How Photography Helped Physicist Bibha Chowdhuri See the Invisible in the 1930s

In 1939, a 26-year-old physicist published an article that contains what, at first, reads like instructions for an eccentric treasure hunt: place photographic plates in a refrigerator in Calcutta, in Darjeeling or in a Dak Bungalow at Sandakphu. But Bibha Chowdhuri was not looking for buried treasure. She was looking for traces of the invisible. The plates had been prepared with samarium sulphate and exposed for months to cosmic radiation. When two of those placed at Sandakphu were developed, they revealed 30 “star-like tracks” – microscopic records of nuclear disintegration. The photographs transformed an otherwise invisible encounter between cosmic rays and matter into something that could be seen and studied. Chowdhuri’s experiment belonged to a new kind of scientific image-making, pioneered in part by physicist Marietta Blau, in which photographic emulsions became detectors of subatomic events. Working with DM Bose at Calcutta’s Bose Institute, Chowdhuri would go on to develop methods for distinguishing particles by the tracks they left in photographic plates. Her work formed part of a chain of experiments that culminated in Cecil Powell’s Nobel Prize-winning research on mesotrons. Much of what is known about Chowdhuri’s life and work has been compiled by physicists Suprakash C Roy and Rajinder Singh in their biography A Jewel Unearthed: Bibha Chowdhuri (2018). She was the third of six children, born in 1913 into an upper-caste family with progressive views on women’s education. Her father was a doctor from a zamindar family, while her mother was a staunch follower of the Brahmo Samaj. In 1936, Chowdhuri became only the third recorded woman to receive a postgraduate degree in physics from Calcutta University. She subsequently joined the research group of an initially reluctant DM Bose – the first scientist to photograph H- and gamma particles in a cloud chamber – first at her alma mater and later at Bose Institute, of which he became director in 1938. That year, Chowdhuri conducted a series of experiments in Calcutta and Sandakphu over several months, using photographic plates whose emulsions had been soaked in samarium. A repetition at higher altitude of a recent study by HJ Taylor, D Fraser and VD Dabholkar, the experiment’s premise was that charged particles generated during the disintegration of samarium nuclei by bombardment with high-energy radiation would ionise the emulsion and leave tracks. Chowdhuri was interested in isolating tracks left by cosmic particles then becoming known as “heavy electrons” (or mesotrons, later understood to be muons) and distinguish them from those of other charged particles, some of which, like the alpha particles spontaneously emitted by radioactive samarium, had recognisable track signatures. Subatomic trails In 1938, these photography-based studies were at the very cutting edge of particle physics experiments internationally. The use of light-sensitive emulsion to observe cosmic-ray-induced nuclear disintegration was still relatively new and had gained legitimacy only in the couple of years preceding the Indian scientists’ experiments. As Chowdhuri notes in her first article, the advantage of the photographic emulsion method over the established one of cloud-chamber observation was that “the period of exposure can be extended over several months, and in this way evidence of very seldom occurring nuclear disintegration can be obtained.” This was the approach she would eventually deploy with Bose to differentiate between particles based on the scattering of their emulsion tracks, including protons and mesotrons. In their essay “D.M. Bose and Cosmic Ray Research”, Roy and Singh cite Powell describing it as the first approach to the scattering method of determining particle momenta from emulsion tracks, thereby contributing to the development of photographic methods later used by Powell in his mesotron research. From June to October 1938, three pairs of photographic plates were half-soaked in samarium sulphate – one pair was refrigerated in Calcutta and the other two were kept horizontally at Sandakphu. While the Calcutta plates were damaged due to a malfunctioning refrigerator, two of the Sandakphu plates revealed “30 star-like tracks,” interpreted as evidence of the nuclear disintegration. A microphotograph accompanying Chowdhuri’s 1939 article shows the traces of the samarium nucleus’s disintegration in an asterisk-patterned arrangement. The black dots arranged in a starburst pattern are deposits of metallic silver formed from the silver bromide grains that constitute the photographic emulsion. As the charged nuclear particles streak through the emulsion like subatomic comets, their high energy frees electrons within the silver bromide crystals in their path. These electrons are taken up by silver ions and converted to their metallic state, creating microscopic silver clusters that register the particle’s trajectory latently. Once the invisible track is brought to light through chemical development in a darkroom, the physical properties of the “comet’s tail” help identify the particle. Extending the extraterrestrial simile, Chowdhuri’s 1938 microphotograph resembles the negative image of a constellation – almost like a visual recursion, as though particles from outer space were assuming related shapes in earthly images. The discovery of this starburst shape was a crescendo in Blau’s career. Exemplifying the perpetual feedback loops of scientific research, she credited the “scintillation and ionization” of H-particles revealed by Bose in a cloud chamber for prompting speculation that “their interaction with photographic material would exist” in the first place. Years later, in 1937, reports physicist Brigitte Strohmaier in her essay “Marietta Blau and the Photographic Method of Particle Detection”, Blau and her colleague Hertha Wambacher recorded cosmic rays on photographic plates at an observatory near Innsbruck. They detected a new pattern that “provided experimental evidence” for cosmic-ray-induced nuclear disintegration – Zertrümmerungssterne, or disintegration stars. This would be the discovery that brought Blau’s method widespread acceptance in the now-exploding field of cosmic ray physics that Chowdhuri entered at this point. Scientific vision Advancing to original experiments with DM Bose, Chowdhuri co-authored a series of papers in the journal Nature between 1940 and 1942 that examined the physical properties of the Zertrümmerungssterne to determine their origin and properties. In “Photographic Plates as Detectors of Mesotron Showers” (1940), they