How particle detectors capture matter’s hidden, beautiful reality

By | August 5, 2021

At every moment, subatomic particles stream in unfathomable numbers through your body. Each second, about 100 billion neutrinos from the sun pass through your thumbnail, and you’re bathed in a rain of muons, birthed in Earth’s atmosphere. Even humble bananas emit positrons, the electron’s antimatter counterpart. A whole universe of particles exists, and we are mostly oblivious, largely because these particles are invisible.

When I first learned, as a teenager, that this untold world of particles existed, I couldn’t stop thinking about it. And when I thought about it, I could barely breathe. I was, to steal a metaphor from writer David Foster Wallace, a fish who has only just noticed she’s swimming in water. The revelation that we’re stewing in a particle soup is why I went on to study physics, and eventually, to write about it.

To truly fathom matter at its most fundamental level, people must be able to visualize this hidden world. That’s where particle detectors come in. They spot traces of the universe’s most minuscule constituents, making these intangible concepts real. What’s more, particle detectors reveal beauty: Particles leave behind graceful spirals of bubbles, flashes of light and crisp lines of sparks.

Renee Jones measures particle tracks on a cloud chamber scan
Tracks from bubble chambers and cloud chambers typically had to be inspected by eye. In this June 1984 image, Renee Jones, a bubble chamber scanner working at Fermilab, measures the details of the tracks, including length and curvature.David Parker/Science Source

As a physics student, I spent hours examining these stunning pictures in my textbooks. I went on to build particle detectors in graduate school, and to make my own images of particles wending their way through our world.

As a particle moves through a material, it drops bread crumbs that can give away its path. Those bread crumbs come in a variety of forms: light, heat or electric charge. “Basically, every particle detector that exists is looking for one or more of those three things,” says particle physicist Jennifer Raaf of Fermilab in Batavia, Ill. Particle detectors translate the bread crumbs into signals that can be recorded and analyzed. Such signals helped reveal the physics of the standard model, a crowning achievement of science that describes the particles and forces of nature. They’re also likely to be key in the discovery of physics beyond the standard model.

As time has passed, technologies for detecting particles have vastly improved. Here are a few types of detectors that have made the invisible visible.

Through a cloud

One of the first ways scientists visualized particle tracks was with cloud chambers. Developed more than a century ago, cloud chambers are filled with a gas — often a vapor of alcohol — on the verge of condensing into liquid. When a charged particle passes through the chamber, it strips electrons from the air within, creating an electric charge that initiates condensation. A wispy line forms along the particle’s path, like a miniature contrail.

black and white image of positron particle track
A particle track in a cloud chamber in the early 1930s was the first evidence of a positron, a positively charged particle with the mass of an electron. The track is curved due to a magnetic field that surrounded the chamber.C. D. Anderson, courtesy of Emilio Segrè Visual Archives

Scientists often surround cloud chambers and other detectors with a strong magnetic field, which bends particles’ paths into curves or spirals. Negatively charged particles curve in one direction, positive particles go the opposite way. Other details further characterize the particle: The amount of curvature indicates a particle’s momentum, for example.

Cloud chambers revealed a variety of previously unknown particles, including the positron and the muon, a heavy cousin of the electron, in the 1930s. These particles were mostly unexpected. At the time, physicists were barely coming to grips with the fact that particles besides electrons and protons existed.

Cloud chambers are simple enough that you can make one in your own home, using alcohol and dry ice.

black and white image of Clifford Butler and another scientist working in a lab
In this 1948 image, physicist Clifford Butler (center) is adjusting the instruments on a cloud chamber intended to track particles in cosmic rays. These showers of particles are produced when a high-energy particle from space slams into Earth’s atmosphere.Picture Post/Hulton Archive/Getty Images

Bubble trails

The 1950s were all about bubble chambers.

When charged particles pass through liquid in a bubble chamber, they leave tiny vapor bubbles, like iridescent orbs trailing a soap bubble wand. Although the chambers are typically filled with liquid hydrogen, a variety of liquids can be used; one early prototype even used beer. Bubble chambers could be made bigger than cloud chambers, and produced sharper tracks, making it possible to observe more particles in more detail.

black and white image of a kaon particle track
A subatomic particle called a kaon decays into other particles that leave distinct spirals in this bubble chamber image from the 1970s.CERN

In the same decade, particle accelerators came to the fore. These accelerators produce energetic beams of particles that scientists can crash into other particles or into targets. Those collisions whip up a flurry of new particles. Scientists sent those beams into bubble chambers to watch what happened.

black and white image of the Big European Bubble Chamber
The Big European Bubble Chamber, pictured during installation of the vessel, started up at CERN near Geneva in 1973.CERN

The resulting images were not only scientifically illuminating, they were stunning: If Raaf were going to get a tattoo, she says, it might be a bubble chamber image. I’ve so far resisted the temptation to get ink.

