Nobel Prize in Physics: A Right Recipient but a Wrong Citation

In 1938, the Nobel Prize in Physics was awarded to a scientist "for his demonstrations of the existence of new radioactive elements produced by neutron irradiation, and for his related discovery of nuclear reactions brought about by slow neutrons". The scientist was Enrico Fermi, and the citation was partly wrong by implication. What makes the story even more fascinating is that Fermi and the world knew it was almost certainly wrong by the time he went to collect the Prize in Stockholm.

Enrico Fermi (1901-1954)

Enrico Fermi was an Italian scientist who was working with neutrons. In 1934, he was bombarding uranium with slow neutrons, a process he had earlier found highly effective in inducing nuclear reactions. In a sudden burst of intuition, he had used paraffin to slow down the neutrons and found that the probability of reaction, cross-section in the jargon of physics, goes up enormously. He decided to bombard the heaviest element known to man at that time, uranium, with the slow neutrons produced in his laboratory.

What did he expect? The Curies, Irene Curie, daughter of the celebrated Pierre and Marie, and her husband, Frederic Joliot-Curie, had already demonstrated artificial radioactivity by bombarding aluminium with alpha particles. What emerged from their experiments was a radioactive isotope of phosphorus. This feat would earn the duo the Nobel Prize in Chemistry in 1935. Isotopes of an element have different neutron numbers but the same proton number. For example, the phosphorus isotope commonly found in nature has 15 protons and 16 neutrons and does not exhibit radioactivity. Irene and Frederick created a new isotope of phosphorus with 15 protons and 15 neutrons.

Fermi knew that alpha particles are positively charged and are repelled by the positively charged nucleus of the atom. Hence, it isn't easy to cause nuclear reactions with them. The alpha particles have to have sufficient energy to overcome the Coulomb force between themselves and the nucleus. Neutrons are uncharged and can therefore easily enter the nucleus to initiate nuclear reactions. He expected that the neutrons would enter the uranium nucleus and be absorbed, creating a new isotope of uranium. The newly created nucleus would then undergo beta decay, a process in which it emits an electron (and an antineutrino). We know that there is no electron in the nucleus, so where does the electron come from in the beta decay? The electron is created in the nucleus by the break-up of a neutron into a proton, an electron and an antineutrino. Uranium has 92 protons. The new nucleus after beta decay would then have 93 protons, a new element not available on Earth. That was Fermi’s reasoning.

How would we know that a new element has been created? After all, one cannot see the nucleus or the atom. The trick was to chemically separate the newly created elements from the sample that had been bombarded with neutrons. Chemical properties are determined by the number of protons, and hence, isotopes of an element show the same chemical properties. Fermi argued that if some new properties are manifested, they must come from an element other than uranium.

Of course, the observed new chemical properties may correspond to some element created by neutrons knocking off protons or alpha particles. In that case, we would have elements, not heavier but lighter than uranium. The proton number would be either 91 or 90, the elements corresponding to proactinium and thorium, respectively.

Fermi found that the uranium sample bombarded with neutrons exhibited radioactivity with different half-lives. He checked the sample for chemical properties of not only proactinium and thorium but also for lighter elements up to lead, which has 82 protons. Since the newly created isotope did not match the properties of any of the known elements between lead and uranium, he reasoned that the radioactive isotopes must belong to elements heavier than uranium. He published his results in the journal Nature in an article entitled ‘Possible Production of Elements of Atomic Number Higher than 92’ in 1934.

Other people were working on the same field, notably the Curies in Paris and Otto Hahn and Lise Meitner in Berlin. The latter group supported Fermi, but the Curies were sceptical. A dissenting voice also came from a very respectable chemist. Ida Noddack argued that Fermi's conclusion was built on an unproven hypothesis. A negative result in chemical analysis did not prove the existence of a new element. In fact, she suggested that the nucleus could break into smaller fragments— a process we now call nuclear fission.

One interesting sidelight in this history is that so many women scientists were involved in the study of radioactivity at a time when there were few women in science. Irene was, of course, influenced by her celebrated mother, but we also have two German ladies, Ida Noddack and Lise Meitner, both of whom were later nominated for the Nobel Prize, working in the field.

