Illustration by Suman Choudhury

Life

Bibha Chowdhuri was born in 1913 in Kolkata into a Bengali family highly educated in the sciences. Her father was a doctor and her mother’s side of the family was related by marriage to scientist Jagadish Chandra Bose and doctor Nilratan Sircar. ¹

Bibha’s parents followed the doctrine of Bramho Samaj, which her father had to convert to in order to marry her mother. Bramho Samaj is a progressive Bengali movement seeking to reform repressive social and religious practices, it abolished traditional Hindu beliefs and rituals in favour of emphasising education, especially women’s education. Hence, Bibha and all four of her siblings – 4 sisters and one brother – received equal education. ²

After receiving her M.Sc. in physics from Calcutta University in 1936, Bibha convinced the Palit Professor Debendra Mohan Bose to take her as research assistant at his institute. Between 1938-1942, they conducted nuclear emulsion experiments, where Bibha was responsible for setting up experiments (emulsion plates) in three high-altitude stations, bringing them back to the institute and observing the plates under the microscope. They published 4 papers in Nature on their process and results, including the discovery of a new particle, the pi-meson. ³

Bibha completed her PhD between 1945-49 at University of Manchester, where she researched in Patrick Blackett’s cosmic ray laboratory. She used cloud chambers to study extensive air showers, and her data contributed insight into the role of particle density and penetrating particles in extensive air showers. She published her thesis findings in Nature in 1948, as well as in the Proceedings of the Physical Society in 1950. ⁴

In 1949, Bibha Chowdhuri returned to India and, with Blackett’s recommendation, was accepted as the first woman to join Homi Babha’s cosmic ray group at the new Tata Institute of Fundamental Research (TIFR) in Mumbai. She continued her thesis research in extensive air showers, and discovered that penetrating particles other than muons, which were known to be the main penetrating component, were associated with 29% of the showers. Her team’s paper was published in the Proceedings of Indian Academy of Sciences in 1952. ⁵

Despite her senior position at TIFR, Bibha left in 1953 and subsequently joined cosmic ray physicist L. Leprince Ringuet’s lab under the Centre National de la Recherche Scientifique (Paris). She studied and identified many new K mesons in cloud chambers on the Alps, publishing the research in the Nuovo Cimento in 1957. She worked alongside American physicist Wayne Hazen, who later invited her to teach at his home college University of Michigan. ⁶

After teaching and researching in the US, Bibha received a position at the Physical Research Laboratory (PRL) in Ahmedabad. During this time, she began her investigation on high-energy muon interactions at the TIFR large shower array in Kolar Gold Fields (KGF) after reconnecting with a colleague. Using a muon detector and plastic scintillators, she recorded and categorised many events and even managed to measure the probability of their occurrence (the photo-nuclear cross sections of high-energy muons). This experiment took many years and was completed in 1976, after which Bibha retired from PRL. ⁷

After her retirement from PRL, Bibha returned to Kolkata, but she continued her work in High Energy Physics until her death. She collaborated with the Saha Institute of Nuclear Physics, the Indian Association for the Cultivation of Science, and the University of Calcutta, and used the Variable Energy Cyclotron Centre’s (VECC) cyclotron particle accelerator. She attended seminars and conferences, and published multiple papers including her last one in 1990, a year before her death. ⁸

In 2019, the International Astronomical Union named a star in the constellation Sextans “Bibha” after Bibha Chowdhuri. Her name was chosen by the Indian National Committee in the Union, who recognised her achievements, especially as the first woman-researcher at TIFR. ⁹

 

Research

Bose institute
Bibha and Bose began their nuclear emulsion experiments in search of a new proposed particle by Japanese physicist Yukawa, which was supposed to have the attractive force that kept the nucleus together. Their experiments consisted of exposing emulsion plates to cosmic radiation in their high-altitude stations in Darjeeling, Sandakphu and Tibet (at 7000 ft, 12000 ft and 14500 ft respectively) for 150-202 days, then examining the cosmic ray tracks left on them as a result of radiation (They used the type of emulsion plate particle detector developed in the 1930s by Marietta Blau and the Radium Institute in Vienna which made calibration of the tracks of fast-travelling protons possible). Another portion of the experiments was doing the same thing except with protons. Bibha played the instrumental role of setting up the experiments at each station, transporting exposed plates back to the institute, and examining them for analysis. She did all of this travelling on horseback. ¹⁰

