Guy Savard, Ben Arend and Chandana Sumithrarachchi install the ANL Gas Catcher at NSCL. Photo courtesy of Stefan Schwartz.
Since physicist Ernest Lawrence invented the first cyclotron in 1929, scientists have worked to engineer machines that could accelerate particles to higher and higher energies.
That first cyclotron, just 5 inches in diameter, boosted hydrogen ions to 80,000 electron volts. Today’s Large Hadron Collider, the most powerful particle accelerator in the world, is more than 5 miles in diameter and accelerates protons to 4 trillion electron volts. The same trend has dominated throughout accelerator history – bigger and faster is better.
And yet, nuclear physicists at Michigan State’s National Superconducting Cyclotron Laboratory are trying to slow particles down.
Slowing particles allows scientists to make some extraordinary measurements. To measure the weight of a nucleus, a scientist at NSCL can place it in a strong magnetic field in a device called a Penning trap. By observing its circular motion in the field, the scientist can determine its weight within 100 parts per billion. That’s like weighing a Boeing 747 and being able to tell how much change is in the pilot’s pocket.
Other experiments excite the orbits of the protons and neutrons within the nucleus using lasers of specific wavelengths. Both the weight of a nucleus and its reaction to these lasers can tell physicists much about their internal structures.
This is not to say that NSCL physicists have no use for particle acceleration. They use accelerators to create the particles they study.
At NSCL, a pair of coupled cyclotrons accelerates nuclei up to half of the speed of light. The speeding nuclei then collide with a solid target, cracking them apart into new isotopes – the same elements you see every day, but with differing numbers of neutrons. For example, at NSCL, physicists can create carbon-22, which has 10 more neutrons than its stable form, carbon-12, the most abundant form of carbon on the planet. By producing these strange, rare forms of elements that exist only for fleeting moments, physicists can learn more about atomic forces and structures than stable isotopes can yield.
So how do scientists transition between the high-speed process that creates the isotopes and the low-speed experiments they would like to conduct? In the last decade, NSCL has pioneered so-called gas stopping technology to put the brakes on their high-energy isotopes.
In a gas stopper, isotopes lose much of their energy in a solid degrader before entering a chamber filled with helium gas. Once there, interactions with the helium cause the isotopes to slow all the way down to room-temperature energies to become “thermalized” at about 60,000 electron volts. Today, these gas stoppers are at work in several laboratories throughout the world.
After nuclei at Michigan State have been sped up, fragmented and slowed down, a linear accelerator called ReA currently under construction will accelerate them up to the same amount of energy found inside stars or supernovae.
Other laboratories also accelerate rare isotopes to these energies without all of the speed changes required at Michigan State. However, the production process used at Michigan State has many advantages. For one, fast fragmentation separates rare isotopes using physical properties rather than chemistry, allowing the production of many isotopes not available at other laboratories. Secondly, because the rarest of nuclei decay in a matter of milliseconds, fast beams are able to deliver them to experiments much more quickly than other methods. And because slowing them down directly to the low energies found in the cosmos would be an imprecise process at best, the lab has opted for this start-stop method.
By creating rare isotopes found only in the cosmos and accelerating them to the same energies that produce them there, nuclear physicists will be able to gain new insights to the inner workings of such astronomical phenomena.