No tour of American science would be complete without a stop in Los Alamos, New Mexico. From 1943 to 1945, the U.S. government sequestered many of the world’s leading physicists on this high desert plateau under the auspices of the Army Corps of Engineers Manhattan Engineer District with the mission to build an atomic bomb before the end of World War II. Until they accomplished their goal, hundreds of scientists, along with their families and a large administrative and technical staff, disappeared from their former lives, leaving behind only an address for a P.O. Box in Santa Fe, New Mexico. (You can check out all their staff badge photos here.)
While most of Los Alamos’s new inhabitants left soon after the use of their invention ended World War II, some stayed. The town of Los Alamos soon became a place with real addresses, accessible roads, great mountain biking, and some of the best public schools in the state of New Mexico. But it still carries the weight of its history, with blueprints of Little Boy and Fat Man (the atomic bombs dropped on Hiroshima and Nagasaki) for sale in the town bookstore, and classified weapons research ongoing at the lab. We went there not really sure what we would be allowed to see or how we would feel about it. But while the history was problematic, the current (unclassified) science we saw exhibited many of the same traits we observed at other labs: creativity, ingenuity, and a lot of foil.
J. Robert Oppenheimer, who was named scientific director of the Manhattan Project in 1942, advocated Los Alamos for the site of the secret atomic city in part because he remembered the impressive and largely inaccessible landscape of northern New Mexico from the time he spent there as a teenager. And despite being literally and figuratively on the map today, the town of Los Alamos is still not very accessible. It’s housed on a series of mesas high in the New Mexican dessert northwest of Santa Fe – and while we weren’t as utterly blown away by the view as Richard Feynman (who had never ventured much outside the borders of New York City), we had to admit that it was one of the most impressive landscapes of our trip.
Our first stop in Los Alamos was the communications office, which is kept separate from the lab to make the flow of visitors easier to regulate and control. For obvious reasons, there are more rules and restrictions to follow when visiting Los Alamos than when visiting the non-weapons national labs. Even though we would only be seeing unclassified physics research, Nick’s photos had to be carefully composed to omit aspects of the experiments we visited, and James Rickman from the communications office had to escort us everywhere.
Our tour started with visits to the people responsible for our invitation to the lab: Melynda Brooks and Mike Leitch, two collaborators on the PHENIX detector at Brookhaven. Melynda showed us the muon tracking chambers she was working on.
As I mentioned in our accelerator technology post, particle detectors work by picking up the signals left behind by high-speed particle collisions. When particles smash into each other going at almost the speed of light, the energy released in the collision is converted into a shower of new particles. As time passes, those new particles decay into other particles, and so on and so on. This process usually happens so fast that by the time the detector picks up a signal a fraction of a second after a collision, the particles have already decayed several times. When physicists decide what particles they are interested in studying in a particular experiment, they must calculate their likely decay patterns and scour the data for those expected signatures.
Melynda explained, “There are just particular decay channels where the particles that are produced are muons. By taking like pairs of muons and reconstructing what they must have originated from we can calculate what the mass was, we can select particular particles. Here we work a lot on a particle called the J-psi.”
The muon tracking chambers Melynda and Mike’s team is working on will be able to tell exactly where the original particle decayed into muons, thereby allowing them to further differentiate the particles that result from a collision in the PHENIX detector and improve their samples of each type. Melynda gave us the example of distinguishing between the J-psi and the open charm samples produced at PHENIX: “The J-psi decays basically instantaneously….The open charms tend to decay a few microns away from the primary vertex,” she explained. This increased precision will allow the PHENIX team to construct better hypotheses about the quark-gluon plasma it is studying.
The PHENIX collaboration is somewhat of an anomaly at Los Alamos: not only is it unclassified, but it has nothing at all to do with weapons science or the military. As James explained, “There’s some straight science for science’s sake that comes out of Los Alamos, but most everything that we have here is a product in one way or another of the laboratory’s nuclear weapons mission. And so a lot of the really excellent science that comes out of there is a result of kind of leveraging that mission science and using it and applying it to problems of national importance, especially in the physics realm.” We saw that kind of applied “mission science” during our next stop: muon radiography.
