Friday, July 3, 2009

Angels and Demons

Even though the movie has been out for a while now, I still receive questions from visitors to Fermilab about our production of antimatter, and whether there is any "validity" in the premise that a "bomb" could be produced using our supply, or the supply that the movie suggests will be generated by the LHC at CERN.

Besides the slight change in the ending (the movie's ending is much better than that in the book!), it was great to see some actual views of CERN and the LHC tunnel in the movie "Angels and Demons". One has to realize that the "control" of the accelerator is not underground nor just outside of the detector, but rather several kilometers away in an above-ground building. And the accelerator operators don't wear lab coats. And... But, the film has enough going for it to make it interesting. What's also interesting, which most people know by now, is that (a) the LHC at CERN will not be used to make antimatter, except as a result of individual particle collisions in the experiments, and (b) even if it were optimized to make antiparticles at an appreciable rate, it would take far too long to get the amount presented in the movie.

Let's make a simple calculation, using the highest antimatter production going on in the world today, the Fermilab Antiproton Source. At Fermilab, beams of protons are accelerated within several stages of accelerators to a final energy of 120 GeV per proton -- that means that each proton is accelerated through a net voltage of 120 Billion volts. The beam of protons is focused and sent into a target made of a Nickel alloy, where the energy of the collisions of the protons with the nuclei of the target is high enough that new particles can be created (E = mc^2). Any particles with a negative charge, and with a momentum of 8.9 GeV/c are collected in a storage ring that is made up of electromagnets and tuned to operate for that particular momentum. Many of those particles, like pions and kaons, etc., will very quickly decay away -- however, stable particles -- like antiprotons -- will remain forever (as far as we know) and can be collected in the ring. In the Fermilab facility, about 20 antiprotons with this particular momentum are collected for every one million (10^6) protons that hit the target.

Now, the facility can produce about 8 trillion (8 x 10^12) protons each with 120 GeV energy every 2.2 seconds to send to the target. That means that 8 x 10^12 x 20/10^6 = 160 x 10^6 antiprotons are produced every 2.2 sec. OR, 0.16 x 10^9 /2.2 s x 3600 s / hr = 26 x 10^10 antiprotons every hour (260 billion/hr).

Thus, if the facility ran non-stop (present conditions generate an effective "up-time" of about 70% throughout a typical year), for 1 billion years, then roughly 26 x 10^10 /hr x 24 hr/day x 365.24 days/ year x 10^9 (1 billion) years x 0.70 = 1.6 x 10^24 antiprotons would be generated.

Coincidentally, the mass of an antiproton is the same as the mass of a proton: 1.6 x 10^-24 gram. Thus, in a BILLION YEARS of running, we could produce *** 1 gram *** of antimatter!

OK, so the 1/4 gram of antimatter that goes "missing" in the movie would only take 250,000,000 years to generate...

"Yes, but couldn't we upgrade the Fermilab accelerator to do better?"

Indeed, if the targeting stations were upgraded, and we could use the full power of the "Main Injector" accelerator (which operates at 120 GeV/proton), then one could imagine 4 x 10^13 every 1.5 seconds at best -- a rate that would be 40/8 x 2.2/1.5 = 7 times better. THUS, it could generate 1/4 gram in "only" about 36 million years!

The other question often asked is, "Isn't the LHC much higher energy? So couldn't it make antimatter that much faster?"

First of all, the LHC can't ramp up and down in 2.2 seconds or anything close to that. It takes many minutes for the accelerator to reach full energy. So, even if it does have over 50 times the energy of the Fermilab Main Injector (it will contain, ultimately, roughly the same number of particles), it takes about 500 times longer to ramp up and down to its final energy. So, the "rate" that it could produce antiprotons is far less than what would be done at Fermlab's machine.

- - -

It's still cool that antimatter exists at all. And, that most of the antimatter produced and accumulated for scientific use in the world so far has been produced right outside of Chicago...

Aerial view of part of the Fermilab accelerator complex. The "oval"-shaped accelerator at the top is the Main Injector, which typically operates at 120 GeV per proton. The small "triangular" arrangement of buildings near the bottom house equipment for the Antiproton Source, located about 20 feet below the surface.

No comments: