ABSTRACT
It is in the presence of this applied voltage that a small quantity of hel ium ga s i s ad mitted i nto t he s ource region, w hich i s maintained at c ryogenic tem peratures. e hel ium ato ms a re neutral, but they are polarized in the presence of the electric eld and are drawn to t he apex of the needle. When a hel ium atom approaches one of the three terminal atoms, one of its electrons can tunnel out, leaving the positively charged helium ion to b e accelerated away from the positively biased apex. As the helium atoms become ionized, each of them is accelerated away from the needle, forming three beamlets of singly ionized helium atoms. One beamlet i s produced for each of t he t hree ter minal atoms and a simple phosphor can be employed to observe the emission pattern (Figure 39.4). e production rate per atom is commonly as large as 108 s−1-corresponding to a b eamlet c urrent on t he order of 100 pA. And since all the ions are created at a potential of 15 kV (in this example), their ultimate energy when they strike a grounded specimen is 15 keV.