Muon radiography at Los Alamos is a homeland security project designed to help find what our tour guide Andrew Green called “special nuclear materials” – from chunks of lead or uranium to suitcase bombs – that are being smuggled into the U.S. After the last decade, the words “homeland security project” usually make me think of human rights violations, not cosmic rays, but cosmic rays were exactly what were on display here. The team had just finished building a detector that used the muons passing through the atmosphere as cosmic rays to scan vehicles for hidden nuclear materials. It was en route to the border crossing at San Diego by the time we visited, but we still got a good sense of the project.
In 1946, as the world was struggling to understand the titanic changes caused by the development of the bomb, a senator asked Oppenheimer what tools existed to prevent a small group from smuggling a bomb in a crate into America and using it to completely destroy a major city. “A screwdriver,” Oppenheimer replied, to open every crate entering the country. Today, if anything, the risks of living in the nuclear age are even more terrifying than they were in the 1940s: as this New Yorker article about the black market trade of uranium refinement machinery demonstrates, the equipment and know-how required to build a bomb have been in the hands of the wrong people for decades.
Los Alamos’s muon radiography detector is essentially Oppenheimer’s screwdriver, without the screwdriver. Using techniques to measure how much cosmic ray muons scatter as they travel through the vehicle in the detector would allow border patrol agents to quickly catch any high density materials that may be hidden inside. The detector has a top and a bottom but no sides and is big enough that a truck or a shipping container could be driven into it. Best of all, it is completely passive – cosmic rays pass through us regardless of whether there’s a detector in place to observe them – and easily fast enough to be used at a border crossing.
“For the same reason that they [muons] are very useful for doing relativistic heavy ion physics, they are useful for this too,” Andrew said. “Muons are able to go through lots of material. And going through most detectors, they are just minimum ionizing. You see a track, and you’re able to reconstruct that track pretty much fully to high precision.” He continued, “To see a smoking gun signal takes only about a minute.”
With detectors installed at ports and commercial border crossings, we stand a real chance of catching potentially fissile materials before they make it into the United States. And as someone who has spent a good chunk of her life waiting to cross the Tijuana/San Diego border, the idea of a precise scan that would target dangerous materials and could actually speed up the line seems like a dream. “[Drivers] always have to stop [at the border] anyway to deal with customs,” Andrew said. “While they’re stopping, they get scanned, and it’s free – except for these million dollar detectors.”
We spent most of the summer exploring basic science, so this new realm of applied physics seemed a little strange. For starters, it was actually profitable. Some of the funding for the muon radiography detectors came from a company called Decision Sciences Corporation, which stood to make money from selling the technology. (At the time of this writing, it had an image of the muon radiography detector on its home page.) “We always like it when people can make money off of our ideas,” Andrew told us.
I was also enchanted by the creativity behind the project. Harnessing cosmic rays to easily, quickly, and precisely scan vehicles in border lines for smuggled nuclear materials just isn’t an idea that occurs to everyone. All in all, the project seemed like the perfect model of applied science: using particle physics detector technology to build a machine that solves a real world problem. (You can read Los Alamos’s press release about the muon radiography project here.)
Our next stop at the lab took us back to the world of basic science with the group searching for the electric dipole moment of the neutron, called the nEDM. An electric dipole moment is an expression of the separation between the positive and negative electrical charges within the same system. While it is hypothesized that the neutron experiences such a separation, the nEDM has never been experimentally observed. In part, physicists are interested in finding the nEDM because it would violate a law of classical physics called time-reversal symmetry, which posits that time cannot move backwards.
Yet strange as it may seem, time-reversal symmetry has actually already been disproven: according to quantum mechanics the only reason that a glass you’ve dropped on the floor doesn’t reassemble itself and jump back into your hand is that such an event is extremely improbable, not actually impossible. So why are physicists still looking for the nEDM? For one thing, finding the nEDM could be a step towards experimentally proving supersymmetry, a popular Beyond the Standard Model theory. No matter what happens, the nEDM is probably part of the next theoretical step: as our guide Martin Cooper told us, “You almost can’t invent a theory beyond the standard model which gets rid of the neutron EDM….Supersymmetry is just the popular one.” Additionally, identifying a new source of time-reversal symmetry breaking may help explain why there is so much more matter than antimatter in our universe, one of the great mysteries of physics.