After their experiments, Bibha and Bose discovered that most of the tracks left behind from cosmic ray exposures were from particles considerably lighter than protons and matching the description of Yukawa’s particle. It was a type of mesotron, also known as muon (Bibha and Bose preferred the named “mesotron”). ¹¹

Between 1940 and 1941, Bibha and Bose published their first two papers in Nature detailing their experimental setup and some collected data up until that point. The latter highlighted their observation that the emulsion plates exposed to cosmic rays had higher mean separation (more scattering) than the plates exposed to protons, suggesting the presence of particles with higher velocities and lower kinetic energies – thus lighter – than protons. Their third and fourth papers published in late 1941 and 1942 focused on calculating the mesotron’s mass. Using the ratio between proton and mesotron energy levels, they calculated the mass using three different data sets collected from their three locations, which came out to be 217, 336 and 313 electron mass. They were planning to refine this estimation by decreasing proton tracks from the higher calculations to narrow the range of masses, but the development of WWII prevented them from accessing equipment for better emulsions. ¹²

This “mesotron”, as Bibha and Bose called it, was in 1948 formally “discovered” by British physicist Cecil Powell, who used their exact technique to find it, as the “pi-meson”. ¹³

Manchester
For her thesis research, Bibha Chowdhuri came up with her own cloud chamber configuration, shown in Fig. 1. Bibha wrote that “a coincidence is recorded between the shielded (with iron or lead) counter tray and the four other counters,” when a penetrating particle is present in the extensive air shower, their paths (tracks) were “recorded on the cloud chamber by means of an indicator lamp.” ¹⁴ The four unshielded Geiger-Müller (GM) counters would simultaneously detect particles in every shower, but when the shielded counter also registered a hit, it would mean that the shower was accompanied by a penetrating particle. ¹⁵

From these experiments, she chose 1460 photographs of showers. She chose the ones where the tracks were not too old to too new and categorised them into events with and without penetrating particles in order to find patterns in the distribution of particle densities (number of tracks per unit volume of the cloud chamber). As shown by the graphs in Fig. 2, she found that particle densities follow a power law, and that “the exponent of extensively penetrating Showers was…of the same order as for all particle extensive Showers,” even though it “should be relatively lower” due to the absorbing effect of the iron and lead shields. ¹⁶ Bibha would go on to further investigate this observation later at TIFR. She also found that the probability of a penetrating extensive shower, which was 300 times lower than that of non-penetrating showers, was “approximately constant” for all particle densities, which indicated that penetrating events were created locally mostly by “the electric components of Extensive Showers”. ¹⁷

Tata Institute of Fundamental Research (TIFR)

Alongside a group of research students, Bibha used the TIFR’s 12-inch cloud chamber to take and analyse photographs of penetrating showers. She used the same set up as her thesis experiment, except this time the cloud chamber was shielded by 10cm of lead on all sides. This led her to discover new penetrating particles that can produce nuclear interaction inside the led shields, and that they were behind 29% of penetrating showers whereas muons were only associated with 0.7%. Her findings helped improve theoretical models of cosmic ray showers and understanding of secondary particles. [“Bibha Chowdhuri and Her Remarkable Scientific Endeavours.” 1493-1494.]

