An accelerating electrode field is set up in the chamber by one or more electrodes, and the chamber is sealed by {HEAD ~ an electronically conductive barrier} so as to prevent {TAIL ~ gas} leaking into the chamber, whilst being sufficiently thin to enable charged particles to travel across the barrier and thereby be detected by the detector 
to prevent
An accelerating electrode field is set up in the chamber by one or more electrodes , and the chamber is sealed by { HEAD ~ an electronically conductive barrier } so as to prevent { TAIL ~ gas } leaking into the chamber , whilst being sufficiently thin to enable charged particles to travel across the barrier and thereby be detected by the detector
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