A nuclear emulsion plate is a type of
particle detector
In experimental and applied particle physics, nuclear physics, and nuclear engineering, a particle detector, also known as a radiation detector, is a device used to detect, track, and/or identify ionizing elementary particle, particles, such as t ...
first used in nuclear and
particle physics
Particle physics or high-energy physics is the study of Elementary particle, fundamental particles and fundamental interaction, forces that constitute matter and radiation. The field also studies combinations of elementary particles up to the s ...
experiments in the early decades of the 20th century.
[ https://cds.cern.ch/record/1728791/files/vol6-issue5-p083-e.pdf][''The Study of Elementary Particles by the Photographic Method'', C.F.Powell, P.H.Fowler, D.H.Perkins: Pergamon Press, New York, 1959.][Walter H. Barkas, ''Nuclear Research Emulsions I. Techniques and Theory'', in ''Pure and Applied Physics: A Series of Monographs and Textbooks, Vol. 15'', Academic Press, New York and London, 1963. http://becquerel.jinr.ru/text/books/Barkas_NUCL_RES_EMULSIONS.pdf] It is a modified form of
photographic plate that can be used to record and investigate fast charged particles like
alpha-particles,
nucleon
In physics and chemistry, a nucleon is either a proton or a neutron, considered in its role as a component of an atomic nucleus. The number of nucleons in a nucleus defines the atom's mass number.
Until the 1960s, nucleons were thought to be ele ...
s,
lepton
In particle physics, a lepton is an elementary particle of half-integer spin (Spin (physics), spin ) that does not undergo strong interactions. Two main classes of leptons exist: electric charge, charged leptons (also known as the electron-li ...
s or
meson
In particle physics, a meson () is a type of hadronic subatomic particle composed of an equal number of quarks and antiquarks, usually one of each, bound together by the strong interaction. Because mesons are composed of quark subparticles, the ...
s. After exposing and developing the emulsion, single particle tracks can be observed and measured using a microscope.
Description

The nuclear emulsion plate is a modified form of
photographic plate, coated with a thicker
photographic emulsion of
gelatine
Gelatin or gelatine () is a translucent, colorless, flavorless food ingredient, commonly derived from collagen taken from animal body parts. It is brittle when dry and rubbery when moist. It may also be referred to as hydrolysis, hydrolyzed col ...
containing a higher concentration of very fine
silver halide grains; the exact composition of the emulsion being optimised for particle detection.
It has the primary advantage of extremely high spatial precision and resolution, limited only by the size of the
silver halide grains (sub
micron); precision and resolution that surpass even the best of modern
particle detector
In experimental and applied particle physics, nuclear physics, and nuclear engineering, a particle detector, also known as a radiation detector, is a device used to detect, track, and/or identify ionizing elementary particle, particles, such as t ...
s (observe the scale in the image below, of K-meson decay).

A stack of emulsion plates, effectively forming a block of emulsion, can record and preserve the interactions of particles so that their trajectories are recorded in 3-dimensional space as a trail of silver-halide grains, which can be viewed from any aspect on a microscopic scale.
In addition, the emulsion plate is an integrating device that can be exposed or irradiated until the desired amount of data has been accumulated. It is compact, with no associated read-out cables or electronics, allowing the plates to be installed in very confined spaces and, compared to other detector technologies, is significantly less expensive to manufacture, operate and maintain. These features were decisive in enabling the high-altitude, mountain and balloon based studies of
cosmic rays
Cosmic rays or astroparticles are high-energy particles or clusters of particles (primarily represented by protons or atomic nuclei) that move through space at nearly the speed of light. They originate from the Sun, from outside of the Solar ...
that led to the discovery of the
pi-meson[G. P. S. Occhialini, C. F. Powell, ''Nuclear Disintegrations Produced by Slow Charged Particles of Small Mass'', Nature 159, 186–190 & 160, 453–456, 1947] and
parity violating charged
K-meson decays;
shedding light on the true nature and extent of the subnuclear "
particle zoo", defining a milestone in the development of modern experimental
particle physics
Particle physics or high-energy physics is the study of Elementary particle, fundamental particles and fundamental interaction, forces that constitute matter and radiation. The field also studies combinations of elementary particles up to the s ...
