Nuclear Experiment nucl-ex

At Jefferson Lab, electrons are accelerated to 12 GeV and smash into protons to pry out their secrets. But existing detectors are like cameras with a narrow field of view: you either see a lot but dimly, or brightly but only a tiny patch. Electron scattering is the main tool, but statistics are insufficient for subtle effects.

A thin film of thorium dioxide, just 10 nanometers thick, is deposited on a steel disk. A laser pulse strikes it, but instead of the expected glow, the detector catches a stream of electrons.

A tuning fork cut from a perfect metal rings at a single frequency. Its sound is as pure as a mathematical formula. But clamp it in a vise or dip it in a viscous liquid, and the tone smears, picks up noise, and loses its sharpness. The nucleus of scandium-45 behaves much the same way when placed in a crystal lattice.

Measured dipole moment of tantalum-181: 3.39 × 10⁻³² e·cm — like spotting a one-atom shift across the entire Solar System. An ordinary atomic nucleus is perfectly balanced, like a spinning top. In tantalum, that balance is broken: the positive and negative charges are slightly offset from each other.

Deep inside stars, light nuclei fuse, giving birth to helium and streams of neutrons. On Earth, we recreate this process by combining deuterium and tritium — special isotopes of hydrogen.

Conventional atomic clocks are essentially optical counters: they rely on spectroscopy — a method for measuring the frequency of radiation with high precision as electrons jump between energy levels.

The muon is a tiny spinning top, twirling in the turbulent sea of the vacuum. Its magnetic axis points slightly away from what the Standard Model predicts. The deviation is just a couple of parts per million, but for physicists, it's a full-blown mystery. The vacuum is not a dead calm.

For two atomic nuclei to fuse, they must overcome their fierce electric repulsion — like trying to bring two angry bulls together. Normally, only monstrous energy does the trick, as in stellar cores, where Hans Bethe explained fusion back in the 1930s.

Our Sun emits streams of neutrinos — particles that barely interact with matter. Passing through the dense solar material, they change their type — scientists call this 'flavor.' It's like a voice that changes as it moves through a crowd: the thicker the crowd, the more it gets distorted.

Inside a giant spectrometer in Karlsruhe, shaped like a huge metal barrel, scientists are catching particles that are nearly impossible to catch. Neutrinos are the ghosts of the microcosm: every second, trillions of these particles fly through your body, and you don't even notice. But they do have mass, and scientists want to measure it.

When a neutron slams into a nucleus, it can knock out a chunk — an alpha particle, which is a helium nucleus. Such collisions happen all the time in nuclear reactors, but they are hard to measure precisely: a neutron leaves no track in a regular detector, and you can only catch it by its aftermath.

Just as in a crowded room each new person sharply increases the pressure on the walls, in a neutron star increasing density leads to a rise in internal resistance. But even this resistance has an absolute ceiling.

When an extra neutron is added to an atomic nucleus, it starts behaving like an overfilled cup: one more drop and it spills over. Physicists at the French facility GANIL orchestrated such a 'pass' of a neutron from a deuteron (the nucleus of heavy hydrogen) to an oxygen-19 nucleus.

Heavy water (the kind where ordinary hydrogen is replaced by its beefier cousin, deuterium) was subjected to an electric current. Palladium settled on the cathode, and in this modest electrochemical bath, detectors repeatedly caught something unexpected: traces of slow neutrons.

Water freezes at zero and boils at one hundred degrees. Matter made of quarks and gluons also has boundaries between states, but instead of a thermometer, the temperature here is measured in millions of electron-volts.

Inside a seven-meter spectrometer that looks like a tall column, fragments of a decayed neutron are born. To catch them, you need silicon wafers the size of a dining table and as thick as a finger.

Water in a kettle heats to 100 degrees, and stops being water, turning into steam. Something similar happens to atomic nuclei when they are smashed together at enormous speeds in the Large Hadron Collider.

Deep inside an underground laboratory, a room-sized detector waits for an event that might happen once in a hundred million billion years. That event is neutrinoless double beta decay — a process that would break one of physics' fundamental laws. To catch it, you need perfect silence: not a single stray radioactive atom in the detector's materials.

The atomic nucleus is not just a bag of protons and neutrons. It is a quantum system where particles are bound by invisible threads of entanglement. When two protons or neutrons interact, their states become intertwined so that describing one without the other is impossible.

Every atom has its own 'voice'—the frequency of light it emits. For isotopes, atoms with different numbers of neutrons, the voices differ slightly. It's like a choir: if you know the score precisely, the slightest false note reveals external interference. The King plot is a way to compare how harmoniously two notes from different isotopes sing together.

Atomic nuclei are home to protons and neutrons. But sometimes a guest flies in — a particle called the lambda. It's like a neutron, but heavier and with a special property physicists call strangeness. A nucleus with such a guest is called a hypernucleus. At the MAMI accelerator in Mainz, researchers created the lightest hypernucleus — a triton with a lambda inside.

