High-precision construction of sPHENIX detector components wraps up at NPL

A team of scientists and research technicians at Illinois Physics has played a key role in the construction of the upcoming sPHENIX experiment at Brookhaven National Lab (BNL), by manufacturing the new electromagnetic calorimeter (EMCal) for the sPHENIX detector system. 

Garrett Williams
for Illinois Physics Condensate

 

 

 

 

 

 

 

The sPHENIX experiment is a collaboration of about 300 scientists who are exploring some of nature’s most basic and intriguing phenomena through study of the quark-gluon plasma (QGP). The QGP—an extremely hot and dense liquid that filled the universe for a few millionths of a second after the Big Bang—can only be produced in the high-energy collisions of heavy ions such as gold or lead. The planned sPHENIX experiment’s new sophisticated detectors will enable the search for more detailed descriptions of the physics governing the QGP. 

Illinois Physics Professor Anne Sickles comments, “The new sPHENIX detector is designed to enable us to measure the properties of the QGP in ways that we haven’t been able to achieve before. The QGP is the high temperature state of matter where protons and neutrons centrally melt, and the matter is composed of free quarks and gluons. This matter lives for an extremely short time, about 10-22 of a second [that’s 1 /10,000,000,000,000,000,000,000!], so studying it is extraordinarily difficult and requires highly specialized detectors. The new detector will enable us to do the same kind of measurements at RHIC that we could previously only do at the Large Hadron Collider (LHC) at CERN.”

“Electromagnetically interacting particles traveling through a tungsten block cause an electromagnetic shower, a cascade of bremsstrahlung—high energy photons—and electron-positron pairs produced from photons. Some photons are then guided by the scintillating fibers toward the detector, where their energy is converted to an electrical signal. Tungsten is used because it’s dense and stops the particles over a short distance. The EMCal is one of several sPHENIX detectors that measures the position and energy of particles in order to characterize the collisions. ”

Illinois Physics Professor Anne Sickles

Work on the major upgrades to the detectors could easily have ground to a halt early in the effort as a result of manufacturing challenges. When it became apparent that private companies would be unable to manufacture at sufficiently high quality the necessary specialized components for the EMCal subsystem of the sPHENIX detector, Sickles proposed to the collaboration that the components be built at the Nuclear Physics Lab (NPL) at the University of Illinois Urbana-Champaign campus. 

“We have an extensive history of building large detector components at high quality,” Sickles notes. “NPL is one of the only university labs in the country that can build things like this—we have both the technicians and facilities.”

After sPHENIX tested 16 UIUC-produced prototype blocks at Fermilab in 2018, Illinois Physics Research Professor Caroline Riedl and Research Engineer Eric Thorsland put together a team to construct the more than 5,000 detector blocks for the EMCal. The two have overseen the day-to-day operations and coordinated the prototyping and materials acquisition for the project. 

“My work with the sPHENIX project began in late 2014,” Thorsland says, “and as we wrap everything up now at the end of 2021, I can say, it has been quite the effort. There were early development and R&D challenges with several prototypes. I worked to develop a literal factory at the Nuclear Physics Lab, creating a process for manufacturing thousands of functioning calorimeter blocks. When Dr. Riedl came on board, we were fortunate to employ many students from UIUC and an excellent group of technicians recruited from the Parkland College Industrial Technology Program.”

The COVID-19 pandemic brought extra challenges to an already daunting production schedule, but Sickles, Riedl, and Thorsland made sure work moved forward with every safety measure in place to protect the team.

“Everyone worked extremely hard—even throughout the COVID lockdown—to keep the calorimeter production going,” notes Thorsland. 

Sickles adds, “We were one of the first labs to open up after the COVID closures, having developed rigorous safety protocols. We have been working safely since May 2020 and have not had to close the lab for COVID infections at any time. Timeliness of production is very important to the progress of the experiment, and where many research projects slowed down during COVID, this team kept going.”

