The X rays are emitted in response to {HEAD ~ an impinging beam}, such as {TAIL ~ an electron beam}, directed towards the sample surface where a defect resides 
such as
The X rays are emitted in response to { HEAD ~ an impinging beam } , such as { TAIL ~ an electron beam } , directed towards the sample surface where a defect resides
OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH REL REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH