The hunt for Marie Curie's radioactive fingerprints in Paris

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 '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 ed 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 ed 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, which they named after Marie Curie's homeland, Poland; and radium. 

"It was a very toxic environment," says Huynh. "Because there were not only radioactive vapours, and radioactive dust, but they also used many chemical products to break up the pitchblende that are banned from laboratories today, such as mercury."

The shed no longer exists, having been torn down; the lab in the museum is where Marie Curie later worked. In recent years, Ammerich conducted a wide-ranging inspection and safety review for the museum. He and his team removed surface contamination, such as weakly radioactive dust, from furniture in the preserved office. The remaining, faint radioactivity is from traces that sunk into the wood or metal and are now inside it, meaning that even if someone were to now touch the furniture, they would not transfer any contamination.

"The lab was already decontaminated in the 1980s," says Huynh. At the time, the practice in the museum was to "try and scrub off the contamination with abrasive sponges, and if radioactivity was then still detected, it meant it had sunk into the material, and they'd throw away the whole thing and replace it" with a copy, he says.

The lab bench, for example, was replaced with a replica, Huynh explains. Today, weakly radioactive traces such as the ones on the chair and doorknob are allowed to stay in place, he says, and are considered as heritage.

"These historical traces of radioactivity are so important because they show the working conditions of Marie Curie at the time. They should be preserved at all costs," says Thomas Beaufils, a professor and museologist at the University of Lille who specialises in the conservation and protection of radioactive heritage. "There is no other place in the world where radioactivity has been spread throughout a lab and office by Marie Curie. It has a huge heritage value."

Today, radium – which Marie Curie discovered during her PhD research – is no longer used in , having been replaced with safer and more manageable elements, Ammerich explains. And the way scientists work with radioactive elements has of course fundamentally changed, he says. 

"If Marie Curie were a PhD student today, she would first of all need to apply for a range of permits to work with these radioactive materials," says Ammerich. "And she would only be allowed to do her research in an authorised lab, with all the necessary safety and ventilation equipment. She certainly wouldn't be handling the materials on a table or lab bench, she would be using a glove box," a sealed container with the radioactive material inside, he adds.

When we planned the visit, Huynh said we could measure anything we wanted in the lab, office and archive, as long as it could be done safely. Two photographers accompanied me and documented our tests – which ended up taking six hours, as we journeyed through Marie and Pierre Curies' spellbinding research and discoveries. Given the wealth of objects in the archive, I decided to focus on things that might transport us to two crucial periods in the Curies' story: their early years, discovering radium together; and Marie Curie's time leading research at the Radium Institute, alone, after Pierre's death in 1906.

Ammerich shows me how to take meaningful measurements. He has brought a briefcase filled with different detectors. One is a yellow Geiger counter, about palm-sized, for two types of measurements. The first of these tests, measured in "counts per second", detects whether radiation is present, in the form of alpha, beta or gamma rays. This overall test helps detect whether the object we are measuring is radioactive, or not.

If an object does above-background radioactivity, the second measure we take shows the potential impact of these rays on the human body, measured in microsievert per hour. This helps check if an object's level of radioactivity poses a risk to human health, by potentially raising the long-term risk of cancer. We also use a spectrometer, which can pick up more detailed information, such as which radioactive element is being measured. And we test some non-radioactive surfaces, as a control.

Ammerich had already previously tested all the objects we look at, as part of his evaluation of the collection. He has also assessed if there was a risk to museum visitors or staff, from the weakly radioactive objects. "There's no danger, nothing," he says, for either of those groups, based on his assessment. Nor was there any risk to us as we measured these objects – which we simply did to allow me to understand, and report on, the radioactive traces.

In an office above the museum's archive, Huynh opens a box with a small radioactivity warning sticker on the side. It contains a handwritten, faded document, a lab note written by Marie and Pierre Curie in 1902.

"On the top of the page you see Pierre Curie's handwriting, and below that, the neater handwriting, that's Marie Curie's," he says, pointing at the faded lines.

The two habitually shared notebooks, he explains: "In their lab notes, you see very clearly how they worked together as equals, with mutual respect. It was a really intense and very respectful scientific collaboration, a real exchange."

When Pierre Curie was put forward for the Nobel Prize together with Becquerel in 1903, "it was he who insisted that his wife should also be included", Huynh adds – leading to all three tly winning the prestigious prize, which had never been awarded to a woman before.

Dating from those early years in the hangar, the note captures a crucial moment in their research, Huynh explains: "It's where she calculates the atomic weight of radium," a key step in their quest to prove that this new element exists, he says. Marie Curie writes the result down as 223.3 – very close to the weight as it is known today, of 226. 

"It's a remarkable document," Huynh says. "It's the calculation proving that, yes, it has an atomic weight that makes it different from other elements, that gives it a place in the periodic table, and it's also written during a period of such intellectual energy."

