A detector that can wait on a mountain
In 1947, Cecil Powell’s Bristol group found that a particle stopping in photographic emulsion could give rise to another particle with a new track. The reports by César Lattes, Hugh Muirhead, Giuseppe Occhialini and Powell helped establish the pion and separate it from the muon. The detector did not need to catch the moment with a camera. It stored the passage inside a layer of photographic material, ready to be developed and examined later.
An emulsion contains tiny crystals of silver halide suspended in gelatin. A charged particle disturbs crystals along its path. Chemical development turns the affected crystals into a trail of dark silver grains that can be followed under a microscope. Ordinary photographic film had inspired the technique, but nuclear emulsion needed suitable sensitivity and enough thickness to preserve useful stretches of particle motion.
A plate could therefore be carried to a high mountain, exposed to cosmic radiation and brought back to a laboratory. Higher altitude offered more access to short-lived particles before they disappeared in the atmosphere. The work continued at a microscope: focus through the layer, find a grain, follow the next one, and keep the path from being confused with an unrelated track crossing at another depth. The apparent simplicity of the plate concealed demanding scanning and measurement.
Follow the track to its end
The key event had a connection one could point to. An incoming track grew to a stopping point, and a secondary track started there. Measuring both paths supplied the evidence needed to interpret the connection. Tracks of slow charged particles become more densely marked as their energy loss increases near the end of the range. Range is the distance a particle travels before stopping in the material. Grain density, range and scattering together gave clues to its speed and mass.
The later 1947 report described forty cases producing secondary mesons; in eleven, the secondary stopped inside the emulsion. Those stopping tracks were especially valuable because their full ranges could be measured. A daughter emitted when a parent decays at rest can have a well-defined energy. Similar daughter ranges across different examples were therefore a clue to a repeatable decay process, not just random fragments from an arbitrary nuclear collision.
The explanation separated two particles that had once been grouped under the broad word meson. The pion could participate strongly in nuclear interactions and could decay into a muon and a neutrino. The muon was the penetrating particle already familiar in cosmic rays. The neutrino was neutral and left no continuous charged track in the emulsion. Its absence from the photograph is compatible with a decay whose visible daughter has a characteristic energy.
A grain trail needs a scale
A long track does not automatically mean a heavier particle. Range depends on the initial energy, the particle species and the material. The same species can stop after different distances when it starts at different speeds. To infer mass, experimenters combined several measured features and compared them with the known behaviour of particles slowing in emulsion.
Scattering offered another clue. Many small encounters bend a path away from a smooth direction; the amount depends on the particle’s momentum and on the material crossed. Grain density supplied information about ionisation. A complete stopping range supplied another relation. Their combination was much more informative than treating a dark or a long track as a visual label for a particular particle.
Sources: [3]
The familiar three-part picture came later
Many introductions show a pion track, a short muon track and then a thin electron track. It is a beautiful complete decay sequence, but it should not be placed unchanged in the first 1947 discovery plate. Emulsions able to record the final electron became available in 1948. CERN’s archive explicitly distinguishes those later pictures from the emulsion used for the initial pion discovery.
That improvement matters because a detector decides which parts of a process become visible. A missing line can reflect a neutral particle, a particle leaving the sensitive volume, or insufficient sensitivity to a particular charged particle. The interpretation must use the properties of the material that actually made the record. Changing the emulsion changed what experimenters could follow to the end.
A photographic method earns the 1950 prize
Powell’s 1950 Nobel Prize recognised development of the photographic method and discoveries concerning mesons. The discovery arose from materials work, high-altitude exposure, patient microscopy and the interpretation of connected tracks. A few microscopic grains carried the decisive geometry: something arrived, stopped, and gave rise to a different particle whose behaviour could be measured.
Sources: [3]