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Bibha Chowdhuri at the International Conference in Pisa, Italy |
Introduction
Modern scientific research arrived in India in the second half of the nineteenth century with principally two men, JC Bose and PC Ray. They were followed early in the next century by people like CV Raman, MN Saha, SN Bose, S Ramanujan, PC Mahalanobis and others. However, experimental research, barring a few examples such as JC Bose and, more prominently, CV Raman, remained a difficult endeavour in the country in the absence of governmental or industrial support. Unfortunately, we seem to have forgotten people like KS Krishnan or DM Bose, who achieved international recognition for their accomplishments in the laboratory despite all odds. In this article, we will look back on one incident where two investigators from India came close to a major achievement in the nascent field of experimental particle physics but faltered in the final step due to circumstances beyond their control.
This story is more intriguing as one of the two investigators was a woman, Bibha Chaudhuri, the first Indian woman to obtain a doctorate degree in Physics. Science in our country remained an exclusive male bastion for the first three decades of the twentieth century, and the few women who joined a career of research in science pursued higher studies abroad. These include Prabhabati Dasgupta, who studied in the USA and obtained a doctorate in psychology from Germany, and the first woman to earn a doctorate degree in Botany in the USA, Janaki Ammal. Opportunities for women were few in the country. Chameli Dutta stood first in the MSc Physics Examination of CU in 1933; this was the first instance of a girl student successfully completing a Master’s degree in science in India. The first woman to receive a doctorate in science from an Indian university was Asima Chatterjee, who was admitted to the DSc degree of the University of Calcutta (CU) in 19442. Bibha, a few years senior to Asima Chatterjee, worked in the laboratory of the Nobel Laureate Patrick Blackett in Manchester to obtain her doctorate. However, before going to the UK, she spent six years at the Bose Institute, and nowadays, she is remembered chiefly for the work carried out in that period in which she and her guide DM Bose came close to a very fundamental discovery.
A major source of information for this article is the book by Rajinder Singh and Suprakash Roy mentioned in the references. Other information has been obtained from the Bi-Annual Reports of the Bose Institute for various years between 1938 and 1951, and the Archive of the University of Calcutta.
Background
Debendramohan Bose (1885-1975), generally known as DM Bose, completed his Master’s degree in physics from Presidency College in 1906. He then went to the UK and worked there with two Nobel Prize-winning scientists, JJ Thomson, the discoverer of the electron and CTR Wilson, the inventor of the cloud chamber. After coming back to India, he briefly served in the City College, Kolkata, before being appointed a professor of physics in the newly created University College of Science of CU. Following the conditions of appointment, he went to Germany in 1914 and obtained a doctorate degree there. Due to the First World War, he could come back and start teaching only in 1919. He was a nephew of the pioneer scientist, JC Bose, who had founded the Bose Institute. DM Bose left CU to take up the directorship of the institute after the death of his uncle.2
Bibha Chaudhuri (1913-1991) graduated from City College in 1934 and did her MSc from Science College, CU two years later. She was the only girl in her MSc class. DM Bose refused her request for research guidance at first, but relented later. Bibha was a relative of DM Bose, and that probably was the fact that tilted the scales in her favour. DM Bose started her in cosmic ray research.
DM Bose, a versatile physicist, was an internationally acknowledged expert in cosmic rays and built the first cloud chamber for this purpose in India. However, for Bibha, he chose a new approach. In 1938, Walther Bothe, a future Nobel Laureate, and Geoffrey Taylor attended the Indian Science Congress. There, they demonstrated the use of photographic plates in cosmic ray studies, a technique developed by the Austrian woman scientist, Marietta Blau. A photographic plate is a glass plate coated with an emulsion, which generally contains a salt of silver. When a cosmic ray passes through the emulsion, it breaks up the salt to release silver, which then deposits on the plate. Thus, a record of the cosmic ray, called its track, is retained in the plate for later examination. This method has the advantage that very little supervision is needed; one can keep the plates for months at a time in a suitable location and later develop them to study the signs of cosmic rays.
