Nuclear emulsion
A nuclear emulsion plate is a type of particle detector first used in nuclear and particle physics
experiments in the early decades of the 20th century.[1][2][3] It is a type of photographic plate,
coated with a thick photographic emulsion of gelatine containing a high concentration of very
fine silver halide grains; the exact composition of the emulsion being optimised for particle
detection. Nuclear emulsion can be used to record and investigate fast charged particles like
alpha-particles, nucleons or mesons. After exposing and developing the emulsion, single particle
tracks can be observed and measured using a microscope.
It has the advantage of extremely high spatial precision, limited only by the size of the silver
halide grains (a few microns), a precision that surpasses even the best of modern particle
detectors (observe the scale in the image below, of K-meson decay). A stack of emulsion plates
can record and preserve the interactions of particles so that their trajectories are recorded in 3dimensional space as a trail of silver-halide grains, which can be viewed from any aspect on a
microscopic scale.[3] 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 that led to the discovery of the pi-meson[4][5]
and parity violation in K-meson decays;[6] shedding light on the true nature of the subnuclear
"particle zoo" and a milestone in the development of modern, experimental particle physics.[1]
The chief disadvantage of nuclear emulsion is that it is a dense and complex material (silver,
bromine, carbon, nitrogen, oxygen) 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, is a slow and labour intensive process.
These disadvantages, coupled with the emergence of new particle detector and particle
accelerator technologies, led to a decline in use of nuclear emulsion plates in particle physics
towards the end of the 20th century.[1] 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
Barkas[3] and by Powell, Fowler and Perkins.[2] For an extensive review of the history and wider
scientific context of the nuclear emulsion method, refer to the book by Galison.[7]
History
Following the 1896 discovery of radioactivity by Henri Becquerel[8] 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.[9] 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.[9] 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,[10] to investigate
in more detail the photographic action of the alpha-particles.
Physicist Kinoshita Suekiti at the University of Manchester in 1910.
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 𝛂particles 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,[11] 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”.[12] This was the
first time that the observation of individual charged particles by means of a photographic
emulsion had been achieved.[1] 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[13] 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.[14][1]
The next steps would naturally have been to apply this technique to the detection and research
of other particle types, including the Cosmic Rays newly discovered by Victor Hess in 1912.
However, progress was halted by the onset of World War I 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[15] - was taken up again by various physical research laboratories in the
1920s.[1]
In particular Marietta Blau, working at the Institute for Radium Research, Vienna in Austria,
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, 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.[16] By an ingenious example of lateral
thinking, she applied a similar method to make the first ever ‘observation’ of a neutron 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.[17] 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 fastproton tracks.[18] 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.[19] 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.[20][21]
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,[22][7] 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.
Cecil Powell
This intrigued Powell, who convinced Heitler to travel to Switzerland with a batch of llford halftone emulsions[23] 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.[24][25][26]
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.[27]
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.[4]
K meson decay in Nuclear Emulsion
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, a research student in Cecil Powell’s group at Bristol.[6]
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. Rosemary Brown called the striking fourtrack emulsion image,[1] 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 and particle accelerator 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.[1] However there remained a continuing
use of the method in the study of rare interactions and decay processes.[28][29][30][31][32] 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. Examples are the OPERA experiment,[33] 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.[34]
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
Biological research
Metallurgy
Reactive Surface chemistry
Radiation protection
Muon tomography (Muography)
Archaeology.[35]
References & Footnotes
1. Herz, A.J.; Lock, W.O. (May 1966). "Nuclear Emulsions". CERN Courier. 6: 83–87.
https://cds.cern.ch/record/1728791/files/vol6-issue5-p083-e.pdf
2. The Study of Elementary Particles by the Photographic Method, C.F.Powell, P.H.Fowler,
D.H.Perkins: Pergamon Press, New York, 1959.
3. 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
4. C. Lattes, G. Occhialini, H. Muirhead and C. Powell (1947). "Processes Involving Charged
Mesons". Nature. 159: 694–697.
