The unknown radioactive element is present in pitchblende.
Explanation:
Marie Curie was the scientists that mainly worked on the radioactive element. She was the first scientist who won the noble prize in two different fields of science.
The pitchblende was the mineral sample that was noticed by Marie curie during her experiment. Pitchblende contains a more radioactive element that uranium and confirms that she had found a new element.The new element was polonium with hundred times more radioactive than uranium.
Thus, the answer is unknown radioactive element is present in pitchblende.
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