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When a uranium-235 nucleus absorbs a slow neutron and splits, it does not break into the same two pieces every time. Some splits give a heavy and a light fragment, others two nearly equal ones. Explaining that spread was the scientific problem Raja Ramanna is best known for. He was also a builder of Indian physics: a research group, a training school and, later, national scientific bodies. This post covers his physics, works through the energy of fission with numbers, and draws out what students can learn from how he worked.
Who was Raja Ramanna?
According to the Physics Today obituary by V. S. Ramamurthy, Ramanna was born on 28 January 1925 in present-day Karnataka and went to school in Mysore and Bangalore. He took a BSc in physics with honours from Madras Christian College, then went to King's College London, where he completed a PhD in nuclear physics in 1948 on the design of a modified ionisation chamber for studying ionising particles.
He came back in 1949 to join the Tata Institute of Fundamental Research in Bombay, in the team led by Homi Bhabha. Our profile of Homi Bhabha and the institutions he built covers that wider story. Ramanna became a leader in organising the physics programme at the Atomic Energy Establishment, Trombay, which was later renamed the Bhabha Atomic Research Centre (BARC). The same obituary describes him as a gifted musician and a scholar of Sanskrit literature and philosophy as well as a physicist.
What did he discover about nuclear fission?
His early work, the obituary says, was on how neutrons slow down in different moderating materials, as part of designing India's first research reactor, Apsara. Once Apsara was running, it gave intense beams of slow (thermal) neutrons, and Ramanna started experiments on thermal-neutron-induced fission of uranium-235. The Trombay fission group that grew from this became internationally recognised, working with equipment it largely designed and built itself.
His main theoretical contribution was a stochastic theory of how fragment masses and charges are distributed in fission. He published an early version as Mass distribution in fission and the theory of random flights in Physics Letters in 1964. In the picture described by Ramamurthy, the two fragments-to-be swap nucleons at random before they finally separate (scission). Random exchange means the final mass split is spread out around typical values instead of being fixed. The theory explained most observed features of mass and charge distributions in low-energy fission and how they change with the excitation energy of the splitting nucleus. Ramamurthy and Ramanna later extended the idea in a 1989 paper in Pramana on nucleon exchange in fission and other binary nuclear reactions.
How much energy does one fission release?
Fission is in the Nuclei chapter of Class 12 physics, and a standard calculation shows why Ramanna's field mattered. The World Nuclear Association gives about 200 MeV per fission of a uranium-235 nucleus.
Worked example. How much energy is released if every nucleus in 1 g of uranium-235 undergoes fission?
Moles of U-235: 1 g ÷ 235 g/mol = 4.26 × 10⁻³ mol.
Number of nuclei: 4.26 × 10⁻³ × 6.022 × 10²³ = 2.56 × 10²¹.
Energy in MeV: 2.56 × 10²¹ × 200 MeV = 5.13 × 10²³ MeV.
Convert, using 1 MeV = 1.602 × 10⁻¹³ J: 5.13 × 10²³ × 1.602 × 10⁻¹³ = 8.2 × 10¹⁰ J.
In kilowatt-hours (1 kWh = 3.6 × 10⁶ J): about 2.3 × 10⁴ kWh.
A common mistake is to forget step 1 and multiply Avogadro's number directly by 200 MeV, which gives the energy per mole, not per gram. Another is to mix up MeV and eV in the conversion. Our guide to fission and fusion energy has more problems of this type.
What did he build?
Ramamurthy writes that Ramanna "singularly contributed" to creating the Atomic Energy Training School at Trombay in 1958, which trained staff in many disciplines for the country's nuclear establishment. That school mattered as much as any single experiment, because a national programme needs a steady supply of trained people.
India's first nuclear test took place in the Rajasthan desert on 18 May 1974. The obituary notes that the credit belongs to many people across several institutions, but that the design and development work was done at BARC under Ramanna's direct leadership. He went on to serve as scientific adviser to the defence minister (1978 to 1982) and as chairman of the Atomic Energy Commission (1983 to 1987), and he had a brief term as minister of state for defence in 1990. He died in Mumbai on 24 September 2004.
What can a student take from how he worked?
Start with the instrument. His PhD was about building a better detector, and his group at Trombay designed much of its own equipment. Knowing how a measurement is made helps you trust, or question, the result.
Look for the pattern behind messy data. Fission fragments vary from event to event. Instead of treating that as noise, he asked what simple process would produce the spread.
Use simple models first. Random exchange of nucleons is an idea you can state in a sentence, yet it explained a lot. When a problem feels complicated, try the simplest picture that could work.
Teach and train. The training school outlasted any single project. Explaining physics to others is one of the fastest ways to understand it yourself.
If the idea of fragment mass distributions is hard to picture, the Fermi Simulation Lab has interactive simulations for ideas that are easier to see than read. To read about his contemporary on the engineering side, see our profile of Homi Sethna and India's nuclear fuel cycle.
Redo the worked example for 1 kg of uranium-235, and check that your answer is a thousand times larger.
