Liquid hydrogen was employed as the visualizing medium in the bubble chamber, making it sensitive enough to detect high-energy subatomic particles while providing nuclear collision targets. Up to this time, the detection of atomic and subatomic particles was a critical problem in experimental nuclear physics.
In experimental nuclear physics, detecting atomic and subatomic particles has been problematic. One of the earliest techniques to examine the properties of these particles was to use an apparatus known as the cloud chamber
While studying the properties of cosmic rays and their constituent muon particles in the late 1940’s, Carl David Anderson set one of his graduate students, Donald A. Glaser, to the task of developing a better method of applying the cloud chamber to cosmic-ray physics. Although Glaser did improve on the cloud chamber by employing two chambers in tandem separated by a powerful electromagnet in order to study high-energy muons, his work eventually led him to invent the bubble chamber.
By 1950, Glaser had completed his graduate work and was free to work on a project of his own—the bubble chamber—which not only was a marked departure from the cloud chamber but also an instrument that, when first used, marked a turning point in physics. With the growing use of particle accelerators during and after World War II, physics was in a transition from a discipline in which a relatively small number of scientists could run experiments with relatively small and inexpensive apparatus. Cosmic-ray physics was one such enterprise. Particle accelerators ushered in the age of high-energy physics, however, so called because of the tremendous energies at which these machines could accelerate atomic and subatomic particles. The bubble chamber became the most powerful detection device for high-energy physics in the 1960’s. Resulting largely from the expense and engineering complexity of both accelerators and bubble chambers, high-energy physics demanded large investments of both money and labor. Therefore, although the bubble chamber was conceived as a solution to the problems of small-scale cosmic-ray physics experiments, it became ultimately the tool of large-scale, high-energy physics experiments.
Given that the cloud chamber exploited the instability of a supersaturated vapor, Glaser reasoned that other instabilities in nature could be exploited also in order to visualize atomic and subatomic particles. Glaser and his research assistant David Rahm
Glaser’s first experimental confirmation of his idea came when he induced boiling in a glass bulb filled with superheated ether. Ether normally boils at 135 degrees Celsius, but Glaser kept the bulb under pressure and was able to maintain the liquid in a superheated state of about 150 degrees Celsius. When he achieved the superheated state, he brought a piece of radioactive cobalt, cobalt-60, near the bulb. Cobalt-60 is a source of gamma radiation, and when brought near the bulb, the source induced violent boiling.
Physically speaking, it is easy to show why this primitive bubble chamber worked. A pure liquid can be brought to a temperature above its boiling point in a very clean container for short periods. The length of time depends on the pressure under which the liquid is kept. If an impurity, such as a piece of broken glass, is introduced into the liquid, boiling begins spontaneously around the impurity. In other words, the impurity disrupts the balance between the liquid and gas phases of the liquid, causing the spontaneous release of the gas, also known as boiling.
With this new experimental knowledge, Glaser embarked on a project to develop a “clean” bubble chamber applicable to particle physics. He believed that the chamber would have to be as clean as possible so that the only boiling observed in the liquid was the result of test particles in the chamber, not stray impurities and imperfections of no experimental interest. From 1950 to 1953, Glaser and Rahm worked on the technical details of the bubble chamber until they had completed a 1.9-centimeter- (0.75-inch-) diameter ether chamber that controlled liquid pressure with a hand-turned piston. After bubble tracks were examined, the piston could be lowered, pressurizing the chamber and thus returning the liquid to its superheated state.
A schematic of a liquid bubble chamber.
In May of 1953, Glaser presented preliminary results of his bubble chamber experiments at the Washington, D.C., meeting of the American Physical Society. Glaser was more concerned with the use of the apparatus in cosmic-ray physics, but there were those in the audience who saw the device as the perfect tool for accelerator-driven, high-energy physics. One of those in attendance was Luis W. Alvarez, a physicist at Ernest Orlando Lawrence’s Radiation Laboratory at the University of California, Berkeley. Alvarez was intrigued by the potential utility of the Glaser invention and particularly by Glaser’s suggestion that ether be replaced by liquid hydrogen as an imaging medium.
It was important to observe the tracks made by particles entering the chamber; however, another important aspect of bubble chamber physics was the ability to observe the interactions of the entering particles with the particles making up the imaging medium. Particle physicists were studying the constituents of matter, and the particles they investigated constituted the bubble chamber liquid as well. Therefore, there was much to be learned from the collisions between the energetic entering particles and the particles making up the liquid. Any electrically charged particle, such as a muon, proton, or electron, made a track in the bubble chamber. Neutral particles such as neutrons and neutrinos left no tracks, but their collisions with the particles of the liquid often created charged particles, leaving evidence of their presence.
For experiments in which a large variety of collisions was essential, it was important to use a visualizing medium that was also a good target for collisions. Liquid hydrogen was the answer—it was the simplest of all elements, with a nucleus consisting of only one proton. In 1953, Glaser began a collaboration with the Darragh Nagle
The Alvarez group was the first to observe particle tracks in a chamber filled with liquid hydrogen. This accomplishment marked the end of a collaboration of physicists and engineers who made some important modifications of Glaser’s original work. Liquid hydrogen poses some unique difficulties because it boils at such a low temperature (-246 degrees Celsius) and because it burns violently in air. When the hydrogen is forced into an unstable superheated state at high pressure, the sudden release of that pressure could result in accidental fires. Therefore, Alvarez had to add experts in cryogenics, the science of extremely low temperatures, to his team.