Curies continued publishing their results on the bombardment of uranium with slow neutrons, but Fermi paid them no heed. He found it inconceivable that a neutron with less than one electron volt of energy could break up the nucleus, whereas alpha particles or protons with millions of times more energy do not succeed. Now, of course, we know that heavy elements like uranium do undergo nuclear fission, and it requires very little external energy. Otto Hahn and Franz Strassmann, in 1938, finally identified barium in the sample and proved that nuclear fission indeed occurs. Uranium, when bombarded with neutrons, breaks up into two big fragments. The heavier fragment is close to barium in proton number, and the lighter fragment is close to krypton in proton number. Fermi was looking for elements with proton numbers between 82 and 92, but the elements being created had proton numbers near 56. i.e. barium, and 36. i.e. krypton. How it happened was explained by Lise Meitner, who, by that time, had fled Germany to escape the Nazi's persecution of the Jews. This discovery earned Hahn the Nobel Prize in Chemistry in 1944. Many people believe that Lise Meitner should have also shared the Prize, but that is another story.

So by the time Fermi went to Stockholm to collect the Nobel Prize, he knew that he had not made any new element. To his credit, he freely admitted it, but he believed that he had already done enough to earn the Prize. He was not wrong. By that time, he had not only discovered the enormous potential of slow neutrons but also produced the first theory of beta decay, a theory which is still taught in class, and also had discovered the statistics obeyed by subatomic particles like electrons, protons and neutrons. In his honour, we call the statistics Fermi-Dirac, and the particles that obey this statistics are called fermions. (Dirac independently discovered the statistics after Fermi.) All fundamental particles are either bosons or fermions.

Fermi had a very important reason to travel to Stockholm. He was escaping the fascist dictatorship of Mussolini, who, prompted by Hitler, had enacted anti-Jewish laws in Italy. Fermi’s wife, Laura, was Jewish. Fermi was permitted to travel to Stockholm with his family to collect the Prize. He did not return to Italy but took this opportunity to escape to the United States of America. After 1938, he would go on to show that neutrons emitted in the fission of uranium nuclei can break up other uranium nuclei, a process called a chain reaction. This led to the construction of the first nuclear reactor by Fermi himself, and later, to the nuclear bomb in whose construction he played a very important role.

  

Gautam Gangopadhyay

A Tale of a Missed Opportunity

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.

1. Photographic plates as detectors of mesotron showers, D.M. Bose and Biva Chowdhury, Nature 145, 894, 1940.

2. 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.

3. A photographic method of estimating the mass of the mesotron, D.M. Bose and Biva Choudhuri, Nature 148, 259, 1941.

4. A photographic method of estimating the mass of the mesotron, D.M. Bose and Bibha Choudhuri, Nature 149, 302, 1942.

5. Cosmic-ray muon spectra, D.M. Bose, Bibha Choudhuri and M. Sinha, Physical Review, 65, 341, 1944.

6. 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.

Remembering Irene Curie

Irene Curie inaugurating the Institute of Nuclear Physics. Besides him are Kailas Nath Katju and A.P. Patro (Photo from the archive of SINP)

 

 It was January 11, 1950. A lady walked up the stairs of the new building inside the Rajabazar campus of the University of Calcutta. She entered the auditorium on the first floor to a thunderous applause. She was only the second woman to receive the Nobel Prize in science. Her field of research was nuclear science, and aptly enough, she had been invited to inaugurate the new building of the Institute of Nuclear Physics of the University by Meghnad Saha, the founder of the Institute. Since then, every year the Institute celebrates January 11 as its foundation day. A few months later, the Institute became an independent institution outside the control of the University of Calcutta. In 1956, it was renamed the Saha Institute of Nuclear Physics after the death of Saha.

 
 The lady was Irene Curie, the elder daughter of the illustrious couple, Mary and Pierre Curie, the first husband and wife team to be awarded the Nobel Prize. Mary, of course, was the first woman to receive the coveted prize and to date the only woman to win it twice. Following the footsteps of such parents is not an easy task. The spotlight of publicity was on her all the time; even her receiving the doctorate became a news item in the New York Times in distant America. It was to her credit that such media attention could not distract her from pursuing the first love of her life, science. 
 Irene was born exactly one hundred and twenty five years ago, on September 12, 1897. Her mother noted down the development of her child, interposed by records on her research. Thus, a note on Irene’s fifteenth tooth by the proud mother was followed by the announcement of the discovery of a new element, radium. Irene had a chequered childhood. Pierre died in an accident in 1906 when Irene was only nine years old. Madame Curie, as Mary was universally known, was devastated by the sudden bereavement, yet she had to bring up her two daughters and continue the research. 
 Madame Curie was fortunate that Pierre’s father, Eugène Curie, came forward to assist her in looking after Irene and her sister, Eve, till his death in 1910. Eugène Curie was a physician with a progressive outlook. He had participated in the Paris Commune uprising in 1971, tending to the injuries of the defenders of the Commune. The Commune had failed, but it left a deep impression on Eugène, which shaped the views of his son and granddaughters. Irene learned to love nature and poetry from her grandfather. Mary was a very secular and progressive social thinker. Irene was also anticlerical and liberal socialist in her views, yet it was Eugène who had shaped her character. To quote Irene, “My spirit had been formed in great part by my grandfather, Eugène, and my reactions to political or religious questions came from him more than from my mother.” The young Irene displayed a great strength of character, standing by her mother, when she went into a depression following the vilification campaign launched against her for the ‘audacity’ of a woman to stand for membership in the French Academy of Sciences in 1912. Mary was defeated in the election, but coincidentally, later that year, she won her second Nobel Prize, the first person in history to do so. Irene never forgot the incident. When she was a reputed scientist, she applied for membership of the Academy three times, knowing fully well that it would be denied, highlighting the misogynist character of the scientific establishment. 
 Irene entered school at the age of twelve. It was not an ordinary school. Called the Co-operative, it was established by a number of leading French scientists for their children. The curriculum included not only the usual subjects but also such diverse topics as Chinese language and sculpture. Classes were held in the houses of the students, and teachers included two Nobel Laureates, Mary and Jean Perrin. Two years later, Irene enrolled in an ordinary school and later went to the University of Paris, Sorbonne. However, her study was interrupted by the First World War, when her mother set up a number of mobile X-ray units for treating the wounded soldiers. Mary personally went to the battlefields, accompanied by Irene, who first served as a nurse, and later found herself lecturing to the physicians, who were much older than her, about X-ray photographic technique. 
 She graduated from the Sorbonne in 1918 and joined the Radium Institute, established by her mother. In 1925, she earned her doctorate degree for her work on alpha decay of polonium, an element discovered by her parents. Her supervisor was Paul Langevin, a brilliant physicist. By this time, she had met Frédéric Joliot, whom she was instructing on radioactivity. She found in him a kindred soul, and they married next year. It was one of the happy marriages where both the husband and the wife complemented each other in all walks of life. Interestingly, Irene, a doctorate in physics, was counted among the greatest radiochemists of the world, while Frédéric, a chemist by training, looked after the physics side. They often signed their names as Joliot-Curie.
 It is not possible to discuss all the achievements of Irene and Frédéric within a short article. We will restrict ourselves to a few cases where they came very close to epoch-making discoveries but stumbled at the last step. However, in science, sometimes failures are almost as important as successes; accounts of their experiments helped others to the final discovery. We will also describe briefly their triumph, which led to a Nobel Prize for the duo, and how that discovery has impacted modern life. 
 Ernest Rutherford, the discoverer of the nucleus, had predicted the existence of the neutron, a neutral constituent of the nucleus, in 1920. Ten years later, the Joliot-Curies found that bombardment of beryllium by alpha particles from polonium results in the emission of some uncharged radiation which can knock out a proton from a block of paraffin. They identified the radiation as gamma-rays of very high energy. Rutherford, learning of the discovery, commented, “I don’t believe it.” His student, James Chadwick, repeated the experiment and correctly identified the new radiation as neutrons. He won the Nobel Prize for his discovery in 1935. Earlier, Irene and Frédéric had seen the track of a positively charged particle in their cloud chamber photographs but had misidentified it as that of a proton. Later in 1932, Carl Anderson saw a similar track in his setup and established that it was due to a positron, the antiparticle of the electron predicted some years back by Paul Dirac. Anderson won the Nobel Prize for his work in 1936. 
