Anuclear emulsion plate is a type ofparticle detector first used in nuclear andparticle physics experiments in the early decades of the 20th century.[1][2][3] It is a modified form ofphotographic plate that can be used to record and investigate fast charged particles likealpha-particles,nucleons,leptons ormesons. After exposing and developing the emulsion, single particle tracks can be observed and measured using a microscope.
The nuclear emulsion plate is a modified form ofphotographic plate, coated with a thickerphotographic emulsion ofgelatine containing a higher concentration of very finesilver 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 thesilver halide grains (submicron); precision and resolution that surpass even the best of modernparticle detectors (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.[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 ofcosmic rays that led to the discovery of thepi-meson[4][5] andparity violating chargedK-meson decays;[6] shedding light on the true nature and extent of the subnuclear "particle zoo", defining a milestone in the development of modern experimentalparticle 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 throughmultiple scattering andionising energy loss. Finally, thedevelopment 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.[7]
These disadvantages, coupled with the emergence of newparticle detector andparticle 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 asautoradiography in medicine and biology.
For a comprehensive and technically detailed account of the subject refer to the books by Powell, Fowler and Perkins[2] and by Barkas.[3] For an extensive review of the history and wider scientific context of the nuclear emulsion method, refer to the book by Galison.[8]
Following the 1896 discovery ofradioactivity byHenri Becquerel[9] usingphotographic emulsion,Ernest Rutherford, working first atMcGill University in Canada, then at theUniversity of Manchester in England, was one of the first physicists to use that method to study in detail the radiation emitted byradioactive materials.[10]In 1905 he was using commercially available photographic plates to continue his research into the properties of the recently discoveredalpha rays produced in theradioactive decay of someatomic nuclei.[10]This involved analysing the darkening of photographic plates caused by irradiation with thealpha rays. This darkening was enabled by the interaction of the many chargedalpha particles, making up the rays, withsilver halide grains in the photographic emulsion that were made visible byphotographic development. Rutherford encouraged his research colleague at Manchester, Kinoshita Suekiti,[11] to investigate in more detail the photographic action of thealpha-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,[12] showing that it was possible “by preparing an emulsion film of very finesilver halide grains, and by using a microscope of high magnification, that the photographic method can be applied for counting 𝛂-particles with considerable accuracy”.[13] 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[14] 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.[15][1]
The next steps would naturally have been to apply this technique to the detection and research of other particle types, including theCosmic Rays newly discovered byVictor Hess in 1912. However, progress was halted by the onset ofWorld 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[16] - was taken up again by various physical research laboratories in the 1920s.[1]
In particularMarietta Blau, working at theInstitute for Radium Research, Vienna inAustria, 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 irradiateparaffin 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.[17]
By an ingenious example of lateral thinking, she applied a similar method to make the first ever observation of the impact ofneutrons 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.[18] 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 (nowIlford 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.[19]
In 1937,Marietta Blau and her former studentHertha Wambacher discovered nucleardisintegration stars (Zertrümmerungsterne) due tospallation in nuclear emulsions that had been exposed tocosmic radiation at a height of 2300m on theHafelekarspitze aboveInnsbruck.[20] 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 toWorld War II, brought a sudden halt to progress in that field of research forMarietta Blau.[21][22]
In 1938, the German physicistWalter Heitler, who had escaped Germany as a scientific refugee to live and work in England, was atBristol University researching a number of theoretical topics, including the formation ofcosmic ray showers. He mentioned toCecil Powell, at that time considering the use ofcloud chambers for cosmic ray detection,[23][8] 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[24] and expose them on theJungfraujoch at 3,500 m. In a letter to 'Nature' in August 1939, they were able to confirm the observations of Blau and Wambacher.[25][26][27]
Although war brought a decisive halt to cosmic ray research in Europe between 1939 and 1945, in IndiaDebendra Mohan Bose andBibha Chowdhuri, working at theBose 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 ofmuons 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' nowmuon) discovered in 1936 by Anderson and Neddermeyer using aCloud 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.[28][29][30][31][32]
Following on from those developments, afterWorld War II, Powell and his research group atBristol University collaborated with Ilford (nowIlford 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 theUniversity of CambridgeCockcroft-Walton generator/accelerator, which provided artificial disintegration particles as probes to measure the required range-energy relations for charged particles in the new emulsion.[33]
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 andHugh Muirhead (University of Bristol), using plates exposed tocosmic rays at thePic du Midi Observatory in the Pyrenees and scanned by Irene Roberts andMarietta Kurz, discovered the chargedPi-meson.[4]
Second, two years later In 1949, analysing plates exposed at theSphinx Observatory on theJungfraujoch in Switzerland, first precise observations of the positiveK-meson and its ‘strange’ decays were made by Rosemary Brown (nowRosemary Fowler[34]), a research student inCecil Powell's group at Bristol.[6] Then known as the ‘Tau meson’ in theTau-theta puzzle, precise measurement of theseK-meson decay modes led to the introduction of the quantum concept ofStrangeness and to the discovery ofParity violation in theweak interaction. Rosemary Brown called the striking four-track 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 newparticle detector andparticle 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.[35][36][37][38][39]
More recently, searches for "Physics beyond the Standard Model", in particular the study ofneutrinos anddark matter in their exceedingly rare interactions with normal matter, have led to a revival of the technique, including automation of emulsion image processing.[7] Examples are theOPERA experiment,[40] studyingneutrino oscillations at theGran Sasso Laboratory in Italy, and theFASER experiment at the CERNLHC, which will search for new, light and weakly interacting particles includingdark photons.[41]
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 implantedradioactive markers byAutoradiography. Examples are:
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