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Where and how do you get your electrons for your accelerator? What kind of machine or process did they use to split the first atom all those years ago and how expensive was it for them?

The answer to this question can be ridiculously simple or almost Star Trekish. When we started Jefferson Lab we used a very simple source for electrons, a hot wire, much like that of a light bulb filament. Electrons fly off the wire in all directions, so we put the wire in an electric field to get them to head off in the right direction. This field sucked the electrons through a hole in a plate and into our accelerator. This is the same kind of device that is used to light the phosphor in your television, computer screen and your dentist's x-rays.

We've become more sophisticated since then though. For most of our experiments we now use a polarized electron source. In this machine most of the electrons' spins are the same. To get spin polarized electrons we shine a laser on a gallium arsenide crystal. The laser light energizes the electrons in the crystal. Some of the electrons are energized enough to break free from their atom and are whisked away in a 100,000 volt electric field. Designing, building and operating the polarized electron gun is a very complex task. The people that do it are considered some of the "rock stars" of this lab.

The first device used to look inside the atom was a tiny accelerator called a Crookes tube, named after Sir William Crookes. This was nothing more than the same hot wire - or more often a flat plate - inside a glass tube. Air was pulled out of the tube and electricity was pumped into the plate. Electrons flew off the plate and into/through the target or experiment that was also inside the tube. The experimenter simply watched the experiment, that probably only cost a few dollars, through the glass. You can still buy them today since their fabrication and operation are not beyond the abilities of the average home experimenter. The early particle physicists were able to squeeze a phenomenal amount of information from these relatively simple devices by placing different targets in magnetic fields. Today, targets vary in different experiments (very thin films of water, thin metal films, or extremely complex machines that generate liquids such as hydrogen just a few degrees above absolute zero).


Brian Kross, Chief Detector Engineer (Other answers by Brian Kross)

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