本底辐射

background radiation

root / stem / origin / source / this / the current / original / inherent / originally / classifier for books, periodicals, files etc

background

(equivalent to 的 as possessive particle)

background / bottom / base / end (of the month, year etc) / remnants / (math.) radix / base

spoke of a wheel

radiation

old variant of 射[she4]

to shoot / to launch / to allude to / radio- (chemistry)