 

MIT

After teaching at UMichigan for 2 years, Bibha worked at Bruno Rossi’s MIT lab, a new extensive air shower array in New Mexico, for a few months before returning to India. This experience introduced her to plastic scintillator detectors, which she used for her later experiments at KGF. ¹⁸

 

Kolar Gold Fields

194m below the TIFR large air shower array, where only high-energy muons (>150GeV) could reach, Bibha set up a muon detector with plastic scintillators – which pulses at the arrival of a particle – as the trigger that set off the camera, and neon flash tubes (NFT) – which emit light when the gas is ionised by penetrating particles and then applied with high voltage – as trails left behind by the muons. She then analysed photographs of the showers, and categorised the pictured events as results of either electromagnetic or photo-nuclear interactions with surrounding rocks. ¹⁹

 

Experience as woman in Physics and Astronomy

An article about Bibha Chowdhuri was published in The Manchester Herald, a local newspaper, during her time at Blackett’s lab. The article, titled “Meet India’s new Woman Scientist – She has an eye for cosmic rays”, included photos and an exclusive interview. ²⁰ The article was also republished in the Indian Express that same year. This media attention shows how forefront Bibha was as a woman in the study of fundamental sciences at the time. 

In her interview, Bibha famously said, “Women are terrified of physics – that is the trouble. It is a tragedy that we have so few women physicists today. In this age of science, women should study atomic power; if they don’t understand how it works, how can they help decide how it can be used?” ²¹ Evidently, her pursuit of physics and astronomy was also her way of struggling against a male-dominated society. 

Chowdhuri taught and led many student researchers during her time at TIFR, PRL and KGF. The former Indian Space Research Organisation (ISRO) chairman Krishnaswamy Kasturirangan, was a student of hers at PRL, and once praised her teaching in an interview: “Her teaching style greatly enhanced our problem solving skills, original thinking, and the ability to apply principles and ideas from different subject areas…this prepared us for the more demanding challenges of research…” ²² However, her abilities were not always recognised due to various biases. Another one of her students at KGF,  the renowned cosmic ray physicist Palahalli Vishwanath, recounted that “due to our lack of maturity, we failed to appreciate Dr Bibha Chowdhuri’s pioneering contributions fully. We heard that she had worked in England, but I, at least, remained ignorant of her achievements.” ²³ As Chowdhuri was in her fifties at the time, it can be easily inferred that a leading factor of her underestimation by her – usually male – students was her gender.

Chowdhuri never married. Roy and Singh speculate it was the difficulty of finding a match from both being a Brahmo, ostracised from mainstream Hindu society, as well as being a woman scientist, 14% of whom remain unmarried. ²⁴

Although Bibha Chowdhuri conducted instrumental work with many “leading lights” in the Indian and global physics field, her contributions remain unrecognised by institutions, making her a much forgotten figure in India and around the world. She was never made a fellow of any of the three Indian scientific academies, nor received any major prize, and no literature on the KGF cosmic particle research mentioned her name. Lilavati’s Daughters, a book about 100 Indian women scientists throughout history published by the Indian Academy of Sciences, also did not include Chowdhuri. ²⁵

Even after Bibha published her discovery of the pi-meson in the journal Nature in 1942, she was never formally given credit, and a British male scientist was awarded the Nobel prize for it years later following her method. ²⁶ Similarly, despite the fact that her PhD experiments were repeated and referenced many times by ray physicists of the western scientific community, with “glowing acceptance,” in a time when cosmic ray extensive showers was one of the most important studies in physics, she received no honourable titles or awards for it. ²⁷

 

 

 

Bibliography

“Bibha Chowdhuri – Celebrating a Forgotten Life in Physics.” Last modified December 5, 2018. https://www.downtoearth.org.in/news/science-technology/bibha-chowdhuri-celebrating-a-forgotten-life-in-physics-62393.

Mondal, Naba K. “Bibha Chowdhuri and Her Remarkable Scientific Endeavours.” Resonance Vol. 28, No. 10 (2023). doi: https://doi.org/0.1007/s12045-023-1686-1.

“Star named Bibha after Indian scientist and first woman researcher-faculty at TISS.” Free Press Journal (Mumbai, India). 18 Dec. 2019, p. NA. Gale OneFile: News. link.gale.com/apps/doc/A608988258/STND?u=mlin_m_wellcol&sid=ebsco&xid=0f834b6e. Accessed 5 June 2024.

Roy, S. C., and Rajinder Singh. “Historical Note: Bibha Chowdhuri – Her Cosmic Ray Studies in Manchester.” Indian Journal of History of Science 53, no. 3 (2018). doi:10.16943/ijhs/2018/v53i3/49466.