In fact, Frédéric Joliot, the Curies' son-in-law, made a print of this lab note with a photographic plate in the 1950s to show the contamination, and also measured it with a ticking Geiger counter – possibly making him the first person to investigate his extended family's radioactive heritage. "It's moving to hear ... the very same radium extracted and handled by Pierre and Marie Curie", making itself known through the detector's sound, he wrote at the time.

As we measure the objects, we also take measurements of surfaces that have nothing to do with the Curies and their legacy, to give us something to compare our other readings against. 

An ordinary Parisian parquet floor, in a building that was never used by the Curies, gives a reading of 0.11 microsievert per hour. This is the background radiation which an average person is exposed to every day, from natural sources such as the ground and cosmic radiation, Ammerich says. He points out that we humans are also radioactive ourselves, as we contain radioactive elements such as potassium. In , the legal limit of a person's exposure to radioactivity, in addition to natural and medical exposure, is 1 millisievert (1,000 microsievert) per year for of the public. For workers in nuclear facilities, it's 20 millisievert (20,000 microsievert) per year. 

Placed on an ordinary stretch of pavement outside the building, the reading rises slightly, to 0.19 microsievert per hour. That's because Parisian pavements are made of granite, which can contain radioactive elements such as uranium, Ammerich says.

We take turns carefully hovering the Geiger counter over the Curies' lab note. It buzzes, having detected above-background levels of radioactivity, especially towards the bottom of the page, where human hands may have touched it more. But the levels are very low, and from a human health and safety perspective, the lab note is not dangerous at all, Ammerich and Huynh say.

In the museum, we measure the public areas where visitors walk – they measure at background levels, of around 0.11 microsievert per hour: "It's the level of natural radioactivity, of the ground, the Sun, the people all around us, with their potassium," says Ammerich. The back of Marie Curie's office chair; the doorknob; and an instrument called a piezoelectric quartz electrometre, which the Curies used to measure radioactivity, all measure somewhat above those background levels, but still well within safe ranges.

The overall safety assessment of any given place or object is not only based on such measurements, Ammerich explains. Instead, it is estimated based on a range of factors, including the length of time of the exposure, the distance to the object, and which parts of the body are exposed to it. The different types of rays also matter: alpha rays can be mostly stopped by human skin, and completely stopped by a sheet of paper, he explains. Gamma rays are more penetrating, but can be stopped by concrete or lead. Radium, the chief source of contamination for the Curies' heritage, gives off alpha, beta and gamma rays, but mostly alpha. Ammerich's risk assessment for the museum's visitors as well as for museum staff was based on thorough measurements of all the objects, along with those comprehensive factors, and found that there was no risk.

The Curies themselves noticed that their radioactive materials, such as radium salts and radioactive gases, were contaminating everything else in the shed.

"The dust, the air in the room, the clothes are radioactive," Marie Curie reports in her doctoral thesis in 1903.

Still, at this stage, the Curies did not seem worried for their safety: their only concern was that the contamination might muddle their scientific results.

Marie Curie and others noticed, over time, that her hands were "calloused, hardened, deeply burned by radium". Pierre Curie repeatedly put radium salts against his skin, to test the effect. Burn-like, red lesions appeared on his skin. This did not seem to scare him; on the contrary, he and other scientists thought this effect could be useful for treating tumours – an insight that led to the first effective cancer treatments. Only later did scientists discover that being exposed to radium, and other radioactive materials, can also raise one's risk of cancer.

Other scientists at the time also experimented freely with radium, for example, dabbing radium salts on their temples or their closed eyelids, and reporting that it filled their closed eyes with light.

Mainly, the Curies observed their newly discovered radium with hope and wonder: it gave off warmth, and glowed beautifully in the dark. They went to the shed at night to marvel at the bottles and tubes of radium salts on the rickety shelves and tables: "Like faint fairy lights," Marie Curie observed. 

Not all of the Curies' heritage is being preserved. Even today, some of it ends up in nuclear waste facilities, in cases where public safety concerns override heritage protection.

The day before my visit the Curie Museum and its archive, I meet up with experts from Andra, 's agency in charge of radioactive waste. Over lunch on a cobblestoned square close to Andra's headquarters just outside of Paris, they tell me about some of the more surprising tasks that fall into their remit.

Andra oversees radioactive waste from 's nuclear power plants, as well as from research labs, hospitals and so on. About once a week, they get a call from people who have found potentially radioactive antiques in their home – alarm clocks, for example, from the 1920s, when radium was considered harmless, and used in paint on clocks. It was even used in cosmetics and special soda fountains with radium in them, to make radioactive water, which was thought to be healthful.

Andra's experts test these heirlooms, and put the contaminated ones into radioactive waste facilities. In some cases, antiques such as the fountains are decontaminated by removing the radium, then given to the Curie Museum.

"We walk in Marie Curie's footsteps," says Nicolas Benoit, a specialist at Andra who oversees the remediation of sites polluted by radioactivity. "Every time we visit a site where there's radium, we think of her. And we aren't angry with her, not at all, because at the end of the day – yes, they handled the radium in a bit of a slapdash way, but at the time, there wasn't an awareness of its dangers."

