The Ray He Could Not Name
On 8 November 1895, the German physicist Wilhelm Röntgen was experimenting with a Crookes tube when he noticed something that should not have been happening. Some form of radiation was passing straight through solid material.
He did not know what it was, and he said so in the name. He called it X-radiation, using X in its mathematical sense — the unknown. Colleagues suggested calling them Röntgen rays instead, and many languages still do, but his placeholder stuck in English and never left.
The naming honesty is worth pausing on. Röntgen had found a form of high-energy electromagnetic radiation with a wavelength shorter than ultraviolet and longer than gamma rays, running roughly from 10 nanometres to 10 picometres, with photon energies from 100 eV to 100 keV. But none of that was known in November 1895. He had a phenomenon, no theory, and the good sense to admit it.
"I Have Seen My Death"
On 22 December 1895, Röntgen made the first medical X-ray photograph — an image of the hand of his wife, Anna Bertha Ludwig.
Her reaction has outlived almost everything else about the experiment. Looking at the bones of her own hand, with the shadow of her ring floating around a finger, she is reported to have said: "I have seen my death." No one had ever seen inside a living body before. The first human response to that was not wonder but dread.
The image was presented to Ludwig Zehnder on 1 January 1896, and the technology spread with extraordinary speed. Röntgen received the very first Nobel Prize in Physics in 1901. Few discoveries have travelled from laboratory curiosity to worldwide medical tool in so few years, largely because the value was instantly obvious to anyone who saw a single picture.
Ninety-Nine Percent Waste Heat
Producing X-rays is a surprisingly brutish process. A modern X-ray tube uses a hot cathode to fire electrons at very high voltage into a metal target, usually tungsten. The X-rays come out of that collision by two different routes.
Characteristic X-rays appear when an incoming electron knocks an inner-shell electron out of a target atom; an outer electron drops in to fill the gap and releases a photon at a specific, characteristic frequency. Bremsstrahlung — German for "braking radiation" — is produced when electrons are sharply decelerated by the electric fields of atomic nuclei, and it yields a continuous spread of energies rather than sharp lines.
The efficiency is dismal. Only about one percent of the electrical energy fed into the tube emerges as X-rays. The other ninety-nine percent becomes waste heat, which is why X-ray tubes need serious cooling and why the target is made of a metal that tolerates being repeatedly hit very hard.
The Decades Before Anyone Was Careful
X-rays are ionizing radiation. Exposure damages DNA and can cause cancer, and at higher intensities it produces burns and radiation sickness. None of that was understood at first, and the people who learned it paid for the lesson personally.
- Clarence Madison Dally, Edison's glassblower, tested X-ray tubes on himself and developed persistent hand cancer. Both arms were amputated in 1904, and he became the first known death attributed to X-ray exposure.
- John Hall-Edwards developed X-ray dermatitis and had his left arm amputated at the elbow in 1908.
- Elizabeth Fleischman, an early X-ray pioneer, died on 3 August 1905 from complications of her work.
Casual public use lasted far longer than it should have. From the early 1920s through the 1950s, shoe shops used shoe-fitting fluoroscopes so customers could watch their toes wiggle inside a new pair. Canberra proposed a ban in 1957; Switzerland did not prohibit them until 1989.
Five Billion Looks Inside
The trade-off is now managed rather than ignored. X-rays penetrate many solids, including construction materials and living tissue, which makes radiography enormously useful in medical diagnosis — checking for a broken bone — and in materials science, where it identifies chemical elements and finds weak points in structures.
Hard X-rays, above roughly 5–10 keV, penetrate well and do the imaging work. Soft X-rays are absorbed easily and are far less useful for seeing through things.
The scale of use is staggering. Up to 2010, roughly five billion medical imaging examinations had been performed worldwide. And in the United States in 2006, medical imaging accounted for about 50% of all ionizing radiation exposure the population received — meaning that looking inside ourselves is now, collectively, one of the largest sources of radiation in our lives. Generation and use of X-rays are strictly controlled by public health authorities for exactly that reason.
