From X-rays to radioactivity

Are you aware that the discoveries of X-ray and radioactivity, both with immense implications, happened in back-to-back years? Better still, it was the discovery of one that inadvertently led to the other. A.S.Ganesh gives you the story that binds together these two great discoveries...

January 21, 2019 11:00 am | Updated November 10, 2021 12:18 pm IST

Henri Becquerel seen in his laboratory.

Henri Becquerel seen in his laboratory.

What happens when a German physicist, a French mathematician and a French physicist are brought together by a letter written by one of them? No, this isn’t a joke or a riddle that you need to solve. This is the one-line plot of a real-life scenario that led to the discovery of radioactivity.

The German in question is William Conrad Roentgen, born in 1845 as the only child of a cloth merchant. After studying physics and taking to academia, Roentgen rose to the Chair of Physics at the University of Giessen in Germany. He had his breakthrough moment after turning 50, when he stumbled upon a new kind of ray, which he called X-ray, while experimenting with Crookes tube (an early experimental electrical discharge tube) on November 8, 1895.

Roentgen’s letter

News spread quickly as newspapers jumped onto the story, leading to a spike in interest about X-ray. Basking in the glory that comes with such a find, Roentgen shot out letters to many, attaching X-ray photographs for good measure.

Among the receivers was Frenchman Henri Poincare, who had a distinguished career as a mathematician and mathematical physicist. Poincare is known for his innovations in geometry, theory of differential equations and the philosophy of mathematics in general, but that didn’t stop him from trying to replicate Roentgen’s results himself.

Poincare makes a point

Surprised and shocked, Poincare brought the attention of the Academie des Sciences (French Academy of Sciences) in Paris to Roentgen’s findings. During a weekly meeting of the Academie on January 20, 1896, Poincare read out the contents of Roentgen’s letter, showed the photographs and also added his own observations. A discussion ensued about the newly discovered X-ray and among those in attendance was French physicist Henri Becquerel.

Becquerel, whose earliest work concerned the phenomenon of phosphorescence and with the absorption of light by crystals, wondered if there was a connection between this newly discovered X-ray and the phosphorescence he was investigating.

Weather’s role

Even though Becquerel’s initial experiments led him to the belief that phosphorescent uranium salts absorbed and emitted a penetrating radiation similar to X-ray, he was forced to reconsider following a quirk of nature. Overcast weather conditions in Paris late in February 1896 meant that Becquerel couldn’t continue his research for want of sunlight.

But when Becquerel returned to the photographic plates and the uranium salts he left in his drawer on March 1, and developed them expecting to see a very weak image, he was stunned by the splendid clear image it produced. Becquerel came to the right conclusion this time that the uranium salts emitted radiation without stimulation from sunlight and when he went on to announce that this radiation was in fact a property of the uranium atom, radioactivity was born.

If their discoveries came in back-to-back years, their Nobel Prizes were separated by a couple of years. While Roentgen won the Nobel Prize for Physics for his discovery of X-ray in 1901, Becquerel’s discovery of spontaneous radioactivity allowed him to share the Nobel Prize for Physics in 1903 along with Pierre and Marie Curie.

The discovery of both X-ray and radioactivity have had a telling effect in the way we live our lives. Studying and researching them further have led to tremendous growth not just in these fields, but have also spilled over to various other branches of science, enhancing quality of life in the society as a whole.

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Niepce’s near miss

Abel Niepce de Saint-Victor

Abel Niepce de Saint-Victor

Abel Niepce de Saint-Victor was a French photographer, inventor and cousin of Joseph Nicephore Niepce (often credited as the inventor of photography) who lived from 1805 to 1870.

Nearly 40 years before Henri Becquerel made his serendipitous discovery, Niepce, while experimenting with various chemicals, observed that photographic emulsions were exposed even in complete darkness.

With time, Niepce was able to zero in on uranium salts as the reason for this behaviour and suspected that he had discovered a new form of invisible radiation.

In fact, by 1861, he clearly mentioned that uranium salts emit a radiation invisible to the human eye, even quoting Edmond Becquerel, father of Henri Becquerel, to confirm that the said activity could not be due to phosphorescence.

Even though Niepce did his bit and reported his findings to the French Academy of Sciences, no one delved deeper, leaving it as is.

This paved the way for Henri Becquerel to stake the claim decades later, through a serendipitous discovery not unlike how Niepce came about it.

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