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.
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| Segrè and Ghoshal with their families in Kolkata |
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| 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.
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| Prof. Ghoshal being felicitated by Prof. Bikash Sinha, The DIrector of Saha Institute and Dr. S. Kailas, Director, Physics Group, BARC |
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
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