He pauses, then adds: "And there's a bit of pride as well, because it's as if we're closing the circle, we're finishing her work", by taking care of the contaminated objects from the radium era. 

In 2020, Benoit led an unusual operation: a visit to the home of Hélène Langevin-Joliot, a nuclear physicist and part of the Curies' dynasty of scientists. She is the granddaughter of Pierre and Marie Curie; her parents, Irène Joliot-Curie and Frédéric Joliot, won a t Nobel Prize in 1935 for their discovery of artificial radioactivity. In fact, Irène thanked her mother, Marie, for sharing her stash of rare polonium with her, which helped h Irène and Frédéric with the research that led to the discovery. A love of science was ed down in the family, along with friendships with other scientists and their families, including Albert Einstein.

Langevin-Joliot had a number of family heirlooms from her parents and grandparents in her home, which she suspected might be slightly contaminated. She was not worried for herself, having lived with them for many years, being in good health, and considering the risk to be low. But she did not want to leave them behind, and force others to deal with them. After talking to Huynh, the museum director, she invited Andra's experts into her home, to measure the heirlooms. 

"It's one of the best memories of my life," Benoit says, of the operation. "If you imagine that Marie Curie used those objects – that was really moving for us, it's not something you get to do every day." 

They tested a cupboard that used to belong to Marie Curie – photos exist of her standing next to it, he says – and had been handed down in the family.

"We emptied it, and tested it. The contamination was above all on the doors, where you open the cupboard. And on the locks, on the drawers ... everywhere she [Marie Curie] touched," he says. "We tried decontaminating the wood, without damaging it, but it wasn't possible," because the radium traces had sunk into it, he adds. The worry was that leaving it in place could mean it would end up with a future owner who might not know about its past, and might use or process the wood in ways that would spread the contamination.

With Langevin-Joliot's agreement, the cupboard was cut into pieces and incinerated in a radioactive waste facility, Benoit explains: "The cupboard doesn't exist anymore. It's sad, it was heartbreaking, but that's how it is."

Today, radium is sometimes described as the most radioactive natural element ever discovered. But Benoit challenges that description. From a safety perspective, "saying that one element is more radioactive than another, doesn't really make sense", he says, since estimating radioactivity is more complex than just measuring an element's level of activity (the rate at which the radioactive element decays, measured by counting the number of disintegrations per second). One must also consider its half-life, the time required for half of it to decay – in the case of radium, 1,600 years – he says, as well as the actual impact on humans, which in turn depends on a range of factors. 

"If you take carbon-14, for example – yes, that's an element that emits radiation. But only over a very weak distance, only a few centimetres," he says. He puts his finger on the table, between our coffee cups. "So if it were placed here, we could sit where we're sitting, and we wouldn't risk anything."

There is a tragic side to the Curies' legacy. Already in the early days of working with radium in the shed, Pierre noticed that he felt increasingly sick. Marie Curie also felt sick, struck by a mysterious fatigue. She died at 66, of leukaemia, a cancer of the blood. It may not have been radium that killed her: Huynh says the culprit was more likely her work with X-Rays during World War One, which would have exposed her to the kind of radiation known to raise the risk of leukaemia. 

Irène and Frédéric Joliot-Curie died in their late 50s, also of cancer. Before his death, Frédéric had been especially active in improving safety regulations and equipment for people working with radium, Huynh says.

Today, the Curies are entombed in a crypt of the Panthéon monument in Paris, in lead coffins, to block any potential radiation from traces inside (or on) their bodies. They were previously buried in a cemetery just outside of Paris. In the 1990s, before they were transferred to the Pantheon, radiation experts exhumed and tested their bodies, detecting some radioactive contamination, before laying them to rest in the lead coffins. 

For Ammerich, the experience of handling the couple's belongings remains very moving. "When I tested Marie Curie's diary, where she writes about her husband's death – I'll be honest with you, I had tears in my eyes," he says.

In his view, it would be a shame to remove the small remaining traces in her Paris office: "Imagine if you cleaned everything off, and then in the future, nothing would prove what happened here."

Beaufils, the museologist, also emphasises how important it is to save and protect this kind of radioactive heritage.

"From a historical point of view, our societies are built on these kinds of objects and memories from the past. If we don't protect our material heritage, we'll be a nation, a society, without any historical depth," Beaufils says. "And a society without depth will struggle to develop, and to thrive," both from a social and technological point of view, he adds.

Huynh sees the Curie Museum as "a link between the past and the future", especially given its location on a busy cancer research campus, the Institut Curie Research Center. When I visit, I see researchers mingling in the rose garden by Marie Curie's lab – dressed in jeans and T-shirts, not suits and long dresses, as they would have been during her time. Huynh tells me there are also active labs on the floors above and below the museum.

"Many researchers here are very proud of that heritage," he says. "It's a kind of 'Curie spirit'."

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