Each calorimeter block has been constructed from scratch by the group’s team of about 20 workers—technicians, undergraduate and graduate students, postdocs, and faculty. The blocks are made of tungsten, a heavy metal, with embedded scintillating fibers. NPL’s block-production protocol was developed and overseen by Thorsland. Once the blocks passed a rigorous inspection—high precision being essential to the successful operation of the new calorimeter—they were shipped to BNL to be assembled to sectors of the calorimeter. 

Riedl notes, “We had in total 72 UIUC students working on this project, mostly undergraduates, though three were physics graduate students. Some stayed with us for more than 3 years. We could not have done it without them. It was not only their sheer number, it was also that a small hand-selected group was trained to take over the coordination of tasks.”

Thorsland sums up, “One of the best parts of the experience has been being able to work with such a dedicated group of students, technicians, grad students and researchers. It has made an exhausting effort enjoyable over the past six years. I’m looking forward to seeing the detector in its final form.”

Studying the quark-gluon plasma and proton structure

The sPHENIX collaboration has scheduled first collisions for February 2023. The overarching goal of sPHENIX is to understand the microscopic structure of the plasma and how its strong interactions arise from the underlying quantum chromodynamics.

The quark-gluon plasma (QGP) generated in the sPHENIX experiment will come from the collision of the nuclei of large gold atoms. The collisions result in jets—sprays of subatomic particles that aren’t stable and evolve into other particles as the jet is occurring. The detector registers the particles and a trigger system decides which data to store on BNL computer disk and tape systems. Electronic devices on the detector front-end convert the analog signal from the detector response to digital form, which can be processed by computers.

“The sPhenix experiment will provide state-of-the-art capabilities for studies of the strongly interacting QGP,” Riedl notes. “The sPHENIX detector is an optimized jet detector. Jets are sprays of particles that emerge in a cone-like structure from the collision interaction point. The EMCal is designed to measure the properties of these jets.”

Sickles adds, “We want to measure how these jets are modified by their interaction with the quark gluon plasma.”

Sickles describes in more detail how the tungsten blocks work in the EMCal’s detection of the particle-spray, or high-energy jets, of heavy-ion collisions:

“Electromagnetically interacting particles traveling through a tungsten block cause an electromagnetic shower, a cascade of bremsstrahlung—high energy photons—and electron-positron pairs produced from photons. Some photons are then guided by the scintillating fibers toward the detector, where their energy is converted to an electrical signal. Tungsten is used because it’s dense and stops the particles over a short distance. The EMCal is one of several sPHENIX detectors that measures the position and energy of particles in order to characterize the collisions,” says Sickles.

Riedl notes that the physics of interest to members of the sPHENIX collaboration comes in the form of two separate projects. With the future collisions of transversely polarized protons in RHIC, Riedl and her sPHENIX colleague, Illinois Physics Professor and Head Matthias Grosse Perdekamp, are interested in investigating proton structure: Where does the proton spin come from? Do quarks and gluons in the proton undergo orbital angular motion? What is the multi-dimensional proton picture in transverse-momentum and position space? Whereas this line of inquiry in  nuclear physics is referred to as “cold QCD,” Sickles is interested in the QGP of “hot QCD” at high temperatures. 

Sickles explains, “How does this low-viscosity fluid arise from the fundamental interactions of quarks and gluons—that’s the question we are trying to answer. The measurements sPHENIX takes will help us to understand the temperature dependence of the quark-gluon plasma. There are currently only two places on Earth where such measurements can be taken, and the LHC is hotter than RHIC, we operate at comparatively moderate and low temperatures. Comparing our measurements with those at the LHC will help us to understand the properties of quark-gluon plasma—an exotic state of the strong nuclear force. One of the fundamental forces, the strong nuclear force is responsible for over 99 percent of visible mass, but is one of the hardest forces to investigate experimentally.”