( ( Beta ( Decay It ) ) ( often ( ( happens ( that ( ( unstable nuclei ) ( emit ( ( high-speed electrons ) ( called ( beta particles ) ) ) ) ) ) ) . ) ) )
(ROOT (S (NP (NNP Beta) (NNP Decay) (PRP It)) (ADVP (RB often)) (VP (VBZ happens) (SBAR (IN that) (S (NP (JJ unstable) (NNS nuclei)) (VP (VBP emit) (SBAR (S (NP (JJ high-speed) (NNS electrons)) (VP (VBD called) (NP (JJ beta) (NNS particles))))))))) (. .)))
Beta Decay It often happens that unstable nuclei emit high-speed electrons called beta particles.
(ROOT (S (NP (NN Beta) (NN decay)) (VP (VBZ occurs) (SBAR (WHADVP (WRB when)) (S (NP (DT an) (JJ unstable) (NN nucleus)) (VP (VBZ emits) (NP (NP (DT a) (JJ beta) (NN particle)) (CC and) (NP (NN energy))))))) (. .)))