Cosmic Ray Research of DM Bose and Bibha Chaudhuri
Let us briefly consider the scenario in the field of cosmic rays when Bibha Chaudhuri started her work. Earth is being continuously bombarded by high-energy radiation from space. We are not yet sure about their origins, more than a century after their discoveries. The principal component of these rays is a number of particles which may produce more particles after collisions with nuclei in the atmosphere. For example, positron, a particle predicted by PAM Dirac, was observed in a cloud chamber by Carl Anderson in 1932. With this discovery, interest in cosmic rays increased, as before the advent of modern day accelerators, they were the only source of new particles. In 1935, Hideki Yukawa predicted a new particle, later called a pi meson or pion, as responsible for the force between nucleons, i.e. protons and neutrons7. He suggested that it has a mass between that of the electron and the proton. A proton is some 1840 times heavier than an electron. No such particle was known at that time. The proposed particle was variously termed meson or mesotron, ‘mesos’ being a Greek word meaning intermediate. Meson is the commonly accepted word nowadays, but it is used to indicate a class of particles, not necessarily lighter than the proton. Anderson and Seth Neddermeyer found a particle two hundred times heavier than the electron in their cloud chamber in 1936, and Yukawa seemed to be vindicated. However, further studies soon showed that the observed particle cannot be the carrier of the force between nucleons. Nowadays, we call it a muon; it is no longer called a meson.
Bibha and DM Bose placed their photographic plates high up in the Himalayas, in Darjeeling, Sandakphu and in Phari Jong, then in Tibet. The advantage of a high altitude is that there is less absorption of cosmic rays by the atmosphere. They devised a method, where from the track density (alternately, spacing between grains of deposited silver), they could estimate the mass of the particle. They could also measure the mass by looking at the deflection suffered by the incoming particle when it collides with a nucleus in the emulsion. They kept the plates there for months, sometimes in the air, sometimes below absorbing materials such as wood, water or lead, before developing them and studying the tracks.
Bose and Chaudhuri could find tracks of protons and muons in their photographs. The mass measurement allowed them to distinguish between the meson and the proton, which is nearly nine times heavier. They published their conclusions in four publications in the famous journal Nature. The two investigators, along with another student of DM Bose, Mriganka Shekhar Sinha, also published a paper in Physical Review, a journal published by the American Physical Society. Bibha published a single-author paper in the Indian Journal of Physics. Below we present the list of papers published by Bibha and her co-authors in this period.
Photographic plates as detectors of mesotron showers, D.M. Bose and Biva Chowdhury, Nature 145, 894, 1940.
Origin and nature of heavy ionisation particles detected on photographic plates exposed to cosmic rays, D.M. Bose and Biva Chowdhury, Nature 147, 240, 1941.
A photographic method of estimating the mass of the mesotron, D.M. Bose and Biva Choudhuri, Nature 148, 259, 1941.
A photographic method of estimating the mass of the mesotron, D.M. Bose and Bibha Choudhuri, Nature 149, 302, 1942.
Cosmic-ray muon spectra, D.M. Bose, Bibha Choudhuri and M. Sinha, Physical Review, 65, 341, 1944.
Mass determination of the ionising particles recorded in photographic plates exposed to cosmic rays, B. Choudhuri, Ind. J. Phys., 18, 57, 1944.
The data from the photographic plates seemed to indicate that there might be another particle slightly heavier than the muon, but its mass measurement varied widely. At that time, there was no theoretical justification for the existence of two particles, both having masses lying between electron and proton but with completely different properties. Bose and Chaudhuri tried to explain away the tracks of this more massive particle as the average between those of muons and protons.
In hindsight, we clearly understand that the slightly heavier pion showed up for the first time in the plates of Bose and Chaudhuri. Plates exposed to air generally did not show tracks of the more massive particles; the tracks showed up only when the plates were placed below some absorbers, such as water or lead. The high energy cosmic rays produced pions in collisions with the nuclei of the atoms of the absorbing material. These pions showed up in the photographic plates. Later evidence and some indications in the single-author paper by Bibha seem to indicate that Bibha was thinking of Yukawa particles, but it was never stated clearly and unambiguously in any of her publications.
