s . ▁ ▁High - vol t age ▁T EM s ▁require ▁ult ra - high ▁vac u ums ▁on ▁the ▁range ▁of ▁ 1 0 − 7 ▁to ▁ 1 0 − 9 ▁Pa ▁to ▁prevent ▁the ▁generation ▁of ▁an ▁elect rical ▁arc , ▁particularly ▁at ▁the ▁T EM ▁c ath ode . ▁As ▁such ▁for ▁higher ▁voltage ▁T EM s ▁a ▁third ▁vac u um ▁system ▁may ▁operate , ▁with ▁the ▁gun ▁isolated ▁from ▁the ▁main ▁chamber ▁either ▁by ▁gate ▁val ves ▁or ▁a ▁differential ▁p ump ing ▁a pert ure ▁– ▁a ▁small ▁hole ▁that ▁prevents ▁the ▁diffusion ▁of ▁gas ▁mole cules ▁into ▁the ▁higher ▁vac u um ▁gun ▁area ▁faster ▁than ▁they ▁can ▁be ▁p ump ed ▁out . ▁For ▁these ▁very ▁low ▁press ures , ▁either ▁an ▁ion ▁p ump ▁or ▁a ▁get ter ▁material ▁is ▁used . ▁ ▁Po or ▁vac u um ▁in ▁a ▁T EM ▁can ▁cause ▁several ▁problems ▁r anging ▁from ▁the ▁de position ▁of ▁gas ▁inside ▁the ▁T EM ▁onto ▁the ▁spec imen ▁while ▁viewed ▁in ▁a ▁process ▁known ▁as ▁electron ▁beam ▁induced ▁de position ▁to ▁more ▁severe ▁c ath ode ▁dam ages ▁caused ▁by ▁elect rical ▁dis charge . ▁The ▁use ▁of ▁a ▁cold ▁trap ▁to ▁ad s orb ▁sub lim ated ▁g ases ▁in ▁the ▁vic inity ▁of ▁the ▁spec imen ▁largely ▁elimin ates ▁vac u um ▁problems ▁that ▁are ▁caused ▁by ▁spec imen ▁sub lim ation . ▁ ▁Spec imen ▁stage ▁ ▁T EM ▁spec imen ▁stage ▁designs ▁include ▁air