How photography helped physicist Bibha Chowdhuri see the invisible in the 1930s

Chowdhuri’s experiment discovered visible evidence of nuclear disintegration caused by high-energy radiation. The finding mattered because it converted an invisible subatomic event into a permanent image. Scientists could then examine particle tracks instead of relying only on brief observations.

From June to October 1938, she placed samarium sulphate-treated photographic plates in Calcutta and at Sandakphu. The Calcutta plates were damaged by a refrigerator malfunction. Two Sandakphu plates, however, revealed 30 “star-like tracks.” Charged particles from disintegrating samarium nuclei ionised the photographic emulsion and created silver deposits along their paths.

This work helped establish photographic emulsions as useful particle detectors. Chowdhuri later worked with DM Bose to distinguish particles by their tracks and scattering. Their approach contributed to photographic methods used in later mesotron research, including Cecil Powell’s Nobel Prize-winning work. Mesotrons were later understood to be muons.

A photographic emulsion is the light-sensitive material coating a photographic plate. In Chowdhuri’s experiment, it contained silver bromide grains. The emulsion acted as a detector because charged particles passing through it changed the material along their paths. It preserved evidence of events too small and brief to see directly.

When a charged nuclear particle crossed the emulsion, its energy freed electrons inside silver bromide crystals. Silver ions absorbed those electrons and became metallic silver. Microscopic silver clusters formed along the particle’s route. After development, those clusters appeared as tracks, dots, or star-like patterns.

This method mattered because plates could remain exposed for several months. That long exposure increased the chance of recording rare nuclear disintegrations. Scientists could study the tracks’ shapes and scattering to distinguish particles, including alpha particles, protons, and mesotrons. Photographic emulsions therefore became lasting records of subatomic events.

The experiment produced 30 recorded tracks, described as “star-like tracks.” They appeared on two photographic plates kept at Sandakphu. These tracks were interpreted as evidence that samarium nuclei had undergone disintegration after exposure to high-energy radiation.

The plates were exposed from June to October 1938, giving an exposure period of several months. Three pairs were prepared. One pair was refrigerated in Calcutta, while two pairs were kept horizontally at Sandakphu. The Calcutta plates were damaged when the refrigerator malfunctioned, but two Sandakphu plates survived and showed the tracks.

The long exposure was central to the experiment’s design. Photographic emulsions could collect evidence over months, unlike an observation limited to a short interval. That made them especially useful for rare nuclear events. The result also supported Chowdhuri’s broader effort to classify particles by the trails they left behind.

Sandakphu was chosen because it offered a higher-altitude location for repeating a recent study. Chowdhuri’s experiment therefore compared plates kept in Calcutta with plates placed high in the mountains. This mattered because the investigation concerned cosmic radiation and the nuclear events it could produce.

Three pairs of plates were prepared with samarium sulphate. One pair was refrigerated in Calcutta. The other two pairs were placed horizontally at Sandakphu from June to October 1938. The Calcutta plates failed after the refrigerator malfunctioned. Two Sandakphu plates survived and revealed 30 star-like tracks.

The article does not give a separate measurement of how altitude changed the radiation exposure. It does establish that the Sandakphu work was a higher-altitude repetition of the study by HJ Taylor, D Fraser, and VD Dabholkar. That setting produced the usable photographic evidence supporting Chowdhuri’s conclusions about nuclear disintegration.

Particle tracks functioned like fingerprints. Their appearance and behaviour in the emulsion gave scientists clues about which particle had passed through. This was important because several kinds of charged particles could appear in the same photographic material.

Alpha particles from radioactive samarium had recognisable track signatures. Chowdhuri and DM Bose also studied how tracks scattered through the emulsion. By comparing the scattering of different tracks, they developed a way to distinguish particles such as protons and mesotrons. The scattered paths could also help determine particle momenta.

The article does not provide a complete visual catalogue of each particle’s exact shape. It does explain that track signatures and scattering were the key evidence. This approach became an early form of the scattering method for photographic emulsions. It helped researchers identify particles from their recorded paths rather than from direct visual observation.

Chowdhuri’s work contributed an important method for studying mesotrons through photographic emulsions. She and DM Bose learned to distinguish particles by the scattering of their tracks. That made the plates more than passive records: they became tools for identifying particles and studying their motion.

Roy and Singh cite Cecil Powell describing Chowdhuri and Bose’s approach as the first use of scattering to determine particle momenta from emulsion tracks. The method helped separate mesotrons from other charged particles, including protons. Their experiments therefore provided an early step toward more developed photographic studies of cosmic-ray particles.

The work formed part of a chain of experiments that culminated in Powell’s Nobel Prize-winning research on mesotrons. Mesotrons were later understood to be muons. The article does not claim Chowdhuri alone caused Powell’s prize; it places her experiments within the broader development of the photographic methods Powell later used.

The main advantage was time. A photographic plate could remain exposed for several months, allowing it to wait for an event that happened very rarely. A cloud chamber, by comparison, recorded particle paths during an observation period, so an uncommon nuclear disintegration might not occur while it was being watched.

Chowdhuri used plates whose emulsions had been soaked in samarium sulphate. From June to October 1938, they were exposed in Calcutta and at Sandakphu. When the plates were developed, two Sandakphu examples showed 30 star-like tracks. The emulsion had preserved the particles’ paths as metallic silver deposits.

This created a durable record that could be examined later under a microscope. It also allowed researchers to compare track shapes and scattering in detail. The method became especially valuable for cosmic-ray research, where rare events needed both long collection times and a permanent image for analysis.