5. 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
6. R.Brown et al. Observations with Electron-Sensitive Plates Exposed to Cosmic Radiation Part
2: Further evidence for the existence of unstable charged particles, of mass ∼1,000 me, and
observations on their mode of decay Nature 163, 82–87 (1949).
https://doi.org/10.1038/163082a0
7. 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.
8. Henri Becquerel (1896). "Sur les radiations émises par phosphorescence" (http://gallica.bnf.fr/
ark:/12148/bpt6k30780/f422.chemindefer) . Comptes Rendus. 122: 420–421.
9. E. Rutherford, Philosophical Magazine, July 1905, January 1906 and April 1906
10. His name is written here in accepted Japanese form: surname followed by given name, rather
than following Western convention.
11. Rutherford communicated Kinoshita's paper to the Royal Society in November 1909
12. Kinoshita, S. (1910). "The Photographic Action of the 𝛂-Particles emitted from Radioactive
Substances". Proc. R. Soc. 83A: 432–458.
13. Maximilian Reinganum (1876-1914) was Professor of Physics at the University of Freiburg im
Breisgau in 1911. He is referenced in "The Collected Papers of Albert Einstein, Volume 1: The
Early Years, 1879-1902", p305. Princeton University Press (1987) ISBN 0-691-08407-6. Edited
by John Stachel, David C. Cassidy, and Robert Schulmann. In a letter to Mileva Marić, Einstein
discusses a paper by Reinganum. The following note is added by the editors: Maximilian
Reinganum (1876-1914) was not Dutch, but the article in Annalen der Physik [*] on the
electron theory of metals is datelined “Leiden Mia 1900”. By use of the equipartition theorem,
Reinganum derived an expression for the ratio between thermal and electrical conductivity,
which was equivalent to that given by Paul Drude, but which could be evaluated more
precisely. Reinganum’s result was in good agreement with experiment. [*] Max Reinganum
(1900): "Theoretical determination of the ratio of heat and electricity conduction of metals
from Drude's electron theory", Annalen der Physik Volume 307 Issue 6 Pages 398-403.
https://doi.org/10.1002/andp.19003070613
14. Reinganum, M. ‘Streuung und photographische Wirkung der 𝛂-Strahlen’ Phys. Z., vol. 12, p
1076 (1911)
15. A doubly ionised Helium ion
16. Marietta Blau, The photographic effect of natural H-rays, (in German), Sitzungsberichte
Akademie der Wissenschaften in Wien, IIa 134: 427 (1925). English translation
(http://cwp.library.ucla.edu/articles/blau/blau-rosenz.html )
17. Marietta Blau and Hertha Wambacher, Photographic detection of protons liberated by
neutrons. II, Sitzungsberichte Akademie der Wissenschaften in Wien, 141: 617 (1932).
18. Ruth Lewin Sime, Marietta Blau in the history of cosmic rays, Physics Today, Volume 65, Issue
10, p.8, October 2012
19. Marietta Blau and Hertha Wambacher: Disintegration Processes by Cosmic Rays with the
Simultaneou Emission of Several Heavy Particles, Nature 140: 585 (1937).
20. Robert Rosner, Brigitte Strohmaier (ed.): Marietta Blau, Stars of Disintegration. A biography of
a pioneer of modern particle physics. Böhlau, Vienna 2003, ISBN 3-205-77088-9 (in German)
21. Sime, R.L. Marietta Blau: Pioneer of Photographic Nuclear Emulsion and Particle Physics.
Phys. Perspect. 15, 3-32 (2013). https://doi.org/10.1007/s00016-012-0097-6
22. C.T.R. Wilson, who won the Nobel Prize for Physics in 1927 for his invention of the cloud
chamber, had been Powell's Ph.D. supervisor at Cambridge.
23. These emulsions were clearly not standard Ilford photographic plates. In their published paper
Heitler et al. state "A set of Ilford half-tone plates (emulsion 70 microns thick and sensitive to
𝛂-particles and protons)", which is almost certainly the type produced to Blau's 1937 research
specifications.
