cannot accommodate the many alternative lines of evidence that demonstrate a great age for the Earth. Gentry rationalizes any evidence which contradicts his hypothesis by proposing three "singularities" - one-time divine interventions - over the past 6000 years. Of course, supernatural events and processes fall outside the realm of scientific investigations to address. As with the idea of variable radioactive decay rates, once Gentry moves beyond the realm of physical laws, his arguments fail to have any scientific usefulness."