Guest post by Rowena Fletcher-Wood

The x-ray has always been a mysterious thing. An invisible beam of high energy electromagnetic radiation that passes through most kinds of matter, it is even named ‘x’ after the mathematical variable used to denote the unknown. And the x-ray itself isn’t the only unknown thing – so are its origins. Sources suggest it was an accidental discovery, but there aren’t as many sources as there should be, due to a very non-accidental fire.

Wilhelm Röntgen, German physicist and discoverer of x-rays, died on 10 February 1923, whereupon all his laboratory records were burnt on his request.

It was an extreme action, but not an unusual one.

While modern science is becoming more and more transparent, not very long ago secrecy was the tool of the inventor’s trade. Through secrecy, successful men were able to preserve their impression of genius, compete against their peers and prevent their ideas from being stolen. The most coveted prize was not scientific elucidation but personal recognition – impossible for those who were too open and lost their ideas to the less scrupulous. It wasn’t just seen amongst scientists; William Howson Taylor, founder of the admired Ruskin pottery, had all his notes burnt at his death in 1935. And so the method was lost with its maker.

We are left with a fuzzy picture, not much easier to illuminate than x-rays themselves, and can only imagine the scene in Röntgen’s laboratory in the winter of 1895…

A dark room, because Röntgen was working with light.

A screen coated with barium platinocyanide.

On the bench top nine feet away, a Crookes cathode-ray tube, a large glass gas-filled bulb that fluoresces when a high-voltage electrical current is discharged through it. But the bulb is not visible, because Röntgen has covered it with thick black cardboard to contain the distracting glow (and because it’s currently switched off.)

Then Röntgen turns on the tube and the screen begins to glow green…

Nine feet was further than the reach of the blocked cathode rays that Röntgen understood, and he quickly concluded that he had made a new, unknown kind of ray that could travel through cardboard. He tried it with aluminium, copper and brick – it travelled through all of those too. In fact, the only material he found that could absorb it was thick lead.

So naturally, he did what any discerning 19th century scientist would do in his position: he stuck his hand in it. On the screen, he could see the image of his own bones, surrounded by a greenish glowing flesh. He seized some photographic film, and took the first x-ray image. When he repeated the procedure to photograph his wife’s hand and rings for his publication on a ‘new kind of rays’, she famously cried, ‘I have seen my own death!’

100 years later, medical physicist Gerrit Kemerink of the Maastricht University Medical Center thought to piece together some of the missing evidence, and recreated the setup of some of the very first x-ray machines. With a hand he borrowed from medical supplies, he set up the experiment just as Röntgen might have done, and tested the results. Horrifyingly, he found that the hand needed a full 90 minutes of exposure to create a clear image, providing a radiation dose 1500 times more than the dose supplied by a modern x-ray procedure. No wonder early x-ray testers reported burns and hair loss!

Modern x-ray production methods also help us understand what was going on in Röntgen’s Crookes tube: he used a hot cathode to produce electrons, which were then accelerated under a voltage, striking a metal target and knocking off more electrons. Not only were electrons emitted, but the metal was left full of electron vacancies, holes from where the AWOL electrons had been knocked. X-rays are emitted when high energy electrons shift into lower energy vacancies, and so the energy of the x-ray is specific to the metal they came from. Today, copper anode metals are mostly used, but Röntgen probably produced x-rays by ionising the gas inside his tube. If so, he would have produced lower energy x-rays and so required the longer measuring times.

Röntgen may have burned the notes and reports, preventing us from ever understanding the precise details of his experiments, but he did publish three papers on these mysterious new rays, and left us with an invaluable scientific and medical tool.

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