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bubble chamber,device for detecting charged particles and other radiation by means of tracks of bubbles left in a chamber filled with liquid hydrogen or other liquefied gas. It was invented in 1952 by Donald Glaser. The bubble chamber consists essentially of a sealed chamber to be filled with a liquefied gas and constructed so that the pressure inside can be reduced quickly. The liquid is originally at a temperature just below its boiling pointboiling point,
temperature at which a substance changes its state from liquid to gas. A stricter definition of boiling point is the temperature at which the liquid and vapor (gas) phases of a substance can exist in equilibrium.
..... Click the link for more information. . When the pressure is reduced, the boiling point becomes lowered so that it is less than the temperature of the liquid, leaving the liquid superheated. When a charged particle passes through this superheated liquid, it leaves a trail of tiny gas bubbles that can be illuminated and photographed. The track of a charged particle can be used to identify the particle and to analyze complex events in which it may be involved. If a magnetic field is present, the tracks of the particles will be curved, positively charged particles curving in one direction and negatively charged particles curving in the opposite direction. The degree of curvature depends on the mass, speed, and charge of the particle. Neutral particles can be detected indirectly by applying various conservation lawsconservation laws,
in physics, basic laws that together determine which processes can or cannot occur in nature; each law maintains that the total value of the quantity governed by that law, e.g., mass or energy, remains unchanged during physical processes.
..... Click the link for more information. to the events recorded in the bubble chamber or by observing their decay into pairs of oppositely charged particles. The bubble chamber is particularly useful for studying high-energy particles that would pass through a cloud chambercloud chamber,
device used to detect elementary particles and other ionizing radiation. A cloud chamber consists essentially of a closed container filled with a supersaturated vapor, e.g., water in air.
..... Click the link for more information. too quickly to leave a detailed enough track but which pass more slowly through the bubble chamber because of the greater density of the liquid. Liquid hydrogen and helium are commonly used in bubble chambers, with special equipment needed to maintain these gases in their liquid state (see low-temperature physicslow-temperature physics,
science concerned with the production and maintenance of temperatures much below normal, down to almost absolute zero, and with various phenomena that occur only at such temperatures.
..... Click the link for more information. ). For experiments requiring very dense liquids, a variety of organic compounds may be used. See elementary particleselementary particles,
the most basic physical constituents of the universe. Basic Constituents of Matter
Molecules are built up from the atom, which is the basic unit of any chemical element. The atom in turn is made from the proton, neutron, and electron.
..... Click the link for more information. ; particle acceleratorparticle accelerator,
apparatus used in nuclear physics to produce beams of energetic charged particles and to direct them against various targets. Such machines, popularly called atom smashers, are needed to observe objects as small as the atomic nucleus in studies of its
..... Click the link for more information. ; spark chamberspark chamber,
in physics, device for recording the passage of elementary particles produced by reactions in a particle accelerator. Particles pass through a stack of metal plates or wire grids that are maintained with high voltage between alternate layers.
..... Click the link for more information. .
a device for detecting the tracks of fast charged particles based on the boiling of a superheated liquid along the particle’s path. The bubble chamber was invented in 1952 by the American scientist D. Glaser. The superheated liquid can exist from some time τ, after which it starts to boil. If during the time interval τ an ionizing particle enters the chamber, its path will be marked by a line of vapor bubbles that can
be photographed. The bubble chamber may be thought of as a Wilson chamber “in reverse”; that is, instead of drops of liquid in a supersaturated vapor there are vapor bubbles in a superheated liquid. This analogy, however, is purely superficial, since the mechanisms for the formation of drops in the Wilson chamber and of bubbles in the bubble chamber are quite different.