The most important modification of the Glaser apparatus was the development of the “dirty” chamber. A large glass chamber to hold liquid hydrogen under extremes of both temperature and pressure was an engineering impossibility. The only way to keep the inner walls of a chamber perfectly clean and smooth, however, was to construct them of glass. The Alvarez team discovered accidentally that boiling near the imperfections in the chamber’s walls had no adverse effect on the all-important bubble tracks in the center of the chamber. Therefore, they abandoned the quest for the perfectly clean glass chamber and set out to develop a metal chamber with glass viewing ports. This allowed the construction of larger chambers, giving physicists more freedom in the types of collisions they could observe.
From 1954 forward, the Berkeley team dominated bubble chamber research, and by 1955, Alvarez was reporting the results of pion research with his 10-centimeter(4-inch) chamber. In that same year, he planned the design for a 25-centimeter (10-inch) liquid hydrogen chamber. This proposal eventually led to the development of the 183-centimeter (72-inch) chamber at Berkeley to be used in tandem with its large particle accelerator, the Bevatron. Operation of this apparatus began in 1959.
The most important impact of the liquid hydrogen bubble chamber came in the field of high-energy particle physics. The bubble chamber allowed physicists to examine a greater number of collisions between high-energy particles than ever before. Cameras attached to the chambers permitted scientists to examine hundreds of photographs of each experimental run. The tracks shown in these pictures offered evidence of the energy and charge of the particles under investigation. The liquid hydrogen bubble chamber allowed physicists to discover more than three times as many elementary particles than were known prior to 1950.
Alvarez and his team at Berkeley led the discipline in the teaming of bubble chambers and particle accelerators. One problem in the study of high-energy particles is that most of the very interesting particles do not last long after a collision. Alvarez used high-speed cameras triggered by computerized data analysis to photograph the tracks of particles. The complex data-gathering apparatus for this task was dubbed “Franckenstein,” after James Franck, one of the scientists who took on the task of developing the computerized data analyzer.
By 1960, it was becoming clear that the role of the human observer had to be minimized in order to gather and analyze all of the data potentially gathered by a bubble chamber. Interesting events were too numerous and happened far too quickly. By the mid-1960’s, every research center with a particle accelerator either had a bubble chamber or was in the process of building one.
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Aleksandrov, Yuri A., et al. Bubble Chambers. Translated by Scripta Technica. Bloomington: Indiana University Press, 1967. A technical introduction to the physics and engineering of bubble chambers that offers a concise introductory chapter on the history and significance of the bubble chamber. -
Close, Frank, Michael Marten, and Christine Sutton. The Particle Odyssey: A Journey to the Heart of the Matter. 1987. New ed. New York: Oxford University Press, 2002. An overview of the history of particle physics, well illustrated, and written not only for students and specialists but also general readers. The book provides “mysterious, abstract, often beautiful photographs of the tracks of subatomic particles as they speed, curve, dance, or explode through cloud and bubble chambers.” -
Galison, Peter. “Bubble Chambers and the Experimental Workplace.” In Observation, Experiment, and Hypothesis in Modern Physical Science, edited by Peter Achinstein and Owen Hannaway. Cambridge, Mass.: MIT Press, 1985. Details the entire history of the bubble chamber and discusses its effect on physics in terms of both new discoveries and how the practice of physics changed. The best historical account of this invention; should be read by those deeply interested in the subject. -
_______. How Experiments End. Chicago: University of Chicago Press, 1987. Primarily an account of three historical events in twentieth century physics: the discovery of the muon, the determination of the gyromagnetic ratio, and the discovery of weak neutral currents. Galison discusses the role of all experimental apparatus, including the bubble chamber, in the context of the latter two events. He discusses how physics theory and experiment come into agreement, and how the results given by an apparatus such as the bubble chamber are never completely certain. -
_______. Image and Logic: A Material Culture of Microphysics. Chicago: University of Chicago Press, 1997. A study of the material culture of the most basic matter known. Focuses on the interplay of vision and knowledge in microphysics. The chapter “Bubble Chambers: Factories of Physics” is especially relevant to discussions of the bubble chamber. -
Glaser, Donald. “The Bubble Chamber.” Scientific American 192 (February, 1955): 46-50. Written by the bubble chamber’s inventor, this article is the best possible introduction for general readers. It is amply illustrated, and Glaser spares the highly technical details, opting instead for a discussion of how he developed the apparatus and its uses. -
_______. “Elementary Particles and Bubble Chambers.” Glaser’s Nobel Prize in Physics lecture, December 12, 1960. Available from the Nobel Foundation at http://nobelprize.org/nobel_prizes/physics. -
Pickering, Andrew. Constructing Quarks. Chicago: University of Chicago Press, 1984. Devoted not to the bubble chamber but instead to the sociological development of particle physics. Pickering discusses the role of apparatus, including the bubble chamber.
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