24. W. HEITLER, C. F. POWELL & G. E. F. FERTEL, Heavy Cosmic Ray Particles at Jungfraujoch and
Sea-Level, Nature volume 144, pages 283–284 (1939)
25. Owen Lock ‘’Half a century ago - The pion pioneers’’ CERN Courier vol. 37 no. 5 June 1997 pp
2-6.
26. Curiously, although Galison notes that "Dispatched to expose plates [at the Jungfrau], one of
Powell’s colleagues returned on 20 December 1938" he does not name that colleague as
Heitler and makes no reference to the joint paper which was Powell's first using the Nuclear
emulsion method.
27. C.M.G. Lattes, R.H.Fowler, and R.Cuer, "Range-Energy Relation for Protons and a-Particles in
the New Ilford 'Nuclear Research' Emulsions", Nature 159 (1947), 301-2
28. Nuclear Emulsion Evidence for Parity Nonconservation in the Decay Chain π + → μ + → e + π
+ →μ + →e + , J.I. Friedman(Chicago U., EFI), V.L. Telegdi(Chicago U., EFI) (Jun, 1957)
Published in: Phys.Rev. 106 (1957) 1290-1293
29. A measurement of the magnetic moment of the Λ 0 hyperon, G. Charrière, M. Gailloud, Ph.
Rosselet(Lausanne U), R. Weill, W.M. Gibson(Bristol U) et al. (1965) Published in: Phys.Lett. 15
(1965) 66-69
30. Adamivich, M.I.; et al. (Photon Emulsion and Omega Photon Collaborations) (1981).
"Observation of Pairs of Charmed Particles Produced by High-energy Photons in Nuclear
Emulsions Coupled With a Magnetic Spectrometer". Physics Letters B. 99: 271–276.
doi:10.1016/0370-2693(81)91124-2 (https://doi.org/10.1016%2F0370-2693%2881%29911242) .
31. Nuclear Interactions of Superhigh-energy Cosmic Rays Observed by Mountain Emulsion
Chambers, Pamir and Mt. Fuji and Chacaltaya Collaborations•S.G. Baiburina(Lebedev Inst.) et
al. (Feb, 1981) Published in: Nucl.Phys.B 191 (1981) 1-25
32. Particle production in interactions of 200-GeV/nucleon oxygen and sulfur nuclei in nuclear
emulsion, KLM Collaboration•A. Dabrowska(Cracow, INP) et al. (1992) Published in:
Phys.Rev.D 47 (1993) 1751-1761
33. Agafonova, N.; et al. (OPERA Collaboration) (26 July 2010). "Observation of a first ντ candidate
event in the OPERA experiment in the CNGS beam". Physics Letters B. 691 (3): 138–145.
arXiv:1006.1623 (https://arxiv.org/abs/1006.1623) . Bibcode:2010PhLB..691..138A (https://u
i.adsabs.harvard.edu/abs/2010PhLB..691..138A) . doi:10.1016/j.physletb.2010.06.022 (http
s://doi.org/10.1016%2Fj.physletb.2010.06.022) .
34. Feng, Jonathan L.; Galon, Iftah; Kling, Felix; Trojanowski, Sebastian (2018-02-05). "FASER:
ForwArd Search ExpeRiment at the LHC". Physical Review D. 97 (3): 035001.
arXiv:1708.09389 (https://arxiv.org/abs/1708.09389) . doi:10.1103/PhysRevD.97.035001 (ht
tps://doi.org/10.1103%2FPhysRevD.97.035001) . ISSN 2470-0010 (https://www.worldcat.or
g/issn/2470-0010) . S2CID 119101090 (https://api.semanticscholar.org/CorpusID:119101
090) .
35. Morishima, K., Kuno, M., Nishio, A. et al. Discovery of a big void in Khufu’s Pyramid by
observation of cosmic-ray muons. Nature 552, 386–390 (2017).
https://doi.org/10.1038/nature24647
External links
See also Scanpyramids
Nuclear emulsions by Ilford (https://web.archive.org/web/20120204205614/http://www.ilford
photo.com/Webfiles/2006214150271441.pdf)
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