proposed that many of the tracks were those of secondary mesotrons associated with cosmic ray disintegration. The following year, they published two papers: “Origin and Nature of Heavy Ionization Particles Detected on Photographic Plates Exposed to Cosmic Rays” (1941) and “A Photographic Method of Estimating the Mass of the Mesotron”. The first examined the curved tracks and using the average distance between grains as an indicator of velocity, compared their scattering with that of proton tracks of similar velocity to conclude that some of the cosmic ray particles had a smaller mass and were probably mesotrons. The second paper offered a method of estimating the mass of these mesotrons based on their velocity and the scattering of their tracks. In this way, Chowdhuri and Bose used photographic emulsions to differentiate between particles based on the scattering of their tracks, including protons and mesotrons, thereby contributing to the development of methods later used by Powell in his mesotron research. In 1945, Chowdhuri built on these significant contributions to the burgeoning experimental field of photography-based particle research and published a solo article titled “Study of Multiple Ionisation Track Spectra on Photographic Plates Exposed to Cosmic Rays at Different Altitudes”. She gave an account of the multiple ionisation tracks on “plates under different absorbing layers at different altitudes – Pharijong (14,500 ft), Sandaphaku (12,000 ft.) and Darjeeling (7,000 ft.).” The population and properties of particles in a cosmic-ray shower would vary at different points in the Earth’s atmosphere as they travelled downwards. Her article is accompanied by photographs of ionised tracks at Sandakphu and Darjeeling, featuring different ionic patterns, including a single curve, a pair track and two star-like ones. The abstract black-and-white visual composition of these images, later photographed for publication, brings to mind photograms, like those being created during those decades by artists like Man Ray, László Moholy-Nagy and Lucia Moholy. These involved placing the object of photography on a light-sensitive surface and exposing it to light. Just as in cosmic-ray imaging the high energy of the particles ionises the silver bromide emulsion along their trajectory, in the case of photograms, it is light particles that induce chemical change, causing the occluded area to remain shadowed and giving rise to a silhouetted image. The cameraless process that unites both types of images represents what art historian Geoffrey Batchen, in his book Emanations: The Art of the Cameraless Photograph, calls “direct contact” – between not just the world, as in his formulation, but, in the case of particle physicists like Chowdhuri and Bose, between the cosmos and light-sensitive matter. Cameraless photography was part of a wider scientific effort to make manifest physical phenomena beyond the capacity of human vision. For example, the Indian physicist JC Bose, who founded the Bose Institute, once described the electromagnetic waves he transmitted as part of a demonstration in Calcutta in 1895 as adrishya alok, or “invisible light”. The same year, physicist Wilhelm Röntgen’s cameraless radiographs registered another type of adrishya alok – X-rays, with wavelengths too short for human vision and able to penetrate soft tissue. As Batchen notes, “Science had now demonstrated that a completely invisible emanation could manifest its presence on a photographic plate.” In the context of 20th-century cosmic ray physics, Blau’s groundbreaking use of photographic plates to trace particles offered a way for cameraless photographic technology to illuminate aspects of the dark subatomic world. And through her experiments, Chowdhuri was among the scientists who played a key role in using this method to visualise the invisible. Lasting legacy But not all attempts at producing visibility were equally facilitated. Complex photographic ensembles require resources and specialised equipment that have to be carefully calibrated. In her book Photography and Science, art historian Kelley Wilder points out that while particle physicists used commercial film, even treated with pinakryptol yellow dye to improve its sensitivity to particle tracks, most emulsions were too thin and unstable to be optimal. Following Blau, Chowdhuri and Bose used the thicker Ilford halftone plates for their experiments through the early 1940s. However, with the intensification of the Second World War, this became challenging. In their Annual Report and Accounts of 1943-’44, the Bose Institute reported “difficulty…obtaining photographic plates from England suited for cosmic ray investigations”. Roy and Singh quote Bose from a 1951 article: “We realized that for further progress, improved types of emulsions were necessary, but we left it at that. On the other hand C F Powell of Bristol University started in 1939 collaborating with Ilford Ltd. of London for the manufacture of improved nuclear emulsions.” The contrast marks an asymmetry of access to the material and industrial resources on which science depended. With cosmic ray research discontinued at Bose Institute, in 1945, Chowdhuri left to pursue her PhD at the University of Manchester under another future Nobel laureate, PMS Blackett. She carried on her use of photographic methods for particle research, though this time camera-based ones, to observe air showers in cloud chambers. In 1949, she joined the Tata Institute of Fundamental Research in Bombay, where she was in charge of the Cloud Chamber Group. After brief stints in Calcutta, France and the United States, in the late 1950s, Chowdhuri started working at the Physical Research Laboratory in Ahmedabad, where she was a key scientist in the Kolar Gold Field Project, detecting and studying high-energy muons associated with extensive air showers hundreds of feet underground – this time with a neon flash-tube technique. Retiring from the Physical Research Laboratory in 1976, Chowdhuri returned to Calcutta and continued her research with track-detection technologies rather than the photographic methods that had defined the beginning of her career. Nevertheless, this early work with DM Bose, deploying photographic material in the service of scientific investigations, is now widely recognised as valuable. Her very name, Bibha, means “light” – appropriate for the trailblazing experiments she conducted, tracing the paths of cosmic particles and making their passage visible to the human eye. The writer would like to thank Soumen Ghosh and Jayanta Patra of the Birla Industrial & Technological Museum, Kolkata, for their help.