.
The chief disadvantage of nuclear emulsion is that it is a dense and complex material (
silver
Silver is a chemical element; it has Symbol (chemistry), symbol Ag () and atomic number 47. A soft, whitish-gray, lustrous transition metal, it exhibits the highest electrical conductivity, thermal conductivity, and reflectivity of any metal. ...
,
bromine
Bromine is a chemical element; it has chemical symbol, symbol Br and atomic number 35. It is a volatile red-brown liquid at room temperature that evaporates readily to form a similarly coloured vapour. Its properties are intermediate between th ...
,
carbon
Carbon () is a chemical element; it has chemical symbol, symbol C and atomic number 6. It is nonmetallic and tetravalence, tetravalent—meaning that its atoms are able to form up to four covalent bonds due to its valence shell exhibiting 4 ...
,
nitrogen
Nitrogen is a chemical element; it has Symbol (chemistry), symbol N and atomic number 7. Nitrogen is a Nonmetal (chemistry), nonmetal and the lightest member of pnictogen, group 15 of the periodic table, often called the Pnictogen, pnictogens. ...
,
oxygen
Oxygen is a chemical element; it has chemical symbol, symbol O and atomic number 8. It is a member of the chalcogen group (periodic table), group in the periodic table, a highly reactivity (chemistry), reactive nonmetal (chemistry), non ...
) which potentially impedes the flight of particles to other detector components through
multiple scattering and
ionising energy loss. Finally, the
development and scanning of large volumes of emulsion, to obtain useful, 3-dimensional digitised data, has in the past been a slow and labour intensive process. However, recent developments in automation of the process may overcome that drawback.
These disadvantages, coupled with the emergence of new
particle detector
In experimental and applied particle physics, nuclear physics, and nuclear engineering, a particle detector, also known as a radiation detector, is a device used to detect, track, and/or identify ionizing elementary particle, particles, such as t ...
and
particle accelerator
A particle accelerator is a machine that uses electromagnetic fields to propel electric charge, charged particles to very high speeds and energies to contain them in well-defined particle beam, beams. Small accelerators are used for fundamental ...
technologies, led to a decline in use of nuclear emulsion plates in particle physics towards the end of the 20th century.
However there remains a continuing use of the method in the study of rare processes and in other branches of science, such as
autoradiography in medicine and biology.
For a comprehensive and technically detailed account of the subject refer to the books by Powell, Fowler and Perkins
and by Barkas.
For an extensive review of the history and wider scientific context of the nuclear emulsion method, refer to the book by Galison.
[Galison, Peter (1997). ''Image and logic: a material culture of microphysics. Chapter 3, Nuclear Emulsions: The Anxiety of the Experimenter.'' Chicago, Illinois: University of Chicago Press. ISBN 9780226279176.]
History
Following the 1896 discovery of
radioactivity
Radioactive decay (also known as nuclear decay, radioactivity, radioactive disintegration, or nuclear disintegration) is the process by which an unstable atomic nucleus loses energy by radiation. A material containing unstable nuclei is conside ...
by
Henri Becquerel
Antoine Henri Becquerel ( ; ; 15 December 1852 – 25 August 1908) was a French nuclear physicist who shared the 1903 Nobel Prize in Physics with Marie and Pierre Curie for his discovery of radioactivity.
Biography
Family and education
Becq ...
using
photographic emulsion,
Ernest Rutherford
Ernest Rutherford, 1st Baron Rutherford of Nelson (30 August 1871 – 19 October 1937) was a New Zealand physicist who was a pioneering researcher in both Atomic physics, atomic and nuclear physics. He has been described as "the father of nu ...