 Another discovery which eluded Irene was that of nuclear fission. In Rome, Enrico Fermi and in Berlin, Otto Hahn and Lise Meitner were bombarding uranium with neutrons in the hope of creating new elements. Fermi was thought to be successful, and his Nobel citation actually mentions this, along with his other achievements. The experiment was repeated in Paris by Irene and her collaborator, Pavel Savitch, but they found no trace of new elements. Instead, they found traces of the already known lighter element, lanthanum, though they could not explain its presence. Hahn was livid by Irene’s objection to his results and refused to read her articles. By this time, Lise Meitner had fled Germany to escape the Nazis. Hahn’s new assistant, Fritz Strassmann, insisted that Hahn go through the paper by Irene and Savitch again. Hahn relented and, being struck by its arguments, repeated the experiment. They established that the uranium nucleus had been split by the neutron, a discovery that fetched Hahn the Nobel prize in Chemistry in 1944, and set up a chain of events that led to the two bombs dropped on Hiroshima and Nagasaki in 1945. Fearful of the military consequences of their research in war-torn Europe, Irene and Frédéric stopped publishing results on fission and put all their documents in the vaults of the French Academy of Sciences two months after the beginning of the Second World War. Those were retrieved ten years later. 
 The two particles which eluded the Joliot-Curies were associated with their triumphal experiment. In 1934, they bombarded aluminium with alpha particles, which resulted in the emission of a neutron and the formation of a new isotope of phosphorus. This isotope was radioactive and emitted a positron. This was the first observation of radioactivity created in the laboratory. Madame Curie was ecstatic about the discovery. The Joliot-Curies were awarded the Nobel Prize in chemistry next year; unfortunately, Irene’s mother was not alive to see the day. 
 The importance of artificial radioactivity can hardly be overestimated. Artificial radioactive isotopes are now regularly used in cancer therapy and medical imaging, such as Positron Emission Tomography (PET) and Single Photon Emission Computed Tomography (SPECT) scans. Industrial uses include food preservation and tracing of leaks and blockage in pipelines, etc. Branches of science that routinely utilise artificial radioactivity include genetic engineering, plant biology, animal physiology, biochemistry, geology and even subjects such as archaeology. 
 In spite of such dedication to science, neither Irene nor Frédéric was an ivory-tower scientist. They held progressive views and were not afraid to articulate them or to put them into practice. Irene and her husband were active against the rising tide of fascism in Europe. Irene served as a minister in the Popular Front government formed by a coalition of communist, socialist and democratic parties, led by Leon Blum, before the Second World War. She played an important role in setting up the National Centre for Scientific Research (CNRS), which later served as a model for many other countries. After Germany occupied France in the Second World War, Frédéric joined the French Communist Party and became active in armed resistance against the Nazis. It was a traumatic time for Irene, who had to bring up her children alone. She was also suffering from tuberculosis. Once Irene and her children had to flee to neutral Switzerland, crossing the border illegally, while Frédéric went underground. The role Frédéric played in the street fighting against the German army, employing his knowledge of chemistry to manufacture bombs, when Paris was on the verge of liberation, has been immortalised in the book ‘Is Paris Burning?’ by Dominique Lapierre and Larry Collins. 
 After the war, both the Joliot-Curies became members of the World Peace Council. Frédéric was appointed the first High Commissioner of Nuclear Energy to head the French nuclear power programme. Irene was also made a commissioner. However, their outspoken opposition to secrecy in nuclear research and to nuclear weapons, as well as sympathy towards socialism, led to their removal within a few years. 
 Over-exposure to radioactivity took its toll, and both Irene and Frédéric became seriously ill. Irene was diagnosed with leukaemia and passed away on March 17, 1956. Frédéric also passed away in 1958. 
 Irene was strong in her convictions. She never patented her discoveries, which would have brought her huge amounts of money. She strongly believed that research is a legacy of mankind as a whole. Her parents, who often had to survive on very little money and carry out research on a shoestring budget, had also refused to take patents on the extraction process of radium. Irene believed in the ideal of a just and fair society. Her sympathy towards the refugees fleeing the fascist regime in Spain was well known, and she lectured on their plight in various countries to draw public attention towards the problem. She was passionate about education for women and served on the National Committee of the Union of French Women. On her 125th birth anniversary, we pay homage to a legendary scientist and great human being. 

