The device is detecting beta radiation from the Uranium glaze on antique fiestaware. Most of the detected radiation is emitted by Uranium-238's decay products: Thorium-234 and Palladium-243(m). It may look alarming, but these plates are not that dangerous. In the unlikely event you managed to ingest all of the glaze from a single plate (~4.5 grams of U-238), the radiation dose would be under 10 mSv. That is only 1/5th the annual dose limit for a radiation worker.