, working first at
McGill University
McGill University (French: Université McGill) is an English-language public research university in Montreal, Quebec, Canada. Founded in 1821 by royal charter,Frost, Stanley Brice. ''McGill University, Vol. I. For the Advancement of Learning, ...
in Canada, then at the
University of Manchester
The University of Manchester is a public university, public research university in Manchester, England. The main campus is south of Manchester city centre, Manchester City Centre on Wilmslow Road, Oxford Road. The University of Manchester is c ...
in England, was one of the first physicists to use that method to study in detail the radiation emitted by
radioactive
Radioactive decay (also known as nuclear decay, radioactivity, radioactive disintegration, or nuclear disintegration) is the process by which an unstable atomic nucleus loses energy by radiation. A material containing unstable nuclei is conside ...
materials.
[E. Rutherford, Philosophical Magazine, July 1905, January 1906 and April 1906]
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
Radioactive decay (also known as nuclear decay, radioactivity, radioactive disintegration, or nuclear disintegration) is the process by which an unstable atomic nucleus loses energy by radiation. A material containing unstable nuclei is conside ...
of some
atomic nuclei
The atomic nucleus is the small, dense region consisting of protons and neutrons at the center of an atom, discovered in 1911 by Ernest Rutherford at the University of Manchester based on the 1909 Geiger–Marsden gold foil experiment. Aft ...
.
This involved analysing the darkening of photographic plates caused by irradiation with the
alpha rays. This darkening was enabled by the interaction of the many charged
alpha particles, making up the rays, with
silver halide grains in the photographic emulsion that were made visible by
photographic development. Rutherford encouraged his research colleague at Manchester, Kinoshita Suekiti, 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.
However, that was the detection of individual particle impacts, not the observation of a particle's extended trajectory. Soon after that, in 1911, Max Reinganum 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.
The next steps would naturally have been to apply this technique to the detection and research of other particle types, including the
Cosmic Rays
Cosmic rays or astroparticles are high-energy particles or clusters of particles (primarily represented by protons or atomic nuclei) that move through space at nearly the speed of light. They originate from the Sun, from outside of the Solar ...
newly discovered by
Victor Hess in 1912. However, progress was halted by the onset of
World War I
World War I or the First World War (28 July 1914 – 11 November 1918), also known as the Great War, was a World war, global conflict between two coalitions: the Allies of World War I, Allies (or Entente) and the Central Powers. Fighting to ...
in 1914. The outstanding issue of improving the particle detection performance of standard photographic emulsions, in order to detect other types of particle - protons, for example, produce about one quarter of the ionisation caused by an 𝛂-particle - was taken up again by various physical research laboratories in the 1920s.
In particular
Marietta Blau, working at the
Institute for Radium Research, Vienna in
Austria
Austria, formally the Republic of Austria, is a landlocked country in Central Europe, lying in the Eastern Alps. It is a federation of nine Federal states of Austria, states, of which the capital Vienna is the List of largest cities in Aust ...
, began in 1923 to investigate alternative types of photographic emulsion plates for detection of protons, known as “H-rays” at that time.

She used a radioactive source of 𝛂-particles to irradiate
paraffin wax
Paraffin wax (or petroleum wax) is a soft colorless solid derived from petroleum, coal, or oil shale that consists of a mixture of hydrocarbon molecules containing between 20 and 40 carbon atoms. It is solid at room temperature and melting poi ...
, which has a high content of hydrogen. An 𝛂-particle may collide with a hydrogen nucleus (proton), knocking that proton out of the wax and into the photographic emulsion, where it produces a visible track of silver halide grains. After many trials, using different plates and careful shielding of the emulsion from unwanted radiation, she succeeded in making the first ever observation of proton tracks in a nuclear emulsion.
