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A Brief History of Radiation

Though radiation and nuclear technology may seem like a hallmark of the modern era, we have known about radiation for quite some time. Let’s take a look at the history and the people behind this amazing branch of physics.

The Discovery of X-rays

The discovery of radiation goes all the way back to 1895, when German scientist Wilhelm Röntgen first1 discovered X-rays.

Wilhelm Röntgen

In 1895, Röntgen was experimenting with Crookes tubes, a predecessor to the more modern cathode-ray tube). A Crookes tube is, essentially, a sealed glass tube filled with low-presure gas with metal electrodes at either end. When a high voltage is applied between the electrodes, a stream of negatively-charged particles called a cathode ray (what we now know to be electrons) is ejected from the negatively-charged cathode to the positively-charged anode. As the electrons travel through the tube, they collide with the gaseous atoms, transferring some of their energy to the electrons in the gas atoms, causing them to excite and de-excite (fluorescence), releasing visible light. Depending on the geometry of the tube, some of the higher-energy electrons miss the anode and instead collide with the back wall of the tube, causing it too to fluoresce and produce a visible glow.

Schematic of a Crookes tube

Röntgen was running an experiment with a Crookes tube that he had covered up with cardboard, when he noticed that a fluorescent cardboard screen nearby lit up when the tube was on. This was puzzling because the cardboard covering should have prevented light or the cathode rays from escaping, yet something else was passing through it! No matter where he moved the screen in his laboratory, the same effect occurred, even with objects placed between the tube and the screen! Eventually, he tried placing his own hand in front of the screen, and saw an image of the bones in his hand projected onto the screen. He spent weeks investigating these new rays, which he named X-rays (where “X” meant “unknown”), discovering that objects varied in their transparency to X-rays, that these rays could not be reflected or refracted (bent), that they were not deflected by a magnetic field, that photographic plates could be exposed by X-rays, and that the source for the rays was the spot where the cathode ray collided with the glass wall of the Crookes tube. He made the first-ever X-ray radiograph of his wife’s hand, thus demonstrating its potential use as a medical diagnostic tool. Röntgen would later receive the first Nobel Prize in Physics in 1901 for his work with X-rays. Element 111, roentgenium would later be named in his honor in 2004. His name is also borrowed for the unit of equivalent dose, rem, mostly used in the United States.

The discovery of radioactivity

Henri Becquerel

After Röntgen published his findings on X-rays, a French scientist named Henri Becquerel began searching for other materials that might emit X-rays. He initially thought that it was the phosphorescent and fluorescent substances that were emitting the X-rays as they fluoresced, so he began to experiment with various fluorescent materials to try to detect any X-rays that they would give off. He was unsuccessful at first, until he came across uranyl potassium sulfate.

To test these materials for X-ray emission, he covered a photographic plate (which was known to be sensitive to X-rays) in black paper, sprinkled a powder of the substance on top of the paper, and placed the arrangement in the sunlight for a few hours. When he developed the photographic plate after testing the uranyl potassium sulfate, Becquerel saw that the area of the plate below the uranyl salt had been exposed. He wanted to repeat this experiment, but because Paris was cloudy that day, he put the plate and the uranyl salt in a drawer and waited for a sunny day. He decided to develop the plate anyway, and was surprised to that the silhouette on the plate beneath where the salt sat was stronger than ever! From this, he concluded that X-rays were coming from the uranium itself, even without prior exposure to sunlight! This marked the discovery of spontaneous radioactivity, later shortened to radioactivity.

Exposed Becquerel plate

For his work in discovering radioactivity, Becquerel would share the 1903 Nobel Prize in Physics with Pierre and Marie Curie. The SI unit for radioactivity, the becquerel, is named after him.

Curious Curies

Pierre and Marie Curie

Marie Curie (née Skłodowska) was born in Warsaw, Poland in 1867 as the youngest of five children to Władysław and Bronisława Skłodowski, both teachers. At the time, Poland was divided between the kingdoms of Russia, Prussia, and Austria, yet there were periodic (ultimately unsucessful) uprisings for Polish independence. Marie’s parents lost money and property due to their support of the uprisings, which made for a difficult upbringing for Marie and her siblings. She would lose both her oldest sister, Zofia, to typhus when Marie was 7, and her mother Bronisława to tuberculosis when Marie was 10. Though her mother was a devout Catholic, her death and the death of Marie’s sister caused Marie to give up Catholicism and become an agnostic.

Despite these hardships, Marie graduated from a girls’ gymnasium in 1883 with high marks at age 15. However, she and her sister Bronisława were denied enrollment into the Russian-run universities because they were women; instead they undertook studies at the Flying University, a clandestine educational program which defied the ideology of the Russian authorities. Fortunately for Marie and Bronisława, The Flying University also accepted women into their ranks.

