coaxing high-energy ions from our ancient tandem particle accelerator
Would that have been a tandem Van de Graaff? Many decades ago when an undergrad at CalTech, I designed and built beam-chopping circuitry for one under the Sloan/Bridge labs. It was used (IIRC) because of its ability to produce beams of very accurately controlled energy.
Well, yes, it was designed by Robert J. Van de Graaff, and others. And, yes, it was a tandem, a small heavy-ion research accelerator built in the 1950s by
High Voltage Engineering Corporation, a company that eventually went out of business and whose designs were absorbed by High Voltage Engineering Europa. Their "compact" tandem accelerator was called a Tandetron. Mine was capable of 1.7 megavolts positive terminal potential (with respect to ground) on a good day.
Several of these machines still exist and, AFAIK, are in use today. If memory serves me correctly, it was a tandem, similar to the Tandetron, that was used to first demonstrate the use of an accelerator mass spectrometer for age-dating microgram sized specimens, including small snippets from the Turin Shroud. I investigated the cost to "upgrade" my tandem to perform accelerator mass spectrometry (you need to carefully separate isotopes and measure their relative quantities) and was told by someone "in the know" at National Electrostatic Corporation that it would cost more than a new machine bespoke for that purpose... which NEC would gladly build for me. At that time, the only job we had for the accelerator was oxygen-ion implantation into 5-inch GaAs wafers containing integrated-circuit semiconductors for device isolation in lieu of chemical etching. The Department of Defense helped us develop the isolation process at the Air Force Institute of Technology, located on the campus of Wright-Patterson AFB near Dayton, OH. So, no need to look for business dating ancient relics. The DoD customer sent me a box of twenty or so patterned wafers, coated with a layer of developed photo-resist, every month for several years. I loaded the wafers into a purpose-built wafer carousel and implanted ten or twelve different energies to obtain a uniform isolation depth between "devices" on the wafer. I never was told what the devices were, but they had to be pretty hot stuff to go to that much trouble to make them!
The High Voltage Engineering Corporation was founded in 1947 by three MIT graduates: Robert J. Van de Graaff, Denis M. Robinson, and John G. Trump. It was a well-designed solid-state (Cockroft-Walton) accelerator, and several models are still in use AFAIK. One, at Oak Ridge National Laboratories, was decommissioned when its operator took another job in Knoxville to make radioisotopes with a small cyclotron for PET scanners. Another was in use at a university in Kansas, but I forgot the name of the nice young fellow who inherited it. There was a very nice machine installed at the Ion Beam Laboratory in Ann Arbor operated by Dr. Ovidiu Toader. and another exists here in Florida near Orlando that was in the process of being moved to a different building the last time I talked with them before retiring in December 2014.
The problem with any tandem, my Tandetron included, was it requires negative ions to be accelerated first, stripped of some of their negative electrons, and then accelerated again as positive ions. Unfortunately negative ions are not easy to produce, and some elements do not produce a stable negative ion. National Electrostatic Corporation is the world renowned expert at building high-voltage tandems based on their
Pelletron chain charging system. They built the now de-commissioned 25 MV Holifield Radioactive Ion Accelerator at Oak Ridge National Laboratories. You can find NEC pelletrons all over the world. They sponsor the international Symposium of North Eastern Accelerator Personnel (SNEAP) every year with hosts located around the world. SNEAP is an informal gathering of scientists and engineers who operate electrostatic particle accelerators for the purpose of exchanging operating notes, and procedures and to have a good time touring accelerator facilities and nearby "tourist type" attractions with their families. My wife and I have attended meetings at ORNL (twice) and once at the Michigan Ion Beam Laboratory near Ann Arbor, MI. We were especially impressed with our tour of the spallation neutron source at Oak Ridge and the superconducting RF accelerator at The National Superconducting Cyclotron Laboratory (NSCL) on the campus of Michigan University near East Lansing.
Eventually our DoD customer moved on to larger GaAs wafers, which we could not efficiently implant even if we extended our beam line to scan the larger area. Our tandem used a nitrogen gas stripper line, operating at very low pressure, to "knock off" two or more electrons from the accelerated negative ions, thus creating a positive ion for further acceleration by the positive terminal voltage. Unfortunately, the process was very inefficient so it was very difficult to obtain positive ions with more than +1 or +2 charge state. I needed the +2 charge state to reach the maximum required implant energy. On a really good day, I could get about one microampere of +2 charged oxygen ions, so a box of twenty or so wafers would take all day and a good part of the evening to complete. Still, it was steady and predictable work and I enjoyed it... mostly. In my "spare time" I was tasked with keeping the rest of the electronics equipment (including the high-vacuum systems) working. You would be amazed at what can go wrong when a scientist tries to operate 120 vac synchronous clock motors inside a vacuum vapor deposition chamber without the benefit of using an isolation transformer. And I never did get across the idea that you don't control the speed of such motors with a Variac. I don't miss that part of the job.
Hop -- AC8NS