Question: what is the term for the amount of time it takes 50 of an element to decay
[{'answer': 'half - life', 'provenance': [{'wikipedia_id': '197767', 'title': 'Radioactive decay', 'start_paragraph_id': 139, 'start_character': 862, 'end_paragraph_id': 139, 'end_character': 875, 'bleu_score': 1.0, 'section': 'Section::::Occurrence and applications.\n'}]}, {'provenance': [{'wikipedia_id': '197767', 'title': 'Radioactive decay', 'start_paragraph_id': 52, 'start_character': 0, 'end_paragraph_id': 52, 'end_character': 168, 'bleu_score': 0.8547305998833801, 'meta': {'yes_no_answer': 'NONE', 'annotation_id': 6335906140161174776}, 'section': 'Section::::Radioactive decay rates.\n'}]}]