The practical problem that Bibha and DM Bose faced was the non-availability of good plates. There were two types of plates, full-tone and half-tone. The full-tone plates are coated with emulsions on both sides, unlike the half-tone plates, which have a coating on only one side. The resolution of the latter is poorer; consequently, the measurement of mass from those plates will have a large error. Soon after Bibha began research, the Second World War started, and all the good plates were reserved for military use. Bose and Chaudhuri had access to half-tone plates from the Ilford company only. The Annual Reports of the Bose Institute bemoaned the lack of good photographic plates. They were available only after the War ended. By that time, Bibha had decided to go to Manchester, and DM Bose was too tied up in administrative duties to follow up on their work.
In Britain, Cecil Powell, who started photographic research of cosmic rays nearly at the same time as Bose and Chaudhuri, continued his work after the War. He had access to better plates and also made significant improvements to them in collaboration with the Ilford company. Finally, in 1947, he and his collaborators clearly showed that there are two mesons. One of them, the muon, which does not interact with nucleons, is the one that was observed and identified earlier. The other is a new particle, now called a pion, with a mass 273 times that of the electron. They also showed that a pion decays into a muon. Powell was awarded the Nobel Prize in Physics in 1950 for his study of cosmic rays, one year after Yukawa was so honoured.
Conclusion
The life of Bibha still contains some gaps that remain to be filled. We do not know why she did not submit her work for a doctorate degree at CU despite having made significant progress and published a number of papers in two of the top journals in the world. She went to Manchester in 1945 and worked in the field of cosmic rays using cloud chambers to obtain a PhD degree. She returned to India for a few years but left again to go to first France and later, the USA. She finally came back to India and joined the Physical Research Laboratory, Ahmadabad, first in a temporary position and then as a permanent scientist. After her retirement, she came back to Kolkata, where she continued her research till her death. Very recently, the International Astronomical Union named a star after her.
The present author does not intend to express the opinion that Powell’s Nobel Prize was undeserved, or that Bose and Chaudhuri should be considered as co-discoverers of the pion. In science, it is the concrete proof that counts. For example, it is well known that the positron was observed a few times before Anderson properly identified it. Bose and Chaudhuri never put forward the hypothesis that they had found the particle that Yukawa had predicted. They knew that their mass measurements varied too widely to make an unambiguous identification. Yet, if the Second World War had not intervened and good plates had been available, perhaps we could have had another fundamental discovery from India.
Acknowledgements
Discussions with Anirban Kundu and Sreerup Raychaudhuri are gratefully acknowledged.
References
1. C V Subramanian, Lilavati’s Daughters, Eds. Rohini Godbole and Ram Ramaswamy (Indian Academy of Sciences, 2008), p 1.
2. Gautam Gangopadhyay, Anirban Kundu and Rajinder Singh, The Dazzling Dawn: Physics Department of Calcutta University (1916-36) (Shaker-Verlag, 2021)
3. A Jewel Unearthed: Bibha Chowdhuri, The Story of an Indian Woman Scientist, Rajinder Singh and Suprakash C Roy (Shaker-Verlag, Expanded Indian Edition, 2020)
4. http://www.jcbose.ac.in/annual-report
5. https://www.culibrary.ac.in/
6. Gautam Gangopadhyay, Nakshtrer Nam Bibha (Khoyabnama Prantajaner Kotha, 2022)
7. Hideki Yukawa, Proceedings of the Physico-Mathematical Society of Japan 17, 139-148 (1935).
8. Sreerup Raychaudhuri, CU Physics 100 – Department of Physics – University of Calcutta 1916-2015, Eds. Gautam Gangopadhyay and Anirban Kundu (University of Calcutta, 2016) p. 148.
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