How did Becquerel discover radioactivity when it was already discovered?

so apparently Becquerel was facinated by x-rays(a type of radioactivity). he left a piece of uranium around for a week and found that it put an imprint on a photographic plate. i dont know what a photographic plate is but i think the imprint was caused by radioactivity. How did he discover radioactivity when x-rays(which is radioactivity) was already known