Irene Curie speaking at the inauguration of Institute of Nuclear Physics (Photo from the archive of SINP)

 

 

Gautam Gangopadhyay

Monthly Bulletin of the Asiatic Society, June 2022 

Samarendranath Ghoshal: A Tribute

This year (2023 CE) marks the centenary of the celebrated nuclear physicist and teacher Samarendranath Ghoshal. His seminal work on nuclear reactions is a compulsory reading for undergraduate physics students all over the world. S N Ghoshal, as he is popularly known, was born on February 22, 1923, in the village of Raipur in Birbhum district, about seven kilometres away from the town of Bolpur. He studied in Burdwan Municipal High School, where he won two medals for proficiency in Bengali and Sanskrit in the Matriculation examination. He did his I.Sc. from Burdwan Raj College with a district scholarship. He then came to Kolkata and passed B.Sc. from Scottish Church College under the University of Calcutta. He stood second in the examination and was awarded the Mohinimohan Roy Silver Medal of the University of Calcutta for his results. He then joined the postgraduate department of Physics in the University College of Science, University of Calcutta. He was first class first in the M.Sc. examination of the University in 1944. He then approached Prof. Meghnad Saha for research.

Prof. Saha had come back to the University of Calcutta in 1938 as the Palit Professor after spending fifteen years at the University of Allahabad. He had great plans for establishing a research centre in cutting-edge science. His appeal for grants to the Sir Dorabji Tata Trust and the University of Calcutta for a cyclotron for research in nuclear physics and nuclear medicine had been approved. Simultaneously, he instructed a junior colleague, Nirajnath Dasgupta, to build India’s first indigenous electron microscope. The funds for this were obtained from the Krishnarpan Charity Trust of the Birlas and a private donor. This instrument was meant for the building of a centre for biophysics, a newly developed field. The cyclotron and the electron microscope were to be the nuclei of the newly established Institute of Nuclear Physics under the Department of Physics. Ghoshal was appointed a research fellow in biophysics under Prof. Nirajnath Dasgupta in 1945 with a fellowship of Rs. 150 in place of Pareshchandra Bhattacharya, who had joined as a Palit research fellow.

Ghoshal joined on February 1, 1945, but stayed less than ten months in the position. He applied for and obtained a Government of India Fellowship for study abroad. He joined the Radiation Laboratory of the University of California, Berkeley. Meghnad Saha had close contact with the Director of the Laboratory, Ernest Lawrence. Lawrence had won the Nobel Prize in 1939 for his invention of the cyclotron. Saha had sent his student, Basantidulal Nagchaudhuri, to Lawrence for training in running a cyclotron. Saha also obtained the design of the cyclotron from Lawrence, and Nagchaudhuri was instrumental in procuring the different parts of the cyclotron and shipping them to India. Ghoshal joined Emilio Segrè as a research student. Segrè later went on to win the Nobel Prize in physics in 1959 along with Owen Chamberlain for the discovery of the antiproton in an experiment carried out in 1955.

Although Ghoshal spent only a short time in the Institute of Nuclear Physics, evidence shows that he had some contact regarding research during this period. Ajit Kumar Saha, the eldest son of Meghnad, Ghoshal and a co-worker published a paper on nuclear beta energy systematics [1] in 1948. This was a continuation of a work initiated by MN Saha and Ajit Saha, published in Nature, where they modified the famous Bethe-Weizsacker formula for nuclear masses. In 1948, Ghoshal was abroad; most probably, the work had been carried out earlier. Ghoshal reminisced later that the work was difficult to carry out as the Second World War had made scientific contact with foreign scientists difficult.

