Following the 1896 discovery of
radioactivity by
Henri Becquerel using
photographic emulsion,
Ernest Rutherford, working first at
McGill University in Canada, then at the
University of Manchester in England, was one of the first physicists to use that method to study in detail the radiation emitted by
radioactive materials. In 1905 he was using commercially available photographic plates to continue his research into the properties of the recently discovered
alpha rays produced in the
radioactive decay of some
atomic nuclei. to investigate in more detail the photographic action of the
alpha-particles. Kinoshita included in his objectives "to see whether a single 𝛂-particle produced a detectable photographic event". His method was to expose the emulsion to radiation from a well measured radioactive source, for which the emission rate of 𝛂-particles was known. He used that knowledge and the relative proximity of the plate to the source, to compute the number of 𝛂-particles expected to traverse the plate. He compared that number with the number of developed halide grains he counted in the emulsion, taking careful account of '
background radiation' that produced additional 'non-alpha' grains in the exposure. He completed this research project in 1909, showing that it was possible "by preparing an emulsion film of very fine
silver halide grains, and by using a microscope of high magnification, that the photographic method can be applied for counting 𝛂-particles with considerable accuracy". This was the first time that the observation of individual charged particles by means of a photographic emulsion had been achieved. showed that the passage of an 𝛂-particle at glancing incidence through a photographic emulsion produced, when the emulsion was developed, a row of silver halide grains outlining the trajectory of the 𝛂-particle; the first recorded observation of an extended particle track in an emulsion. - was taken up again by various physical research laboratories in the 1920s. By an ingenious example of lateral thinking, she applied a similar method to make the first ever observation of the impact of
neutrons in nuclear emulsion. Being electrically neutral the neutron cannot, of course, be directly detected in a photographic emulsion, but if it strikes a proton in the emulsion, that recoiling proton can be detected. She used this method to determine the energy spectrum of neutrons resulting from specific nuclear reaction processes. She developed a method to determine proton energies by measuring the exposed grain density along their tracks (fast minimum ionising particles interact with fewer grains than slow particles). To record the long tracks of fast protons more accurately, she enlisted British film manufacturer Ilford (now
Ilford Photo) to thicken the emulsion on its commercial plates, and she experimented with other emulsion parameters — grain size, latent image retention, development conditions — to improve the visibility of alpha-particle and fast-proton tracks. In 1937,
Marietta Blau and her former student
Hertha Wambacher discovered nuclear
disintegration stars (Zertrümmerungsterne) due to
spallation in nuclear emulsions that had been exposed to
cosmic radiation at a height of 2300m on the
Hafelekarspitze above
Innsbruck. This discovery caused a sensation in the world of nuclear and cosmic ray physics, which brought the nuclear emulsion method to the attention of a wider audience. But the onset of political unrest in Austria and Germany, leading to
World War II, brought a sudden halt to progress in that field of research for
Marietta Blau. In 1938, the German physicist
Walter Heitler, who had escaped Germany as a scientific refugee to live and work in England, was at
Bristol University researching a number of theoretical topics, including the formation of
cosmic ray showers. He mentioned to
Cecil Powell, at that time considering the use of
cloud chambers for cosmic ray detection, and expose them on the
Jungfraujoch at 3,500 m. In a letter to 'Nature' in August 1939, they were able to confirm the observations of Blau and Wambacher. Although war brought a decisive halt to cosmic ray research in Europe between 1939 and 1945, in India
Debendra Mohan Bose and
Bibha Chowdhuri, working at the
Bose Institute,
Kolkata, undertook a series of high altitude mountain-top experiments using photographic emulsion to detect and analyse cosmic rays. These measurements were notable for the first ever detection of
muons by the photographic method: Chowdhuri's painstaking analysis of the observed tracks' properties, including exposed halide grain densities with range and multiple-scattering correlations, revealing the detected particles to have a mass about 200 times that of the electron - the same 'mesotron' (later 'mu-meson' now
muon) discovered in 1936 by Anderson and Neddermeyer using a
Cloud Chamber. Distance and circumstances denied Bose and Chowdhuri the relatively easy access to manufacturers of photographic plates available to Blau and later, to Heitler, Powell et al.. It meant that Bose and Chowdhuri had to use standard commercial half-tone emulsions, rather than nuclear emulsions specifically designed for particle detection, which makes even more remarkable the quality of their work. Following on from those developments, after
World War II, Powell and his research group at
Bristol University collaborated with Ilford (now
Ilford Photo), to further optimise emulsions for the detection of cosmic ray particles. Ilford produced a concentrated 'nuclear-research' emulsion containing eight times the normal amount of silver bromide per unit volume (see External Link to 'Nuclear emulsions by Ilford'). Powell's group first calibrated the new 'nuclear-research' emulsions using the
University of Cambridge Cockcroft–Walton generator/accelerator, which provided artificial disintegration particles as probes to measure the required range-energy relations for charged particles in the new emulsion. They subsequently used these emulsions to make two of the most significant discoveries in physics of the 20th century. First, in 1947,
Cecil Powell,
César Lattes,
Giuseppe Occhialini and
Hugh Muirhead (
University of Bristol), using plates exposed to
cosmic rays at the
Pic du Midi Observatory in the Pyrenees and scanned by Irene Roberts and
Marietta Kurz, discovered the charged
Pi-meson.), a research student in
Cecil Powell's group at Bristol. More recently, searches for "
Physics beyond the Standard Model", in particular the study of
neutrinos and
dark matter in their exceedingly rare interactions with normal matter, have led to a revival of the technique, including automation of emulsion image processing. studying
neutrino oscillations at the
Gran Sasso Laboratory in Italy, and the
FASER experiment at the CERN
LHC, which will search for new, light and weakly interacting particles including
dark photons. == Other applications ==