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In 2019 a star, HD 86081, was given the name Bibha, after an Indian physicist most textbooks never mention. Bibha Chowdhuri's early work used one of the simplest particle detectors ever made, a photographic plate, to estimate the mass of particles arriving from space. This post covers what she did, checks her numbers against modern values, and asks what students can learn from her.
Who was Bibha Chowdhuri?
The most careful account is Roy and Singh's 2018 historical note in the Indian Journal of History of Science. Bibha Chowdhuri was born in 1913. She took her MSc in physics at Calcutta University in 1936 and began research under D. M. Bose. When Bose became director of the Bose Institute in 1937, his students, including her, moved with him. At a time when very few Indian women studied physics after a first degree, this was a bold choice.
In 1945 she joined P. M. S. Blackett's cosmic-ray laboratory at the University of Manchester and submitted her PhD thesis there in 1949, on extensive air showers. Blackett won the 1948 Nobel Prize in Physics for cloud-chamber discoveries in nuclear physics and cosmic radiation. Back in India, Roy and Singh report, Homi Bhabha offered her a post at the Tata Institute of Fundamental Research, where she worked from 1949 to about 1957, before years at the Physical Research Laboratory in Ahmedabad. She died in Kolkata on 2 June 1991.
How do you detect a particle with a photographic plate?
Cosmic rays are fast particles from space. When they hit the upper atmosphere they create showers of secondary particles. A charged particle passing through a photographic emulsion leaves a trail of developed silver grains, visible under a microscope.
Bose and Chowdhuri exposed Ilford half-tone plates at high altitude in the Darjeeling hills. In Photographic plates as detectors of mesotron showers (Nature, 1940) and later letters they described the tracks. Their August 1941 letter, A photographic method of estimating the mass of the mesotron, explains the method: they compared the grain spacing and curvature of the cosmic-ray tracks with tracks of protons of known energies in similar plates. "Mesotron" was the period's name for what we now call mesons and muons.
A worked check: how close were their mass estimates?
Roy and Singh report that the analysis split the tracks into two groups with similar grain spacing but different scattering. The average masses came out as 221 electron masses for one group (almost all recorded at Sandakphu, around 12,000 ft) and 278 electron masses for the other (all at Pharijong, around 15,000 ft). Let us compare with modern values.
Electron rest energy. NIST gives 0.51099895 MeV.
Muon. NIST gives the muon-to-electron mass ratio as 206.768. Their 221 is (221 − 206.8) / 206.8 ≈ 0.069, about 7% higher.
Charged pion. The Particle Data Group lists 139.570 MeV. In electron masses that is 139.570 / 0.511 ≈ 273.1. Their 278 is (278 − 273.1) / 273.1 ≈ 0.018, about 2% higher.
The heavier group appeared only at the higher station, and Roy and Singh note the tentative conclusion that the lighter particles lower down were decay products of the heavier ones. With hindsight, that is what pions do: they decay into muons. But in 1941 the plates were not sensitive enough to settle it, and wartime shortages meant better emulsions were not available.
Why did the credit go elsewhere?
Cecil Powell won the 1950 Nobel Prize in Physics "for his development of the photographic method of studying nuclear processes and his discoveries regarding mesons made with this method". The Nobel page notes that in 1947 his group found pi-mesons in emulsions. Roy and Singh point out that Powell used the same basic technique as Bose and Chowdhuri, with much more sensitive plates. Better instruments turned a suggestive result into a clear one.
Recognition came late. In the IAU's 2019 NameExoWorlds campaign, the Astronomical Society of India's page on the naming records that the star HD 86081 was named Bibha, honouring "the unsung Indian physicist Dr. Bibha Chowdhuri", and its planet Santamasa.
What can a student take from her way of working?
Measure with what you have. A plate, a microscope and patient counting of grains produced mass estimates within about 7% of modern values.
Compare with something known. The method only worked because the unknown tracks were compared with proton tracks of known energy. Calibration is the quiet heart of experiment.
Write down tentative conclusions, clearly labelled. Their decay suggestion matches what we now know pions do, and it was stated as tentative.
Know your instrument's limits. The result was held back by the plates, not by the reasoning.
For the school physics behind tracks and energies, see our guides to nuclear binding energy problems and radioactive decay and half-life. When a term like ionisation or rest energy needs a fuller explanation, the Fermi Concept Library has every concept in physics with its own lesson.
Redo the worked check above using 105.66 MeV for the muon instead of the NIST ratio, and see whether you get the same 7% gap.
