Saturday, March 1, 2014

Leak Detector at Last

I've wanted to work on an AMS 10 leak detector since I was in high school,and now, in grad school, finally, I get to!  My relationship with the AMS 10 starts when I was 17 touring a lab at New Mexico State University.  I was working on building a cyclotron back at our highschool lab, this guy specifically, (picture one):

and the AMS 10 looked like it could be insanely helpful in tracking down the leaks in the accelerator vacuum chamber.  The vacuum chamber is the small brass box at the end of the copper pipe sandwiched between the poles of the magnet in the picture.  Although I had youthful dreams of pulling the perfect vacuum with the help of the ASM 10, in reality, the vacuum we were pulling wasn't really good enough for us to benefit from the services of such a precise instrument.  Its main use is for finding the last leak that has eluded you.  We had a variety of leaks, so it was debatable if we would have benefited at all.  Still, it was such a sexy piece of equipment.  The ASM 10 has its own built in roughing vacuum pump.  The roughing pump gets you from room air pressure down to a kind of good vacuum.  The roughing pump provided a background vacuum for the integrated diffusion pump.  This is the pump that can pull down to microns of vacuum!  It also came with its own Pirani vacuum gauge.  For those of you who have never seen one, it looks like a vacuum tube escaped from a ham radio and attached itself to the side of a leak detector.  The gentle glow of the filament just gives you that warm fuzzy ‘I’m in a lab’ feeling.  Finally, the ASM 10 has its own built-in mass spectrometer!

The mass spectrometer is actually the piece-de-resistance of the device.  To find a leak, in a vacuum system, the user sprays a small jet of helium over suspect areas.  Helium is infamous for penetrating well, just about everything, but especially vacuum leaks.  If the helium can make its way into the system, it flows through the vacuum pumps and eventually winds up in the spectrometer.  The spectrometer differentiates it from other gasses that might still be in the system and signals a leak.  A diagram of the mass spectrometer is shown in picture two below.  The design is very cool.  Rather than using an electromagnet as some of its bigger siblings might, the ASM 10s spectrometer uses a very simple permanent magnet to bend the beam of helium ions… but I’m getting ahead of myself.

It’s interesting that cyclotrons came up earlier because the inventor of the cyclotron was also instrumental in the development to the mass spectrometer  Cyclotrons use powerful uniform magnetic fields to bend beams of charged particles into circular orbits.  If you check out the diagram of the mass spectrometer you’ll see that it uses its permanent magnet for the same purpose.  Lawrence had a lab full of electromagnets for his cyclotron work and began to wonder what else he could do with them.  Not long after, the mass spectrometer was born.

Here’s how it works and why in the case of the ASM 10.  When gas enters the mass spectrometer it’s ionized turning it into a positively charged particle.  An electric field accelerates the ions and they are then sent between the poles of the permanent magnet.  The magnetic field will bend the paths of the ions into circular arcs dependent on how heavy they are.  Heavier ions go into wider arcs, and lighter ions like helium go into tighter arcs.  By placing a detector at the end of the arc size that’s specific to helium the ASM 10 sees only helium ions and ideally, not any others.  Hence, if any helium makes its way into the vacuum system then it will be detected as a leak by the mass spectrometer.

Here’s another cool facts about mass spectrometers.  If you’ve ever wondered why the University of California ran Los Alamos Labs for so long, you might be interested to know that Lawrence and the mass spectrometer were the answer.  For the atom bomb, the army needed to know how to separate Uranium 235 from uranium 238.  The answer?  The same mass spectrometer technology described above.  It just so happened that Lawrence who was at UC Berkeley at the time was the expert on building these devices and leveraged this knowledge into control of the lab for his university once the war was over.

If you want to read more about what Lawrence was up to while inventing the cyclotron before World War II, his activities during the war and his post war research, you should check out the book “Lawrence and Oppenheimer”.  It was interesting enough to keep a 17 year old, (me), entranced, and it reveals quite a bit about the friendship between Lawrence and Oppenheimer that many people aren’t aware of.

1 comment:

Harper smith said...

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