While in Berkeley, Ghoshal published two papers. In both papers, he was the only author, though he thanked Segrè for his guidance and encouragement. In the first [2], he bombarded silver foils with energetic alpha particles from the 60-inch cyclotron of the Radiation Laboratory. Alpha particles are nuclei of the noble gas helium. He used an ionising chamber to study the decay of the resulting radioactive nuclei, principally radioactive iodine.

The second work that Ghoshal published [3] was the famous experiment that now bears his name. Though the experiment was carried out in Berkeley, a note attached to the paper shows that by the time the paper was published, he had come back to the Institute of Nuclear Physics. He experimentally verified the hypothesis on compound nuclear reactions proposed by Niels Bohr in 1936. It is not accidental that working under Segrè, Ghoshal came across that problem. Bohr presented his hypothesis for the explanation of neutron-induced nuclear reactions carried out by Enrico Fermi. Fermi was the acknowledged authority on neutron-induced reaction experiments and was awarded the Nobel Prize in Physics in 1938, partly for his work on reactions using slow neutrons. Segrè was a doctoral student of Fermi and collaborated in those experiments; he had to take shelter in Berkeley to escape persecution of the Jews by the fascist government.

In 1936 speaking before the Royal Danish Academy 1936, Niels Bohr proposed the compound-nucleus model with the sentence: “The phenomena of neutron capture force us to assume that a collision between a high-speed neutron and a heavy nucleus will in the first place result in the formation of a compound system of remarkable stability; the later breaking up of this intermediate system . . . must in fact be considered as a separate process which has no immediate connection with the first stage of the encounter” [4]. Victor Weisskopf proposed a mathematical model based on Bohr’s hypothesis, where the formation of the compound nucleus and its decay were treated independently [5].

Bohr hypothesised that when a projectile enters a nucleus, a large number of collisions occur between the constituent nucleons so that the system reaches equilibrium and the excess energy is divided between the nucleons following a statistical rule. This state is called the compound nucleus. Since the system has achieved a statistical equilibrium, the subsequent decay of the nucleus does not depend on its method of formation; we loosely say that the compound nucleus has lost its memory. While Bohr’s arguments were persuasive, experimental confirmation was lacking, which was supplied by Ghoshal.

Ghoshal formed the same compound nucleus of zinc, using two different reactions. (By convention, refers to a nucleus of an element X with Z protons and (A-Z) neutrons.) He bombarded the nickel nucleus,, with alpha particles, and the copper nucleus, , to produce the same compound nucleus. Alpha particles were obtained from the 60-inch cyclotron, while protons were obtained from the linear accelerator in Berkeley. The compound nucleus is an excited system, and it emits particles to come down to lower energy states. In this experiment, the compound nucleus decayed by emitting one neutron, two neutrons or a neutron and a proton together.

Ghoshal’s arguments can be simplified as follows. The total number of nuclei decaying in a specific way is a product of two quantities: how many compound nuclei have been formed and the probability that a compound nucleus will decay in that specific way. While the first factor is different in the two reactions, the second should be the same if Bohr’s hypothesis is correct. Hence, the ratios of the different products in a particular reaction should be independent of the reaction which formed the particular compound nucleus, as the first factor will cancel out. He was able to show that the ratio of numbers of nuclei formed in the three different decays is the same in the case of the two reactions that he studied within experimental errors, thus providing experimental proof of Bohr’s hypothesis. The importance of the paper can be understood from the fact that fifty-five years after his work, a paper was published with a title, “Ghoshal-like test of equilibration in near-Fermi-energy heavy-ion collisions” by a group of scientists from different countries working in the Texas A&M University cyclotron.  