Going digital

Cloud chambers and bubble chambers had a drawback. Tracks were typically recorded with photographs, and each had to be inspected by eye for anything of interest. That process was too slow; it held physicists back from discovering the particles that might show up in only one or two out of myriad photographs, if that. To find the rarest of particles, “you can’t really be looking at pictures. You want to have that information digitized in a smart way,” says Sam Zeller, a particle physicist at Fermilab.

image of tracks of new particles produced after a proton and antiproton collide
In the UA1 detector at CERN near Geneva, high-voltage wires recorded the electric charge produced when incoming particles dislodged electrons from atoms in a gas-filled chamber. In this computer display, a proton and antiproton have collided and annihilated, producing new particles that traced out paths throughout the detector.Peter I.P. Kalmus, UA1 Experiment/Science Source

Enter the multiwire proportional chamber. Invented in 1968, this technology relies on a fine array of high-voltage wires, which record charge produced when incoming particles dislodge electrons from atoms in a gas-filled chamber. This technique could capture millions of particle tracks per second, much more than bubble chambers could achieve. And the data went directly to a computer for analysis. Multiwire proportional chambers and their descendants revolutionized particle physics, and led to discoveries of particles such as the charm quark and the gluon in the 1970s, and the W and Z bosons in the 1980s.

image of CERN's UA1 detector
CERN’s UA1 detector was active from 1981 until 1990; its most notable discoveries were the W and Z bosons, together with the UA2 experiment. This image shows a section of the experiment, strung with many fine wires, on display at the CERN museum.Mark Williamson/Wikimedia Commons (CC BY-SA 4.0)

Some of the most advanced modern detectors trace their lineage back to multiwire proportional chambers, such as liquid argon time projection chambers. These detectors are high-resolution, meaning that researchers can zoom in on the details of an interaction and visualize it in 3-D. Liquid argon time projection chambers will be key to one of the biggest upcoming particle physics experiments in the United States, the Deep Underground Neutrino Experiment in South Dakota. Because neutrinos very rarely interact with matter, the experiment demands such advanced detection techniques.

Shining a light

Scientists have also devised methods to detect particles via light. When a particle moves above a certain speed limit for a given material, it emits light, known as Cherenkov light. It’s analogous to an airplane passing the sound-speed barrier and creating a sonic boom. Charged particles can also emit light when passing through materials laced with certain chemicals, called scintillators.

image of neutrino data from the NOvA experiment
The NOvA experiment at Fermilab uses tubes of liquid scintillator to spot neutrinos interacting inside the detector. In this image of data from the detector, a neutrino, which enters from the left, produces a spurt of charged particles. The neutrino is not visible, due to its lack of electric charge.NOvA/Fermilab

To spot the small amounts of light left behind by individual particles, scientists use photomultiplier tubes, originally invented in the 1930s, which convert light into electrical signals. These tubes could be used to pick up either Cherenkov light or scintillator light.

Scintillator detectors began to prove their worth in 1956 when a tank of liquid scintillator was used to discover the neutrino — once thought to be entirely undetectable. Liquid scintillator detectors are still common — used in the NOvA neutrino experiment at Fermilab, for example — as are detectors made of solid plastic strips with scintillator mixed in.

image of a detector from the NOvA neutrino experiment
The NOvA neutrino experiment at Fermilab uses two detectors, this one located in Minnesota, made up of hundreds of thousands of PVC tubes filled with liquid scintillator.Justinvasel/Wikimedia Commons (CC BY-SA 4.0)

Putting it all together

Modern detectors at the world’s major particle colliders, like the detectors at the Large Hadron Collider at CERN near Geneva, throw in a bit of everything. “It’s this onion of different types of detectors; every layer is a different thing,” Raaf says.

image of a detector from the CMS experiment
Modern detectors at particle colliders, such as the CMS experiment (shown) at CERN, pack in an assortment of technologies to spot the spray of particles from high-energy collisions.Maximilien Brice/CERN

Standing multiple stories tall, these massive machines include an assortment of technologies — plastic scintillator detectors, Cherenkov detectors, descendants of multiwire proportional chambers. They also typically include detectors made from silicon that can precisely measure particle tracks based on small electric currents produced when particle pass through. These detectors all work in concert within a very strong magnet. After particles collide at the center of the detector, computers crunch the data from all the parts and reconstruct what happened in the collision, tracing out the paths the particles took.

No matter the technique, the mesmerizing subatomic hieroglyphs allow physicists to decipher the native language of matter, unveiling its constituents and the forces by which they communicate. “It’s pretty amazing that you can see the invisible,” says Zeller.

computer visualization of data from the CMS experiment
This computer visualization of data from the CMS experiment at CERN near Geneva shows the results from a collision of two protons. The event may show a Higgs boson transforming into two photons, particles of light. Yellow lines are particle tracks, and green and blue boxes relate to the particles’ energies.Thomas McCauley, CMS/CERN