Wikipedia Deep Dive

Ionizing radiation

13 min read

In the summer of 1898, a young doctoral student named Marie Curie was puzzling over a strange phenomenon. Certain rocks—pitchblende ore, specifically—seemed to emit something invisible that could expose photographic plates through thick black paper. Whatever this was, it passed through solid matter as if the matter weren't there. Within two years, she would coin the term "radioactivity" and discover two new elements. Within four decades, this invisible something would level two cities and reshape the geopolitical order of the entire planet.

That invisible something was ionizing radiation. And understanding it—really understanding it, from first principles—turns out to be one of the most clarifying exercises in modern physics.

What Makes Radiation "Ionizing"

Here's the core idea: atoms are mostly empty space, with a tiny dense nucleus surrounded by a cloud of electrons. Those electrons are bound to the nucleus by electromagnetic attraction, but that bond isn't infinitely strong. If you hit an atom with enough energy—through a fast-moving particle or an energetic photon of light—you can knock an electron completely free. The atom, now missing an electron, becomes a charged particle called an ion.

That's ionization. Radiation that can do this is called ionizing radiation.

The threshold turns out to be around 10 to 33 electron volts, depending on which atom you're trying to ionize and how precisely you want to define things. For context, visible light photons carry only about 2 to 3 electron volts of energy. That's enough to excite electrons into higher energy states—which is why things have color—but not enough to knock them free entirely.

Ultraviolet light sits right at the boundary. The lower-energy ultraviolet that gives you a suntan is non-ionizing. The higher-energy ultraviolet that the ozone layer blocks? That's ionizing, which is one reason ozone depletion was such a serious concern. X-rays and gamma rays, sitting even higher on the electromagnetic spectrum, are always ionizing.

But electromagnetic radiation is only half the story.

Particles That Rip Through Matter

The other half involves actual particles—tiny pieces of matter traveling at enormous speeds. When Ernest Rutherford began systematically studying radioactive emissions in 1899, he discovered that the invisible rays coming from radioactive materials were actually three completely different things. He named them after the first three letters of the Greek alphabet: alpha, beta, and gamma.

Alpha particles turned out to be the nuclei of helium atoms—two protons and two neutrons bound tightly together—ejected from unstable atomic nuclei during radioactive decay. They're relatively massive and carry a charge of +2 (having lost both their electrons). This makes them ferociously ionizing. An alpha particle tears through matter like a bowling ball through pins, knocking electrons loose from atom after atom.

But here's the counterintuitive part: that same property that makes them so damaging also makes them easy to stop. Alpha particles interact so strongly with matter that they exhaust their energy quickly. A few centimeters of air will stop them. A sheet of paper will stop them. The dead outer layer of your skin will stop them.

This creates a strange situation. Alpha emitters like plutonium-239 or polonium-210 are nearly harmless sitting on a table across the room. But if you inhale or ingest them? Now those alpha particles are being released directly into your lung tissue or digestive tract, with nothing to stop them. This is why Alexander Litvinenko's assassination with polonium-210 in 2006 was so insidious—the poison was almost undetectable until it had already done catastrophic damage to his internal organs.

Beta particles are electrons or their antimatter counterparts, positrons, ejected during a different type of radioactive decay. They're much lighter than alpha particles and carry only a single unit of charge, so they don't ionize quite as intensely per unit distance. But that same property lets them penetrate deeper—through skin, into the body. A few millimeters of aluminum or a thick piece of clothing will stop most beta radiation.

Gamma rays, the third type Rutherford identified, turned out not to be particles at all. They're high-energy photons—pure electromagnetic radiation—produced during nuclear reactions. Unlike alpha and beta particles, gamma rays have no mass and no charge. They don't rip through matter by direct collision. Instead, they ionize indirectly, through quantum mechanical interactions that knock electrons free from atoms they encounter.

This makes gamma rays far more penetrating. They'll pass through your body, through walls, through several centimeters of lead. Stopping gamma radiation requires dense, thick shielding—which is why nuclear reactor containment structures are made of concrete meters thick.

The Invisible Bullet You Cannot Feel

Here's what makes ionizing radiation uniquely terrifying among environmental hazards: you cannot sense it. At all.

We evolved elaborate sensory systems for threats our ancestors faced. You can see a predator. You can smell spoiled food. You can feel extreme heat or cold. But ionizing radiation? It passes through your body at nearly the speed of light, ripping electrons from your DNA molecules, and you feel absolutely nothing.

This isn't because the damage is subtle. A lethal dose of radiation doesn't feel like anything while it's happening. The firefighters at Chernobyl who received fatal doses initially felt fine—some even reported a metallic taste, but nothing like the overwhelming sensation you'd expect from an injury that would kill them within weeks.