The Los Alamos team we visited was working on the R&D for an experiment that will look for the nEDM at a fraction of a degree above absolute zero, using cryogenic equipment to measure the difference in how ultra-cold neutrons and helium-3 precess around a magnetic field. (Read Martin’s detailed explanation of his team’s work here. ) At time of our tour, the team’s main focus was figuring out how to work at such cold temperatures. Martin explained, “Most of the difficulties of this experiment are understanding how to build low temperature apparatus and the physics of very dilute mixtures of helium-3 and helium-4 in this region from 300 to 450 millikelvin….The neutron part of it only really comes at the end.”
While the final version of the experiment will be housed at the Spallation Neutron Source at Oak Ridge, the techniques that are being developed are so sensitive that the scientists must figure out all the ins and outs before building the final machine. “It’s such a big piece of apparatus that is takes like a month to cool down or a month to warm up, so the problem is that you can’t work on it very well. Every time you decide to make one change you spend $100,000 and two or three months of your time doing it. So we want to test every piece before it goes there,” Martin said. While experimental particle physics is often focused on proving big ideas (in this case, supersymmetry), the bread and butter of its operations are these very precise baby steps.
Somewhat surprisingly, we’d spent nearly two hours at Los Alamos at this point and hadn’t seen any projects related to nuclear weapons. Of course we weren’t going to be invited to see classified weapons research, but “stockpile stewardship”—making sure that the U.S.’s supply of nuclear weapons is understood and maintained—is a big part of the lab’s mission these days, and we got a good look at it during our next and last stop, proton radiography.
Proton radiography is a technique used to image objects and events on very small scales. In some senses it is similar to what we saw at the National Synchrotron Light Source, but instead of using x-rays, the team at Los Alamos uses focused beams of protons to image microscopic events. Also, the events they are looking at are incredibly powerful explosions.
The main goal of proton radiography is to test weapons in the U.S.’s nuclear stockpile on a miniature scale. Our very enthusiastic guide Dale Tupa told us, “We here at proton radiography can test things that are components for nuclear weapons or other weapons system without actually setting off a bomb. We have been able to answer very important questions on the reliability of weapons that sit the stockpile without having them actually set one off.”
Since most of the weapons in the nuclear stockpile are decades old at this point, and the U.S. conducted its last nuclear test in 1992, there are serious questions about how they have changed over time. Will they still work? To use the stockpile as a deterrent, which is the stated policy of the United States, the answer must be a definitive yes. (To put this in perspective, the Air Force maintains 98.5% readiness of its minuteman missiles at all times). Have the weapons grown more or less sensitive over the years? Will it take more or less force to detonate them? How predictable will their yields be under these changing circumstances? In order to establish protocols for handling these weapons without setting one off by accident, we need to know. Dale and her team are able to answer these questions and more by setting off controlled explosions inside a chamber in the lab and taking detailed pictures of those explosions in progress.
To take one example, in the below photo (taken from the DOE Office of Nuclear Physics explanation of proton radiography), a disk of tin reacts to high explosives: “First, the detonation’s spherical shock wave bulges the disk (about 2 inches in diameter and 0.25 inch thick). Then, when the compressive shock wave reflects from the disk’s upper surface, the shock becomes tensile, dislodging and levitating the spall layer (the “flying saucer”). The shock wave’s high pressure and temperature also melt the tin (light gray region connecting the flying saucer and bulge).”
Dale works on the optical diagnostics for the shots, and she was clearly delighted and amazed that her job involved taking pictures of super cool explosions. While showing us some of the videos that her team as her produced, she exclaimed, “And they pay me!” It reminded me of my Physics for Poets professor Hal Evan’s explanation for why he became a physicist: “I have always been interested in smashing things into each other. This narrowed my career options to High Energy Physics or Demolition Derby. Not having good driving skills, I ended up doing research.”
Proton radiography showcased an interesting middle ground between basic science and military research. They employ some fundamentals of experimental particle physics to do truly cutting edge research into areas like shock waves and high pressure environments. But the work is contingent on the military’s needs. Whether political concerns can or should be separated from the discussion of such research, no matter how cool it may be, is a question I still can’t answer. But I did learn that Los Alamos’s “mission science” is a lot more creative and varied than I expected it to be. The lab may never escape its relationship to the Manhattan Project and the military, but that’s certainly not stopping the scientists there from doing research they love.