Tucked beneath the corner of Whitney and Edwards, the Wright Lab is a hidden landmark on Yale’s Science Hill. Little of its bunker-like exterior gives away the centerpiece lying behind the locked bay doors — a massive, 100 foot-long particle accelerator that was, at one point, the world’s most powerful atom smasher of its kind. On Monday mornings for more than 50 years, the building rumbled to life, as teams of engineers and technicians scrambled to ready the device for a new week’s slate of experiments. For 24 hours a day, 5 days a week, the machine rattled and screeched as researchers anxiously awaited solutions to the great mysteries in nuclear physics.
Today, the lab is in a state of transition. Boxes are strewn across the concrete floor, half-filled with instruments to be shipped away or discarded. The various target rooms that once housed millions upon millions of nuclear collisions now lie completely empty, save for a few stacks of concrete shielding blocks. Workers armed with Geiger counters walk among the blocks, checking for any lingering radiation.
The accelerator itself has been partially dismantled, with some of its components salvaged, but its bright blue hull still imposes upon the center of the room. The machine has been dormant since its last run in 2011, but it has indelibly shaped the story of physics at Yale and beyond.
Revolutionizing particle physics
The Wright Lab began with a vision. In 1962, Professor D. Allan Bromley and other faculty sought to transform Yale into a world leader in the field of nuclear physics, and the most direct way to achieve that was to build a state-of-the-art particle accelerator. Construction of the first version of the accelerator — the MP (Emperor) tandem — began in 1964. Simultaneously, lab facilities were built to house the accelerator and the complex data acquisition and control systems needed to run experiments. Founded as the Wright Nuclear Structure Lab (WNSL), the building was a hub of activity as researchers from around the world came to conduct their experiments. In 1967, the first graduate student, current Professor of Physics Richard Casten, earned his PhD studying the structure and behavior of osmium nuclei.
Since its inception, the Wright Lab’s tandem accelerator has continually been the “best of its kind in the world,” as Casten put it. In a typical electrostatic particle accelerator, a standing electric field is used to propel positively-charged ions, or cations, from a high-voltage terminal. These ions are accelerated to incredible speeds — up to 20 percent of the speed of light depending on the accelerator and the ions’ masses. Magnets placed around the accelerator chamber guide and filter the particles, sending a beam of one billion ions per second surging towards the target. Typically, only one in a million ions actually collides with the target nuclei, but the interactions that do occur produce gamma radiation that can be measured by researchers.
Tandem accelerators take advantage of the ability to remove negative electrons from particles in motion. By stripping ions as they fly through the machine, physicists are able to reverse their polarity, from negative to positive, as they proceed from one stage of the device to the next. In this manner, the same electric potential could essentially be used to accelerate the particles twice, radically increasing the possible output energies.
Until the MP tandem was completed in 1966, nuclear physicists were mostly limited to studying light nuclei, or nuclei with low masses, since existing machines were not powerful enough to accelerate heavier particles to the high energies necessary for effective collisions. Because the protons in nuclei are positively charged, a beam of cations hurtling toward a target of nuclei will slow down due to the electrostatic, repellant force between them. “If you roll a ball up the hill, if you don’t roll it hard enough it’s never going to get past the top,” said Casten. “So when you want to study heavier and heavier nuclei that have more and more protons in them, you’re going to need more and more energy to get over that barrier. With the tandem, that was the beginning of a new era in heavy ion science. It revolutionized everything.”
With the success of Yale’s tandem, six or seven identical accelerators were built around the world. For the first time, scientists could study nuclei at excitation states that were previously unattainable. Research flourished, and Yale became an epicenter of work in nuclear physics. In the mid 1980’s, under Bromley’s leadership, the MP tandem was upgraded with the installation of new stainless steel acceleration tubes. With the change, scientists could achieve even higher energies for their accelerated particles. Later, in 1988, the new and improved ESTU, or extended stretch transuranium, tandem began operations. Two years later, it achieved a benchmark terminal potential of 22.4 million volts, and remained the most powerful stand-alone tandem accelerator in the world until its closing in 2011.
By 1995, however, the Wright lab was caught in a midlife crisis. Bromley had left his appointment as director of the lab to serve as science advisor to the Bush administration. In subsequent years, activity in the lab gradually declined to a point where the Department of Energy — the umbrella organization that funds the bulk of nuclear physics research in the nation — sent in a committee to evaluate the situation. At that time, the committee had two choices: close the Wright lab, and with it, the accelerator, or try and rejuvenate it. Against all odds, they chose the latter.
The first order of business was to appoint a new director, someone who could lead the Wright Lab into the 21st century. Coincidentally, Casten — who had continued on to work at Los Alamos and Brookhaven National Laboratory — happened to be visiting Yale around the same time. With his soaring enthusiasm and long-standing ties to the Wright Lab, Casten was the ideal successor to lead it back to prominence.
Reconstruction was not easy. “When I came in there was one graduate student and one postdoc working with the tandem, and that was it,” Casten said. “Back in the early 70’s there were probably 50 or 60 people on the staff. It was huge!”
With renewed funding, creative ideas for new experiments, and a fresh influx of graduate students recruited from around the world, the Wright Lab began to flourish again. Over the course of the next 15 years, the lab made a number of significant contributions to modern nuclear physics. One of the simplest, yet most important developments was the effect of compounding the number of detectors measuring the gamma radiation output of a specific nuclear interaction. By assembling an array of 11 individual detectors within the main instrument, researchers were able to achieve enough sensitivity to study a whole new class of experiments — the effects of fusing nuclei together.
During this period, researchers also began to explore a phenomenon termed “quantum phase transitions,” changes in nuclear structure predicted by J.W. Gibbs professor of physics Francesco Iachello. Similar to physical phase transitions such as water freezing into ice, Iachello’s theoretical work described shape-shifting nuclei, from spheres to footballs to Frisbees and everything in between. These altered shapes resulted from changes in the number of neutrons. Taking advantage of the accelerator’s capabilities, researchers were able confirm the existence of these oddly-shaped nuclei in 1999, providing the first empirical basis for a field that has remained consistently active ever since.
End of an era
“The accelerator itself was only expected to have a 10 year lifetime,” said Jeffery Ashenfelter, associate director for operations at the lab. “We ran for 23 years.”
Ashenfelter arrived at Yale together with the current ESTU accelerator and fondly recalls the enthusiasm and champagne that accompanied the early successes. Nowadays, he is primarily involved in decommissioning the machine and seeing the lab through its transition. “Over time, the combined progress in the field of nuclear structure and nuclear astrophysics created the need to go, just as this machine did, to higher energies.”
Over the course of the accelerator’s lifespan, it found some of its roles supplanted by larger, more powerful accelerators elsewhere. Moreover, in the past decade, the focus in nuclear physics has shifted to studying increasingly exotic radioactive nuclei that call for a new class of accelerators. These types of radioactive beam accelerators require specialized detectors and beam producers capable of handling extremely rare, unstable interactions of interest. They also require hundreds of staff members working to monitor and contain the radiation. Yale decided that its interests lay elsewhere, and without the resources necessary for an upgrade, the accelerator’s fate had all but been decided.
The last days of the accelerator in 2011 presented a unique challenge to the staff. As part of his thesis work, current associate scientist Ke Han sought to discover evidence of strangelets — superheavy oxygen atoms composed of yet-unobserved strange matter. “We were looking for oxygen with mass of 54, which is really, well, strange,” Han said. “This kind of matter can exist in the early universe and travel along with cosmic rays everywhere, including Earth and the Moon.”
Working with lunar soil gathered by the Apollo 11 mission, the team fired particles from the sample at a gold foil in an attempt to isolate traces of this exotic oxygen. Although the researchers did not observe any interactions at their chosen level of specificity, the test guided future explorations for strange matter aboard the International Space Station and served as a final demonstration of the old machine’s capabilities.
As Yale set its sights elsewhere, the accelerator went quiet one last time.
Karsten Heeger is the face of a new direction in the Wright Lab. As a physics professor and lab director since 2013, he wants to put a spotlight on what he terms “the invisible universe,” an enigmatic class of particles made up of neutrinos and dark matter.
“One of the big outstanding questions in physics is: why do we live in a universe of matter and not one of antimatter? The idea was that at the beginning of time there was a Big Bang, we got particles and antiparticles, and if this happened 14 billion years ago, all of it should have been annihilated by now. And we shouldn’t be here. But we are here, and the question is, why are we here?” Heeger said.
Heeger believes that neutrinos — tiny, almost massless particles traveling throughout the universe near the speed of light — may hold the answer to this question. A related field is the search for dark matter, an invisible class of particles theorized to compose more than 80 percent of matter in the universe. Undetectable by instruments in conventional laboratory settings, neutrinos and dark matter particles can usually only be studied by researchers conducting sophisticated experiments in far-flung locales, typically deep underground. Wright Lab personnel currently work in collaboration with projects worldwide, from China to Italy to the South Pole.
In these fields, the most significant challenge is to build devices sensitive enough to detect these otherwise imperceptible particles. Heeger’s plan is to make the Wright Lab a center of technological innovation, a hub for the development of more specialized equipment. Recently, Wright Lab staff built a detector functioning at temperatures close to absolute zero capable of detecting rare nuclear decays — the kind that produce neutrinos. Data collected from these experiments may provide new insights into the nature and behaviour of these elusive particles.
Following extensive renovations of the Wright Lab building, slated to be completed by the spring of 2016, Heeger envisions a transformation of the lab space and its role on campus, with additional workshop, laboratory, and teaching areas. “We can’t change the building completely, but one of the goals is to make it transparent and accessible,” he said. “We have all sorts of plans for new labs, and instead of a closed bunker, make it an open, collaborative space. We have a new concept of an integrated machine shop for teaching, so people can learn how to build instrumentation. There will be an advanced prototyping shop, somewhat similar to what the CEID is doing.”
With the shutdown of a once world-renowned particle accelerator, the university is looking towards the future, which includes furthering an emphasis on science research, participation, and education. “This will be another resource for science at Yale,” Heeger said.
To the long-time veterans of the Wright Lab, the transition is bittersweet. Technician Frank Lopez, who operated the accelerator for 11 years, recalled the long nights and camaraderie of working with the team. “It was definitely fun to work with the machine … There’s talk of preserving certain aspects of the accelerator,” Lopez said, gesturing to the boxes of timeworn instruments. “Right now, we’re kind of just saving everything that we think is historically significant, but most things you just can’t save. Some of this stuff really tells a unique story about what went on here.”
“There was a time for it, sure,” said Ashenfelter. “You can look at the accelerator and see the last 25 years of my life here — I built it, did a lot of work on it, and now we’re around to take it apart. Sure, there are moments of sadness, but I’m ready to move on.”
About the Authors: Andrew Qi is a senior biomedical engineering major in Morse College. He previously served as the News Editor of the Yale Scientific Magazine. Carrie Cao is a senior in Morse College majoring in molecular biophysics & biochemistry. She is the former Production Manager of the Yale Scientific Magazine.
Acknowledgements: The authors wish to thank Karsten Heeger, Richard Casten, Ke Han, Frank Lopez, and Jeffery Ashenfelter for taking the time to share their inspiring stories and experiences.