By an ingenious example of lateral thinking, she applied a similar method to make the first ever observation of the impact of
neutron
The neutron is a subatomic particle, symbol or , that has no electric charge, and a mass slightly greater than that of a proton. The Discovery of the neutron, neutron was discovered by James Chadwick in 1932, leading to the discovery of nucle ...
s 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
Innsbruck (; ) is the capital of Tyrol (federal state), Tyrol and the List of cities and towns in Austria, fifth-largest city in Austria. On the Inn (river), River Inn, at its junction with the Wipptal, Wipp Valley, which provides access to the ...
. 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
World War II or the Second World War (1 September 1939 – 2 September 1945) was a World war, global conflict between two coalitions: the Allies of World War II, Allies and the Axis powers. World War II by country, Nearly all of the wo ...
, 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 chamber
A cloud chamber, also known as a Wilson chamber, is a particle detector used for visualizing the passage of ionizing radiation.
A cloud chamber consists of a sealed environment containing a supersaturated vapor of water or alcohol. An energetic ...
s for cosmic ray detection,
that in 1937 the two Viennese physicists, Blau and Wambacher, had exposed photographic emulsions in the Austrian Alps and had seen the tracks of low energy protons as well as 'stars' or nuclear disintegrations caused by cosmic rays.
This intrigued Powell, who convinced Heitler to travel to Switzerland with a batch of llford half-tone emulsions 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
Kolkata, also known as Calcutta ( its official name until 2001), is the capital and largest city of the Indian state of West Bengal. It lies on the eastern bank of the Hooghly River, west of the border with Bangladesh. It is the primary ...
, 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
muon
A muon ( ; from the Greek letter mu (μ) used to represent it) is an elementary particle similar to the electron, with an electric charge of −1 '' e'' and a spin of ''ħ'', but with a much greater mass. It is classified as a ...
s 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
A muon ( ; from the Greek letter mu (μ) used to represent it) is an elementary particle similar to the electron, with an electric charge of −1 '' e'' and a spin of ''ħ'', but with a much greater mass. It is classified as a ...
) discovered in 1936 by Anderson and Neddermeyer using a
Cloud Chamber
A cloud chamber, also known as a Wilson chamber, is a particle detector used for visualizing the passage of ionizing radiation.
A cloud chamber consists of a sealed environment containing a supersaturated vapor of water or alcohol. An energetic ...
. 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
World War II or the Second World War (1 September 1939 – 2 September 1945) was a World war, global conflict between two coalitions: the Allies of World War II, Allies and the Axis powers. World War II by country, Nearly all of the wo ...
, 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
The University of Cambridge is a Public university, public collegiate university, collegiate research university in Cambridge, England. Founded in 1209, the University of Cambridge is the List of oldest universities in continuous operation, wo ...
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
The University of Bristol is a public university, public research university in Bristol, England. It received its royal charter in 1909, although it can trace its roots to a Merchant Venturers' school founded in 1595 and University College, Br ...
), using plates exposed to
cosmic rays
Cosmic rays or astroparticles are high-energy particles or clusters of particles (primarily represented by protons or atomic nuclei) that move through space at nearly the speed of light. They originate from the Sun, from outside of the Solar ...
at the
Pic du Midi Observatory in the Pyrenees and scanned by Irene Roberts and
Marietta Kurz, discovered the charged
Pi-meson.

Second, two years later In 1949, analysing plates exposed at the
Sphinx Observatory on the
Jungfraujoch in Switzerland, first precise observations of the positive
K-meson and its ‘strange’ decays were made by Rosemary Brown (now
Rosemary Fowler), a research student in
Cecil Powell's group at Bristol.
Then known as the ‘Tau meson’ in the
Tau-theta puzzle, precise measurement of these
K-meson decay modes led to the introduction of the quantum concept of
Strangeness and to the discovery of
Parity violation in the
weak interaction
In nuclear physics and particle physics, the weak interaction, weak force or the weak nuclear force, is one of the four known fundamental interactions, with the others being electromagnetism, the strong interaction, and gravitation. It is th ...