Inside a refrigerator that cools matter to near absolute zero beats a mechanical heart — the pulse tube. It pumps gas and extracts heat, but along with the cold it brings tremors. Each beat of this heart shakes the instruments sitting next to the sample and ruins delicate measurements.

An ordinary piece of germanium the size of your palm can spot a single dark matter particle flying through it. But for that, the crystal must be nearly perfect: free of electrical noise, with a uniform internal field, and contacts that don't spark at high voltage. In the new work, they assembled the KL01 detector.

A muon is like an electron, but about 200 times heavier. When it stops in a piece of silicon, it gets pulled into the atomic nucleus like a nail to a magnet. The nucleus gains extra energy and starts to "boil": various particles fly out — protons, deuterons, tritons, and alpha particles.

Igniting a miniature sun is a long-held dream of humanity. Typically, scientists try to confine scorching plasma (a gas of charged particles) at temperatures of hundreds of millions of degrees. But plasma tends to escape magnetic traps, and confinement comes at enormous cost.

Neutrons in an atomic nucleus aren't all held equally tight. If you peel them off one by one, like leaves from a cabbage, the energy required changes each time. For isotopes with 149 particles in the nucleus (lanthanum, cerium, praseodymium), this energy has just been remeasured, and one number didn't match the previous value.

A neutron star is the ultra-dense remnant of a supernova explosion, whose existence was predicted by Fritz Zwicky back in the 1930s. Later, Jocelyn Bell Burnell detected them as pulsars — rotating sources of strictly periodic radio signals.

In a tank of pure water, particles traveling faster than light in that medium leave bluish flashes, like a plane breaking the sound barrier creates a sonic boom. From these flashes, you can tell what kind of visitor passed through: a gamma quantum or a neutron. This is no idle task: at borders and warehouses, you need to quickly distinguish a harmless radioactive cargo from weapon-usable material.

Inside an atomic nucleus, particles aren't just tossed together any which way. Sometimes two protons and two neutrons huddle into a tight quartet — a mini-collective known as an alpha cluster. Think of it as a family that sticks together in a crowded building. Knocking this quartet out of the nucleus and seeing how it lived there is a job for a fast proton.

A dense piece of plastic the size of a coin flashes when an invisible particle hits it. That's how scintillators work — materials that turn radiation into faint light. They're usually made from fragile crystals or expensive polymers. But now there's a simpler option: plain polyurethane, the stuff used to mold furniture foam and shoe soles.

Ten microseconds after the Big Bang, the Universe resembled a pot of boiling broth. Floating in it were quarks and gluons — the particles that would later assemble into protons and neutrons. As the broth cooled, at some point the quarks clumped together into lumps — the very protons and neutrons that make up all visible matter.

A bell doesn't ring with a single note — after being struck, it sounds with several harmonics at once. An atomic nucleus vibrates in much the same way: after a collision, it can oscillate, and these oscillations show up in the emitted gamma rays.

In a particle accelerator, electrons race around a ring at nearly the speed of light. When such an electron collides with a photon, a particle of light, the photon bounces off like a ball hitting a moving truck and flies away with much greater energy.

Every year, in the United States alone, doctors perform more than 16 million scans using the radioactive isotope technetium-99m. It lights up the heart, bones, and lungs in images, helping to find tumors or check blood flow. But the isotope itself has a half-life of only six hours, so you can't stockpile it: you need a steady supply.

Catching a whisper in the middle of a noisy train station: that's roughly the challenge facing physicists searching for the neutron's electric dipole moment. This is a tiny shift of charge inside the particle. If it exists, it breaks the symmetry between matter and antimatter and could explain why our world exists at all.

A thermometer cooled to near absolute zero picks up the heat from a single particle. That's how cryogenic calorimeters work — instruments that catch ultra-rare events like neutrinoless double beta decay. But old thermistor thermometers respond slowly: in about a millisecond.

Accelerators sometimes produce particles that don't fit the usual two- or three-quark scheme. Two such particles, Zb(10610) and Zb(10650), have been puzzling physicists for over a decade. They look like heavy cousins of pions, but with a clear sign of a four-quark composition. The question is how those four quarks are packed.

In a particle accelerator in Beijing, an anti-Lambda hyperon, a rare antimatter particle, slams into an ordinary proton. Instead of simply disappearing, they transform into a whole bouquet of other particles: a positively charged kaon, two pions, and sometimes additional neutral pions. Such transformations for the anti-Lambda had never been observed before.

The nucleus of a xenon-133 atom lives only a few days before it decays. Making a thin film target out of it and bombarding it with particles is out of the question: the film would vanish faster than it could be produced. So physicists did the opposite: they accelerated xenon-132 nuclei and aimed them at a target made of deuterium (heavy hydrogen).

Growing an ultra-pure germanium crystal is a bit like baking a finicky soufflé: a slight overheating or an extra speck of impurity, and the whole ingot is only good for remelting. Such crystals are the heart of detectors that catch the rarest events, from dark matter to double beta decay.