Marie Curie worked with radioactive material with her bare hands. More than 100 years after her groundbreaking work, Sophie Hardach travels to Paris to trace the lingering radioactive fingerprints she left behind.

The Geiger counter starts flashing and buzzing as I hold it against the 100-year-old Parisian doorknob. I am standing in the doorway between the historical lab and office of Marie Curie, the Polish-born, Paris-based scientist who invented the word "radioactivity" – and here is an especially startling trace of her. The museum that houses the lab has invited me in here to track radioactive handprints left by her when she worked here in the early 20th Century. Here, on the doorknob, is one such trace. There's another one on the back of her chair. Many more of these invisible traces are dotted all over her archived notes, books and private furniture, some only discovered in recent years.

The Geiger counter's reaction, and the numbers on the display, suggest the presence of above-background radioactivity, though only at low and non-threatening levels. In microsievert, which measures the potential impact of radiation on the human body, it comes to about 0.24 microsievert per hour, well within safe limits, according to experts.

Marie Curie worked here from 1914 until 1934, the year of her death, handling radioactive elements including radium, which she and her husband Pierre Curie had discovered in 1898. For most of her life, she did this with bare, increasingly radium-scarred hands. She then transferred traces of these elements onto other things she touched. Tracking the handprints through her work spaces, one can imagine how she might have gone "from the lab to the office, opened the door and pulled out the office chair to sit down", says Renaud Huynh, the director of the Curie Museum, as he guides me from trace to trace.

Some radioactive traces, for example in the Curies' lab notes and notebooks, have long been known about: one analysis in the 1950s made some of them visible by using a photographic plate. The contamination showed up as dots and splodges, suggesting radioactive lab dust settling on the page, or droplets from boiling solutions of radium salts spraying onto it.

Other traces have been revealed in more depth by further tests in recent years: they have been found on the doors of a cupboard from her home, on drawers, on the pages of books, on lecturing notes, and even on an extendable dining table from the Curies' family home.

For every item, experts face the agonising question of whether to save it as heritage – or, in cases where the contamination is considered a public safety risk, put into a nuclear waste facility. The cupboard, for example, ended up being destroyed.

Marie Curie's lab and office, whose tall windows overlook a rose garden she designed, are usually closed off by a red cordon, to be viewed but not entered by the museum's visitors. They were part of the Radium Institute, which she founded, and still sit in the heart of an active, bustling research campus.

"There's a great probability that the radioactive traces were left by Marie Curie, but it could have been her daughter [Irène Joliot-Curie], who later used the same office," says Huynh. "Either way, it's a material trace of the past, it's a form of heritage. If we were to erase these traces, we would lose this memory. It might be a detail, but it evokes a mode of contamination, it evokes a certain way of working – and it also evokes an era."

Huynh has invited me into the museum outside opening hours, and also into the nearby archive, to talk about these traces. Since radioactivity is invisible, I had asked him before the visit if I could bring a Geiger counter to bring the traces to life for our readers. He agreed, and also let me invite Marc Ammerich, a radiation expert, to help me measure and interpret the results.

Ammerich spent 40 years working for French radioprotection agencies, inspecting the safety of France's nuclear power plants. Since 2019, he has been tasked with comprehensive tests on the museum's collection. He has tested about 9,000 items from the Curies and their family so far, including the extendable dining table, where he found two radioactive patches next to each other, like two handprints, where a person would grab the table and pull it out for visitors.

Turning his attention to the Curies' legacy has been a special experience, Ammerich says: "To measure the notebooks where they write about their discoveries of radium and polonium, to measure the instruments they used – it's extraordinary. It's like holding the history of radioactivity in my hands".

Marie Curie was a doctoral student in Paris in the 1890s, when she came across a curious phenomenon.

She was studying the recently discovered mysterious rays emitted by uranium. The scientist Henri Becquerel had described their interesting properties. The rays gave off light, and also, they made air conduct electricity. Curie proposed the word radioactivity for these peculiar rays – coining the word still used today. Testing various ores for their levels of radioactivity, Marie Curie then noticed something surprising: some of these ores were much more radioactive than the known radioactive elements they contained (uranium and thorium).

After checking her measurements, she concluded that there was only one explanation: there must be another, not yet known, highly radioactive element in these ores.

To find this unknown element, she began refining a uranium ore called pitchblende, removing all known elements from it, until only the mystery element would be left. Excited by the project, Pierre joined her. They crushed the ore, dissolved the resulting powder in acid, filtered it in many different steps, and obtained an increasingly concentrated, and increasingly radioactive product, Huynh explains.

It was an arduous process. As Marie Curie herself put it: "The life of a great scientist in his laboratory is not, as many may think, a peaceful idyll. More often it is a bitter battle with things, with one's surroundings, and above all with oneself".

Not having access to a proper lab, they worked in a store room, and then, a leaky shed behind a university building. In Marie Curie's description, the shed was furnished with "some worn pine tables, a cast-iron stove" – and it lacked any safety provisions whatsoever: "There were no hoods to carry away the poisonous gases thrown off in our chemical treatments". And yet, "it was in this miserable old shed that we passed the best and happiest years of our life, devoting our entire days to our work", she writes.

In 1898, at the end of this backbreaking process of refining the pitchblende and then further refining tiny, highly radioactive crystals, they announced that they had discovered two new elements: polonium,........

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