. Rosemary Brown called the striking four-track emulsion image,
of one 'Tau' decaying to three charged pions, her "K track", thus effectively naming the newly discovered ‘strange’
K-meson.
Cecil Powell was awarded 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 emergence of new
particle detector
In experimental and applied particle physics, nuclear physics, and nuclear engineering, a particle detector, also known as a radiation detector, is a device used to detect, track, and/or identify ionizing elementary particle, particles, such as t ...
and
particle accelerator
A particle accelerator is a machine that uses electromagnetic fields to propel electric charge, charged particles to very high speeds and energies to contain them in well-defined particle beam, beams. Small accelerators are used for fundamental ...
technologies, coupled with the disadvantages noted in the introduction, led to a decline in use of Nuclear Emulsion plates in Particle Physics towards the end of the 20th century.
However there remained a continuing use of the method in the study of rare interactions and decay processes.
More recently, searches for "
Physics beyond the Standard Model", in particular the study of
neutrinos and
dark matter
In astronomy, dark matter is an invisible and hypothetical form of matter that does not interact with light or other electromagnetic radiation. Dark matter is implied by gravity, gravitational effects that cannot be explained by general relat ...
in their exceedingly rare interactions with normal matter, have led to a revival of the technique, including automation of emulsion image processing.
Examples are the
OPERA experiment,
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
There exist a number of scientific and technical fields where the ability of nuclear emulsion to accurately record the position, direction and energy of electrically charged particles, or to integrate their effect, has found application. These applications in most cases involve the tracing of implanted
radioactive markers by
Autoradiography. Examples are:
*
Medical research
Medical research (or biomedical research), also known as health research, refers to the process of using scientific methods with the aim to produce knowledge about human diseases, the prevention and treatment of illness, and the promotion of ...
*
Biological research
*
Metallurgy
Metallurgy is a domain of materials science and engineering that studies the physical and chemical behavior of metallic elements, their inter-metallic compounds, and their mixtures, which are known as alloys.
Metallurgy encompasses both the ...
*Reactive
surface chemistry
Surface science is the study of physics, physical and chemistry, chemical phenomena that occur at the interface (chemistry), interface of two phase (matter), phases, including solid–liquid interfaces, solid–gas interfaces, solid–vacuum int ...
*
Radiation protection
Radiation protection, also known as radiological protection, is defined by the International Atomic Energy Agency (IAEA) as "The protection of people from harmful effects of exposure to ionizing radiation, and the means for achieving this". Exposu ...
*
Muon tomography (
Muography)
*
Archaeology
Archaeology or archeology is the study of human activity through the recovery and analysis of material culture. The archaeological record consists of Artifact (archaeology), artifacts, architecture, biofact (archaeology), biofacts or ecofacts, ...
.
[{{Cite journal , last1=Morishima , first1=Kunihiro , last2=Kuno , first2=Mitsuaki , last3=Nishio , first3=Akira , last4=Kitagawa , first4=Nobuko , last5=Manabe , first5=Yuta , last6=Moto , first6=Masaki , last7=Takasaki , first7=Fumihiko , last8=Fujii , first8=Hirofumi , last9=Satoh , first9=Kotaro , last10=Kodama , first10=Hideyo , last11=Hayashi , first11=Kohei , last12=Odaka , first12=Shigeru , last13=Procureur , first13=Sébastien , last14=Attié , first14=David , last15=Bouteille , first15=Simon , date= 2017, title=Discovery of a big void in Khufu's Pyramid by observation of cosmic-ray muons , url=https://www.nature.com/articles/nature24647 , journal=Nature , language=en , volume=552 , issue=7685 , pages=386–390 , doi=10.1038/nature24647 , bibcode=2017Natur.552..386M , issn=1476-4687, arxiv=1711.01576 ]
References & Footnotes
External links
Nuclear emulsions by Ilford
Particle detectors
Nuclear physics