The damage from ionizing radiation is probabilistic and delayed. When a gamma ray ionizes a molecule in your DNA, several things might happen. The cell's repair mechanisms might fix the damage. The damage might kill the cell outright, which sounds bad but is actually the safest outcome—dead cells get cleaned up and replaced. Or the damage might corrupt the cell's genetic instructions in a way that lets it survive but with altered behavior.

That last outcome is where cancer comes from.

A single ionization event might or might not cause cancer. The probability is very low for any individual event. But ionizing radiation creates millions or billions of ionization events, and probabilities compound. This is why radiation exposure follows a linear no-threshold model in most safety calculations—there's no perfectly safe dose, just doses with acceptably low risk.

Where Ionizing Radiation Comes From

You're being irradiated right now, as you read this. You have been your entire life. So were your parents, and their parents, going back to the first life on Earth.

Background radiation is everywhere.

Some of it comes from space. Cosmic rays—primarily high-energy protons but also heavier atomic nuclei—constantly bombard Earth's atmosphere. Most get stopped by the air, but some make it through, and the collisions in the upper atmosphere produce secondary radiation including muons that do reach the surface. The atmosphere provides shielding equivalent to about 10 meters of water, which is substantial but not total.

This is why airline pilots and frequent flyers receive noticeably higher radiation doses than ground-dwellers. At cruising altitude, you're above much of the atmosphere's shielding. A single transatlantic flight exposes you to about the same radiation dose as a chest X-ray.

Some background radiation comes from the ground itself. Uranium, thorium, and their decay products exist in rock and soil everywhere, though concentrations vary enormously by geography. Granite is mildly radioactive. So is concrete made with granite aggregate. The famous Natural History Museum in London is built of granite and gives its occupants a slightly elevated radiation dose.

Some comes from inside you. Your own body contains radioactive potassium-40, about 0.012% of all the potassium in your cells. It's been there since before you were born—potassium is essential for nerve function, and a small fraction of all potassium is this radioactive isotope. You emit about 5,000 beta particles per second from your own tissues.

The most variable source of natural background radiation is radon, a radioactive gas produced by the decay of uranium in soil and rock. Radon seeps up through the ground and can accumulate in basements and ground-floor rooms of buildings. In some areas with uranium-rich geology, indoor radon levels can exceed occupational exposure limits for radiation workers. After smoking, radon is the second leading cause of lung cancer in most developed countries.

Artificial Sources and Human Applications

Beyond natural background radiation, humans have learned to generate ionizing radiation artificially, for purposes both beneficial and destructive.

X-ray tubes, invented in 1895, accelerate electrons and slam them into a metal target. The sudden deceleration produces X-rays—a phenomenon called bremsstrahlung, German for "braking radiation." Within months of their discovery, X-rays were being used for medical imaging. Within a year, people were dying from overexposure, their hands developing radiation burns and cancers from casually demonstrating the new technology.

Nuclear reactors generate ionizing radiation as an unavoidable byproduct of the fission process that produces heat for electricity generation. When a uranium-235 nucleus splits, it releases not only energy but also neutrons, gamma rays, and unstable fission products that continue emitting radiation as they decay. Managing this radiation—containing it, shielding workers from it, dealing with the waste products that remain radioactive for thousands of years—is the central engineering challenge of nuclear power.

Medical applications have proliferated far beyond simple X-ray imaging. Computed tomography scans use X-rays to create three-dimensional images of internal structures. Positron emission tomography injects patients with short-lived positron-emitting isotopes and detects the gamma rays produced when those positrons annihilate against electrons in the body—mapping metabolic activity in exquisite detail. Radiation therapy deliberately exposes tumors to high doses of ionizing radiation, exploiting the fact that rapidly dividing cancer cells are often more vulnerable to radiation damage than normal tissue.

And of course, there are nuclear weapons. A fission bomb works by rapidly assembling enough fissile material—uranium-235 or plutonium-239—to create a runaway nuclear chain reaction. The energy released in that fraction of a second is equivalent to thousands or millions of tons of conventional explosives. But the radiation effects persist long after the blast itself. Fallout—radioactive fission products scattered into the environment—continues emitting ionizing radiation for years or decades, contaminating land and water and food supplies.

Neutrons: The Weird Ones

Most discussion of ionizing radiation focuses on charged particles and electromagnetic radiation. But there's a stranger category: neutron radiation.

Neutrons are electrically neutral—they have no charge at all. This means they don't interact with electrons through the electromagnetic force the way charged particles do. They can pass through matter relatively freely, which sounds like it might make them less dangerous.

It doesn't.

