background radiation
noun [ C ] physics, environment specialized uk/ˌbæk.ɡraʊnd reɪ.diˈeɪ.ʃən/ us/ˌbæk.ɡraʊnd reɪ.diˈeɪ.ʃən/
背景辐射（可在地球检测到的自然发生的辐射，来自外层空间以及土壤和岩石中）
radiation found naturally on earth that comes from some soil, rocks, etc., and from outer space