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Dr. Katsuko Saruhashi: Geochemistry for a Better Future

How a geochemist helped save the world from nuclear fallout.

Illustrated portrait of Dr. Katsuko Saruhashi looking down and to the left. She wears a lab coat over a collared shirt and black sweater. The background looks like the ocean, and a quote by Dr. Saruhashi is to the right of her portrait. It reads "I would like to see the day when women can contribute to science and technology on an equal footing with men."
Dr. Katsuko Saruhashi by Sachi Weerasooriya

Do you know the geochemist who helped ban above-ground nuclear bomb tests and basically saved the world? Her name was Dr. Katsuko Saruhashi, and she studied the radioactive isotopes produced in nuclear blast, which are unstable, dangerous atoms. Dr. Saruhashi looked at the concentration of these radioactive isotopes in rainfall, dust, and the ocean around Tokyo after nuclear testing in the Pacific Ocean.

"By following her curiosity, paving the way for future scientists, and standing her ground in her science for the benefit of all humanity, I really do think she saved the world!"

Watch the video or keep reading the blog version below!


A New Kind of Danger

Nuclear weapons were a brand-new issue during Katsuko’s early career in the 40s and 50s. The immediate dangers of nuclear bombs were devastatingly clear after the bombings of Hiroshima and Nagasaki in 1945, but the longer-lasting effects of radiation weren’t yet fully understood. Between 1951 and 1958, there were over 250 nuclear tests in the atmosphere - that’s a rate of almost 3 nuclear explosions each month.

A picture of the Nuclear Test Ban Treat, inside a binder with yellowed paper
Nuclear Test Ban Treaty, July 26, 1963; Treaties and Other International Agreements Series #5433; General Records of the U.S. Government; Record Group 11; National Archives.

Dr. Saruhashi realized that these tests were dangerous, not just to the people around them. She found that these nuclear tests were putting harmful materials called nuclear or radioactive fallout into the atmosphere - radioactive material that could travel across the oceans and stick around for decades. Thanks to her work, the world understood the real dangers of these tests, and a treaty was signed to ban all testing in the atmosphere, ocean, and outer space, called the Partial Test Ban Treaty.


Chemical Curiosity

Katsuko Saruhashi was born in Tokyo in 1920. She was a curious child, always wondering about the science behind rain and raindrops. She went on to earn a degree in chemistry from the Imperial Women's College of Science in 1943. Next, she worked at the Meteorological Research Institute until completing her Ph.D. in chemistry at the University of Tokyo in 1957, where she invented a new way to measure the amount of carbon in the ocean. Dr. Saruhashi kept that wonder of the natural world as she became a geochemist, a scientist that uses chemistry to understand geology.

After the war, Dr. Saruhashi worked at the Central Meteorological Observatory of Japan, where she was asked to lead a research project on the long-term effects of nuclear testing in the Pacific Ocean. As a geochemist, she had to understand the chemistry of nuclear fallout and the geology of how the fallout would travel hundreds or even thousands of miles away.


Tracking an Invisible Threat

Why is nuclear fallout so dangerous? The immediate dangerous zones of a nuclear explosion range hundreds of miles, but long-lasting isotopes blown into the stratosphere can cause issues even thousands of miles away for decades. An isotope is a type of atom that has a different number of neutrons in its nucleus. If an isotope is in an unstable energy state, it will randomly emit energy in the form of radiation to get to a stable state. This is what makes the isotopes “radioactive”, and that emitted energy can be hugely harmful to our cells.

An animated give of an isotope in the form of blue and red circles decaying
A radioactive isotope decaying by emitting radiation

Each decay is random, but has a certain probability related to something called the half-life of the isotope. The half-life represents the time it would take for the amount of the isotope to be reduced by half. Different isotopes have a huge range of half-lives, from practically instant to many years to even longer than the age of the universe! Two of the isotopes produced in nuclear explosions are Strontium-90 (Sr-90) and Cesium-137 (Cs-137), and they have super long half-lives of 28 and 30 years long! Because they won't decay away quickly, they can travel in the wind for thousands of miles, be absorbed in water droplets in clouds, come down as rain, and even be absorbed in soil.

a cartoon version of the radiation symbol
The symbol for dangerous radiation

Dr. Saruhashi developed a new technique sensitive enough to measure the amount of these isotopes in dust, rainwater, and the ocean after nuclear tests. These measurements depend on the distance and time since the last nuclear explosion. By measuring these trends, Dr. Saruhashi was then able to predict the future amount of these isotopes for decades to come. By taking into account the current rate of nuclear explosions, the prediction of future amounts showed dangerous levels of radioactive Sr-90 and Cs-137.

With these results, Dr. Saruhashi was able to show how dangerous these nuclear tests were, even for the cities thousands of miles away from the testing sites, like Tokyo. Dr. Saruhashi and her colleagues all over the world pushed governments to ban these tests for the collective safety of Earth’s population.


Hero for Humanity

A picture of Dr. Saruhashi looking at the camera and wearing glasses
Dr. Katsuko Saruhashi

Throughout her career, Katsuko Saruhashi pushed for the greater good of humanity and, more personally, for women in science. Katsuko was constantly breaking barriers: she was the first woman to earn a Ph.D. in science at the University of Tokyo, the first woman to win the Miyake Prize for Geochemistry, and the first woman elected to the Science Council of Japan. After all her personal success, she still made sure to support women in STEM all over Japan. She founded the Society of Japanese Women Scientists in 1958, and created a yearly award called the Saruhashi prize which recognizes female scientists for their contributions to natural sciences and for their commitment to mentoring and advocating for other women in STEM.

By following her curiosity, paving the way for future scientists, and standing her ground in her science for the benefit of all humanity, I really do think she saved the world!

Written by Taylor Contreras

Edited by Aizhan Akhmetzhanova

Illustrations by Helena Almazan

Portrait by Sachi Weerasooriya


Meet Katsuko Saruhashi from Massive Science


Learn more about radioactivity and geochemistry!

Define (5-10 minutes): What are radioisotopes and what are they used for? Take a look at this page to explore the answers to these questions.

Examine (10-20 minutes): Dr. Saruhashi extensively studied abundance of Cs-137 and Sr-90 radioisotopes in her research work. Learn more about these radioisotopes and their dangers.

Discover (15-20 minutes): Curious about what other kinds of questions geochemists and cosmochemists are asking? Learn about some of the fascinating topics explored by these scientists by reading this interview with the scientists from the Earth and Planets Laboratory at Carnegie Institution for Science.


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