Segrè and Ghoshal with their families in Kolkata 

 Ghoshal completed his doctorate under Segrè and came back to India in 1950. He was a Reader at Lucknow University from 1951 to 1954. Then Saha invited him to join the post of Sur Reader of the Department of Physics of the University of Calcutta. This position was associated with the Institute of Nuclear Physics, which, by this time, had become an independent institute outside the University of Calcutta. He joined Presidency College in Kolkata as a professor in 1956 and was there till 1975. He served as the Head of the Department from 1966. For a brief period, he also acted as the principal of Presidency College. In 1975, he became the Director of Public Instruction for Secondary Education of West Bengal and in 1979, he joined as the Khaira Professor in the Department of Physics, University of Calcutta, where he also served as the Head of the Department. He retired in 1988. He was an excellent teacher, much admired by the students.

Ghoshal with Hartland Snyder in Berkeley 

Ghoshal continued research in nuclear physics in India. He and his collaborators built a mass spectrometer in the Presidency College laboratory. In an interesting work, they studied the resolution of the spectrometer as a function of pressure in the tube and pointed out the importance of elastic scattering and resonance charge exchange with the gas molecules [6]. A work on scattering of protons by gold nuclei carried out by Ghoshal in Berkeley and briefly included in his thesis was later analysed in detail by Ghoshal and B.B. Baliga. Ghoshal and his student Arjunnath Saxena also extended the previous work [1] on nuclear binding energy systematics.

Prof. Ghoshal also wrote a number of textbooks for undergraduate and postgraduate levels, which have proved to be very popular and are still in use. These include books on quantum mechanics, atomic physics and nuclear physics. He also wrote a book on atomic and nuclear physics in Bengali, which was published by the West Bengal State Book Board and is now available in two volumes.

Prof. Ghoshal was a great human being. He was interested in promoting science through organisations like the Indian Physical Society. He was instrumental in starting the Young Physicists’ Colloquium under the Indian Physical Society, a programme that has been going on for more than forty years now.  

Prof. Ghoshal being felicitated by Prof. Bikash Sinha, The DIrector of Saha Institute and Dr. S. Kailas, Director, Physics Group, BARC

 Prof. Ghoshal suffered a stroke and passed away on July 30, 2007.

There remains one unsolved puzzle. Despite being a researcher whose work was included in textbooks in his lifetime, the name of S N Ghoshal is almost unknown outside of the narrow academic circle, even in his own country. It cannot be that his work was too difficult to understand; we have many examples of scientists catching the popular imagination despite their works being very technical. Is it because the work was carried out abroad? Or is it because traditionally we tend to eulogise pure thinkers and look down upon experimentalists, people who work with their hands? I believe that Indian science needs a satisfactory solution to this puzzle.

The author is indebted to Mr. Swetketu Ghoshal and Ms. Sudakshina Ghoshal, respectively son and granddaughter of S.N. Ghoshal, and Prof. Harasit Majumder, formerly of Saha Institute of Nuclear Physics, for information related to the life of Prof. Ghoshal. Some information has been obtained from the minutes of the Senate and Syndicate of the University of Calcutta.  Ms. Sudakshina Ghoshal has kindly supplied the photographs used in the article.

References

[1] Nuclear energetics and beta–activity (part-II), A.K. Saha, S.N. Ghoshal and S. Das, Transactions of the National Institute of Science India, 3, 1 (1948).

[2] Excitation curves of (alpha,n); (alpha,2n); (alpha,3n) reactions on silver, S.N. Ghoshal, Physical Review 73, 417 (1948).

[3] An experimental verification of the theory of compound nucleus, S.N. Ghoshal, Physical Review 80, 939 (1950).

[4] Neutron capture and nuclear constitution, N. Bohr, Nature 137, 344 (1936).

[5] Statistics and nuclear reactions, V. F. Weisskopf, Physical Review 52, 295 (1937).

[6] Variation of the resolving power of a mass spectrometer with pressure, S.N. Ghoshal, M. Das and N.N. Mitra, International Journal of Mass Spectrometry and Ion Physics, 17, 67 (1975).  

 

                                                                     Gautam Gangopadhyay

 

Published: Monthly Bulletin of the Asiatic Society,  September 2023