Question: what was the source of alpha particles in rutherford's experiment
[{'answer': 'helium nuclei', 'provenance': [{'wikipedia_id': '25985', 'title': 'Rutherford scattering', 'start_paragraph_id': 2, 'start_character': 199, 'end_paragraph_id': 2, 'end_character': 212, 'bleu_score': 1.0, 'section': 'Section::::Abstract.'}]}, {'provenance': [{'wikipedia_id': '25985', 'title': 'Rutherford scattering', 'start_paragraph_id': 2, 'start_character': 0, 'end_paragraph_id': 2, 'end_character': 770, 'bleu_score': 0.965400604802336, 'meta': {'yes_no_answer': 'NONE', 'annotation_id': 13019097307299820097}, 'section': 'Section::::Abstract.'}]}]