Bubble Chamber

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

Figure 1. Diagram of a bubble chamber using liquid hydrogen (H₂). Expansion is effected through the piston (P). Light from impulse source (S) passes through the glass illuminator (I) and condenser (C). After being dispersed by the bubbles, the light is fixed on films (F₁ and F₂) by means of lenses (L₁ and L₂).

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, r_cr is the radius of the critical bubble, σ is the surface tension of the liquid, p₀ is the saturated vapor pressure, p_cr 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 p_i > 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

Figure 2. Detection in a liquid-hydrogen chamber of the nuclear reaction

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 → 2_π⁺ + 2_π^-

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.