During this time, Marie and Bronisława made a pact: Marie would help pay for Bronisława’s tuition at medical school in Paris, and Bronisława would do the same for Marie two years later. While Bronisława went to Paris to begin her studies, Marie took time off from her studies to work as a governess in Warsaw, and then later in Szczuki. She eventually fell in love with the son of one of her employers, Kazimierz Żorawski (who would later go on to become a famous mathematician in his own right), but his parents did not approve of their relationship due to her comparative poverty. Heartbroken, Marie returned to Warsaw in 1889, where she continued to work as a governess while also continuing her studies.

In 1891, Marie finally saved up enough money to join her sister in Paris, where she enrolled in the University of Paris, studying physics. Still destitute, she studied during the day and tutored in the evenings, enduring cold winters and sometimes passing out from hunger. She earned a physics degree in 1893, followed by a mathematics degree in 1894. In the same year, she also met Pierre Curie, at the time a science lecturer at the École supérieure de physique et de chimie industrielles de la ville de Paris (ESPCI). Intending to return to Poland after completing her studies, she turned down Pierre’s first marriage proposal. After she was denied an academic position at Kraków University in 1894 because she was a woman, Pierre wrote to her during this time and convinced her to return to Paris to pursue a PhD. After Pierre received his doctorate, they were married in 1895.

After Becquerel’s discovery of spontaneous radiation in 1896, Marie decided to study uranium rays as the topic of her PhD thesis. Some years prior, Pierre and his brother Jaques had invented an electrometer, a device for measuring electric charge. Marie used Pierre’s electrometer to discover that the uranium rays caused air around a sample to become ionized. From this, she was able to deduce that the radioactivity (a word she coined to describe this phenomenon) of a uranium compound depended only on the amount of uranium present in the sample. This implied that the radiation was coming from the uranium atoms themselves, which meant that atoms were not indivisible, as had been previously thought. She also noticed that two types of uranium ore, pitchblende and chalcolite, were more radioactive compared to pure uranium, implying that there must be another kind of radioactive substance in the minerals that was producing the radioactivity. She then began looking for other substances which were radioactive, discovering that thorium (an element discovered in 1828) was also radioactive. Pierre, who at the time had been studying crystals, grew increasingly interested in Marie’s work and soon abandoned his research to join her.

Marie and Pierre set out to search for the cause of pitchblende’s excessive radioactivity. They optimistically began with only 100 grams of pitchblende, but over the course of their research, they would literally go through tons of pitchblende in the course of their research looking for new elements. In 1898, they announced their discovery of two new elements, which they named Polonium (in honor of Marie’s homeland) and Radium (from the Latin radius, meaning “ray”). In 1902, they managed to extract 100mg of radium chloride salt from one ton of pitchblende. However, the Curies desired to isolate polonium and radium in their pure forms to conclusively prove the existence of their new elements. Marie was not able to isolate pure radium until 1910, but she never succeeded in isolating polonium.

In 1900, Marie became the first woman faculty member of the École Normale Supérieure, and Pierre joined the faculty at the University of Paris. She was finally awarded her doctorate degree from the University of Paris in June 1903, becoming the first woman in Europe to earn a PhD in the sciences. Later that year, Marie, together with her husband Pierre and Henri Becquerel, shared the 1903 Nobel Prize in Physics. In doing so, she became the first woman to win a Nobel Prize in any field, and (most likely) the only person to win a Nobel Prize and defend a PhD thesis in the same calendar year.

Pierre and Marie Curie, 1903

Tragedy struck the Curies in 1906, when Pierre was killed when he was run over by a horse-drawn cart in heavy rain. Though she was devastated by Pierre’s sudden death, she resolved to continue her work, taking over Pierre’s chair at the University of Paris, in honor of his memory. Thus, Marie Curie became the first female faculty member of the University of Paris. She later established the Radium Institute (now known as the Curie Institute) there, a laboratory dedicated to the study of radioactivity and its applications in chemistry, physics, and medicine.

In 1910, Marie finally succeeded in isolating radium, and used it to create a unit of radioactivity, the curie, defined as the activity of 1 gram of Radium-226, equivalent to [latex]3.7\times 10\^{10} , \mathrm{Bq}[/latex]. In 1911, she received a second Nobel Prize, this time for Chemistry, in “in recognition of her services to the advancement of chemistry by the discovery of the elements radium and polonium, by the isolation of radium and the study of the nature and compounds of this remarkable element.” She was the first person to win two Nobel Prizes, and remains the only person to win two Nobel Prizes in two different scientific categories.2