When a fast neutron does hit something, it tends to hit an atomic nucleus. And because neutrons have significant mass, these collisions transfer substantial energy. The most efficient energy transfer happens when the target nucleus has similar mass to the neutron—which means hydrogen nuclei, which are just single protons. When a neutron strikes a hydrogen nucleus in, say, the water molecules that make up much of your body, it can send that proton flying as a high-energy ionizing particle.

This is why hydrogen-rich materials make good neutron shields. Plastics, water, concrete with high water content—all of these slow neutrons down by giving them lots of opportunities to collide with hydrogen nuclei and dump their energy.

But neutrons have another trick. They can be absorbed by atomic nuclei, transmuting stable atoms into unstable radioactive isotopes. This is called neutron activation. Materials that are perfectly safe before neutron exposure become radioactive afterward. The steel structural components inside a nuclear reactor become radioactive over time. The soil beneath a nuclear detonation becomes radioactive. Even the sodium in coolant systems for certain reactor designs becomes fiercely radioactive when activated.

This activation problem is one reason neutron bombs—enhanced radiation weapons designed to maximize neutron emission—were considered particularly awful. They could kill people while leaving buildings standing and, theoretically, render an area safe to occupy sooner than a conventional nuclear weapon. In practice, neutron activation would make surrounding materials radioactive anyway, just through a different mechanism.

Chemistry in the Wreckage

When ionizing radiation tears electrons from molecules, the immediate effect is physical. But the consequences quickly become chemical.

An ionized molecule is chemically different from its original form. Often it becomes a free radical—a molecular fragment with an unpaired electron, desperately seeking to react with something nearby to stabilize itself. These free radicals are extraordinarily reactive. They'll attack whatever molecules they encounter, potentially setting off chain reactions of molecular damage.

This is why ionizing radiation damage in biological tissue often extends beyond the directly ionized molecules. A gamma ray might ionize a water molecule in a cell. That ionization produces hydroxyl radicals. Those radicals then attack proteins, lipids, and DNA throughout the cell. The single original ionization event cascades into hundreds or thousands of chemical modifications.

The same chemistry happens in non-biological materials. Ionizing radiation degrades plastics by breaking polymer chains. It damages rubber through a process called ozone cracking—the radiation ionizes oxygen molecules in the air, forming ozone, which then attacks rubber. Electronic components fail as radiation damages the semiconductor structures. Even glass can discolor and become brittle under intense radiation exposure.

This radiation-induced chemistry creates particular challenges for nuclear waste storage. The containers holding radioactive waste must withstand decades or centuries of bombardment by the radiation from their own contents. Materials that seem durable under normal conditions may degrade unpredictably when continuously irradiated.

Living With the Invisible

The impossibility of sensing ionizing radiation directly led to the development of an entire field of radiation detection instruments. Geiger counters, the iconic clicking devices of nuclear disaster movies, detect individual ionization events and convert them to audible clicks or meter readings. Film badges, worn by radiation workers, record cumulative exposure over time through the fogging of photographic emulsion. Scintillation detectors convert radiation energy into visible light flashes that can be counted electronically.

These instruments have revealed just how variable radiation exposure can be across environments and activities. A dental X-ray delivers about 5 microsieverts. A chest X-ray, about 20 microsieverts. A CT scan of the abdomen, perhaps 10,000 microsieverts—equivalent to several years of natural background radiation in a few minutes.

The average person in the United States receives about 6,200 microsieverts per year from all sources combined, roughly half from natural background and half from medical procedures. That number has increased substantially in recent decades, not because natural background has changed but because CT scans and other diagnostic imaging procedures have become far more common.

Occupational limits for radiation workers are set at 50,000 microsieverts per year, well below levels associated with immediate health effects but acknowledging increased lifetime cancer risk. The acute radiation doses that cause immediate sickness and death start around 1,000,000 microsieverts—about 150 years of normal background exposure, delivered in hours or days instead of over a lifetime.

We live, in other words, in a radiation environment. It has always been there. Life evolved within it, and our cells have repair mechanisms precisely because radiation damage to DNA has been a constant feature of existence on Earth for billions of years. The question is never whether we're exposed to ionizing radiation—we always are—but how much, and whether that amount falls within the range our biology can handle.

What Marie Curie discovered in that Paris laboratory was not something new in the world. It was something that had been there all along, shaping the evolution of life, driving mutations, occasionally killing, occasionally causing cancer. She gave us the tools to see it, measure it, harness it, and fear it appropriately. The invisible became, if not visible, at least detectable—which turns out to be enough to transform it from an unknown hazard into a quantifiable risk, a tool for medicine, and a source of energy powerful enough to reshape civilizations.