The scientific study of Bequerel was going on properties of X-ray (being discovered by Wilhelm Roentgen) with the use of some fluorescent minerals During the investigation he was working on the belief that salt of potassium uranyl sulfate will absorb energy from sun and will emit X-ray in return and for this he used photographic plate. However later studies showed that it emitted some rays but those were not X-ray but are result of radioactivity.
So he failed to succeed!
Answer: X ray