The Curies experimenting with radium

During World War I, Marie was very active in the French war effort. She offerred to donate the gold in her various medals and awards (including her two Nobel Prizes) to the war effort, but the French National Bank refused to accept them. Instead, she used most of her Nobel Prize winnings to buy war bonds. She entreated the government to allow her to create France’s first military radiology centers. As the first Director of the Red Cross Radiology Service, she begged manufacturers, car body shops, and wealthy acquaintances to donate money, cars, and equipment for her project. She designed and built 20 mobile radiography units and 200 ordinary radiography units at field hospitals to treat wounded soldiers. Intending to operate the X-ray machines herself, she learned driving, anatomy, radiology, and auto mechanics, and together with her daughter Irène (who was only 17 at the time) she went to the battle front in 1914 to treat the wounded. As the radiography equipment was very primitive, Marie and Irène were exposed to unsafe levels of X-ray radiation as they examined their patients. At the Radium Institute, Marie and Irène trained about 150 women as radiological assistants by 1916, and would later train visiting American soldiers stationed in Paris in 1919. Over the course of the war, over one million soldiers were treated with her X-ray units, saving countless lives. Marie also developed hollow radon-containing (a radioactive gas given off by the decay of radium) needles which could be implanted in wounded areas to kill infected tissue (remember, this was before the advent of antibiotics). She produced the radon using her personal one-gram supply of radium (at the time, it comprised the entire French radium stock). After the end of the war, Marie wrote about her experiences in her book, Radiology in War. Despite all her efforts, Marie would not receive any official recognition of her military service from the French government, though she later turned down a Legion of Honour award in 1921 as her international fame and reputation grew.

Curie in one of her mobile X-ray vehicles

Marie Curie spent most of her postwar career raising money for her Radium Institute, travelling to the United States in 1921 where she was welcomed by President Harding and presented with the first gram of radium isolated in the US. She also toured and lectured in other countries to raise money and awareness of her Institute and the important applications of radiation. Thanks to her campaign, the Radium Institute became one of the preeminent radiation research laboratories worldwide. She wrote a biography of her late husband Pierre in 1923, and visited Poland again in 1925 to oversee the groundbreaking for the Radium Institute in Warsaw, which opened in 1932 with Bronisława Curie as its director.

In addition to all of their professional accomplishments, Marie and Pierre raised two daughters: Irène, born in 1897, and Ève, born in 1904. Though her children were French, Marie hired Polish governesses so that her children would learn her native language. After Pierre’s death, Marie never remarried3, continuing to raise her children alone. Irène would follow in her mother’s footsteps, becoming a scientist and winning a Nobel Prize in Chemistry in 1935 together with her husband Frédéric Joliot-Curie, for their discovery of artificial radioactivity. Ève took a different path, becoming an accomplished musician and writer, later publishing a biography of Marie in 1937. Though she herself is the only member of the Curie family to not win a Nobel Prize, her husband Henry Richardson Labouisse, Jr. collected a Nobel Peace Prize in 1965 on behalf of UNICEF, of which he was the director. In all, the Curie family has received more Nobel Prizes (5) than any other.

Towards the end of her career, Marie Curie was afflicted with several health conditions, including double cataracts, which nearly left her blind. She eventually succombed to aplastic anemia in 1934, at the age of 66. It is almost certain that her ailments were caused by exposure to ionizing radiation, whose harmful health effects were not well-known at the time. To this day, many of Marie’s possessions, including her notes and cookbooks, are highly radioactive and must be stored in lead-lined boxes and handled with protective clothing. Marie was buried together with Pierre in 1934, but their remains were re-interred in the Panthéon in Paris in 1995 to honor their achievements. Marie Curie is the first and only woman to date interred at the Panthéon on her own merits.

Although she became quite famous, she remained a very modest and generous person throughout her life. She intentionally did not patent the radium-isolation process so that others could benefit from it freely. She and Pierre also refused many awards and gifts, insisting that they be given to her affiliated scientific institutions instead. Albert Einstein once remarked, “Marie Curie is of all celebrated beings, the one whom fame has not corrupted.”

The Curies’ work with radioactivity changed the foundations of atomic physics. The high radioactivity of radium proved to be an invaluable source of high-energy particles used for later experimental work on the study of atoms. Radium was also used to develop radiotherapy as one of the first effective cancer treatments. Marie Curie also saved countless lives during World War I with her mobile radiography units on the battlefield. She also showed the world that a woman could accomplish just as much, if not more, than a man, in spite of the sexist barriers that existed at the time. She is remembered today as the most well-known (by far) woman scientist of all time, by element 96 (Curium) and the unit of radioactivity (curie) named in her honor, the two Radium Institutes (since renamed Curie Institutes) that she founded, and her family legacy in many branches of science.

Further Resources

In-depth biography of Marie Curie

BBC documentary on Marie Curie’s life

  1. There is some debate as to whether Röntgen was really the first person to observe the effects of X-rays, but he was almost certainly the first person to study their effects in detail. 

  2. Linus Pauling won the Nobel Prize in Chemistry and Peace, John Bardeen won the Nobel Prize in Physics twice, and Frederick Sanger is a double-winner of the Chemistry Prize. 

  3. She did, however, have a brief affair in 1910 with Paul Langevin, another French physicist. Funnily enough, their grandchildren, Hélène Langevin-Joliot and Michel Langevin, would later marry.