The action of the bubble chamber is due to the formation of centers of boiling—nucleus bubbles—in the path of a particle and to their growth to dimensions exceeding the critical value:
Here, rcr is the radius of the critical bubble, σ is the surface tension of the liquid, p0 is the saturated vapor pressure, pcr is the critical pressure, p is the vapor pressure in the superheated liquid, V is the specific volume of the liquid, and Vʹ is the specific volume of the vapor. For the formation of a bubble larger than the critical bubble, energy of the order of a few hundred electron volts must be released in a volume of radius ~ 10-6 cm in a time ~ 10-6 sec. This energy is released in the deceleration of the electrons, here delta electrons, that are ejected from the atoms of the liquid by the particle being detected. The time required for the bubbles to grow to the dimensions suitable for photography (0.1-0.3 mm) ranges from a few milliseconds to tens of milliseconds for different bubble chambers.
Liquid hydrogen and deuterium are preferred for cryogenic bubble chambers, and propane (C3H8), various freons, xenon, and a xenon-propane mixture are used in heavy-liquid bubble chambers.
The liquid in a bubble chamber is superheated by rapidly lowering the pressure from the initial value pi > p0 to p < p0. The pressure drop is accomplished in 5–15 msec either by displacing a piston, in the case of liquid-hydrogen chambers (Figure 1), or by discharging external pressure from a cavity bounded by a rubber diaphragm, in the case of heavy-liquid chambers.
The particles are admitted to the bubble chamber when it is at maximum sensitivity. After the time necessary for the bubbles to become sufficiently large, the chamber is illuminated and the tracks are detected through the technique of stereoscopic photography with two to four lenses. After photography, the pressure is raised to the previous level, the bubbles disappear, and the bubble chamber once again is ready for operation. The entire operating cycle of the chamber runs less than 1 sec, and the sensitive time period is ~10—40 msec.
Bubble chambers, except those containing xenon, are placed in strong magnetic fields. This allows a determination of the momentum of a charged particle by measuring the radius of curvature ρ of the particle’s trajectory:
(2) kc= 300Hρ/cos ɸ
Here, ɸ is the angle between the direction of the magnetic field H and the momentum k of the particle and c is the speed of light. Track distortions in the bubble chamber are small and are due primarily to the multiple scattering of particles. With the use of precision measuring equipment it is possible to determine the spatial positions of the tracks and their curvatures with a greater degree of accuracy.
Bubble chambers generally are used to detect either interaction events of high-energy particles with the nuclei of the chamber liquid or the decay events of particles. In the former case, the chamber liquid acts as both a detecting medium and target medium (Figure 2). The size of the chamber determines its effectiveness in detecting the various processes of interaction or decay. Neutral particles, such as gamma rays and neutrons, are detected on the basis of their interaction with the chamber liquid (Table 1). Bubble chambers with a volume of a few hundred liters are the most common, but there are much larger
The antiproton ρ̄, formed upon the decay of the anti-lambda hyperon ⊼° collides with proton ρ and is annihilated as a result of the reaction
ρ̄ + ρ → 2π+ + 2π-
Here ⋀° is a lambda hyperon and π- and π+ are pions.
chambers. The Mirabel’ hydrogen chamber, for example, in the accelerator of the Institute of High-energy Physics of the Academy of Sciences of the USSR, has a volume of 10 m3; the hydrogen chamber in the accelerator of the Fermi National Accelerator Laboratory in the US has a volume of 25 m3.
The main advantages of bubble chambers are an isotropic spatial sensitivity to particle registration and a high precision in the measurement of particle momenta. One drawback is a low degree of control over the selection of necessary events of particle interaction or decay.
|Table 1. Characteristics of liquids used most often In bubble chambers|
|Operating conditions||Probability of recording a 500-MeVγ quantum over a length of 50 cm||Probability of recording a 1-GeV neutron over a length of 50 cm|
|Liquid||Pressure (atm)||Temperature (°C)||Density (g/cm3)|
REFERENCESGlaser, D. A. “Some Effects of Ionizing Radiation on the Formation of Bubbles in Liquids.” The Physical Review, 1952, vol. 87, no. 4.
Puzyr’kovye kamery. Moscow, 1963.
Trudy Mezhdunarodnoi konferentsii po apparature v fizike vysokikh energii, vol. 2. Dubna, 1971.
S. IA. NIKITIN