ight.
Let us look for a moment at the events they are trying to observe and at the observing devices that have been available up to now. Physicists are probing the nucleus by bombarding it with particles, preferably particles with enough energy to break up the nucleus into its constituent parts.
He has had two ways of seeing and measuring these happenings. The first is the Wilson cloud chamber. In a chamber supersaturated with a vapor, a flying charged particle leaves a visible trail of liquid droplets, which condense on the ions the particle has produced by hitting vapor and gas atoms in its path.... Sometimes the particle breaks down ("decays") into lesser particles which make divergent tracks. But these interesting events occur only rarely in a vapor-filled chamber, because collisions in the gas are infrequent.
The second device for recording nuclear events is the photographic emulsion. A particle charging into the dense emulsion has a high probability of colliding with nuclei; hence there is a good chance that the emulsion will show interesting events, including scattering, disintegrations and the formation of new particles. However, the emulsion also has its drawbacks. Its very density makes collisions so frequent and the particle path so crooked that the effect of a magnetic field cannot be measured. And from the moment it is manufactured an emulsion begins to collect random particle tracks from cosmic rays and terrestrial radioactivity
Could some compromise be found which would eliminate the defects and combine the respective virtues of the cloud chamber and the emulsion? In May, 1952, I began to try a new approach to the problem, and I soon decided to explore the possibility of a liquid medium.
What kind of reversible process in a liquid could show the path of a flying particle and quickly erase the track after its passage? It would have to be a process that magnified the tiny effect of the atomic particle itself, as the condensation of droplets in supersaturated vapor magnifies the ionization produced by a particle in a cloud chamber. It occurred to me that a superheated liquid, like a supersaturated vapor, might provide the desired unstable equilibrium that could be triggered by a small stimulus to yield a large effect. Physical chemists have long known that in a clean, smooth-walled vessel a very pure liquid may be heated above its usual boiling point without boiling. I wondered whether a flying particle might, under suitable conditions, trigger the formation of the microscopic bubbles that start the boiling process. If so, it might make a visible track in a superheated liquid.
I used a bulb (half an inch in inside diameter) filled with ether; it was connected by a capillary tube to a piston-fitted cylinder with a hand crank which could quickly lower the pressure. High-speed movies, at the rate of 3,000 pictures per second, were made of the happenings in the bulb when the pressure was reduced. Sure enough, the pictures disclosed a track of tiny bubbles when a particle darted through the superheated ether. The bubble type of chamber soon proved to be a very sensitive recorder. Even fast mu mesons, which ionize only lightly, made visible tracks in the superheated liquid.
Having demonstrated that the bubble chamber idea worked, we proceeded to the task of building one large enough for practical laboratory use. We first built a two-inch chamber of duralumin and glass, with a diaphragm, actuated by compressed air, which could fully expand the chamber in five thousandths of a second. The liquid remained sensitive for seven thousandths of a second. We then incorporated the same design features in a larger pentane-filled version in which the liquid volume is six inches long, two inches wide and three inches high. This chamber is now in use with the Cosmotron at the Brookhaven National Laboratory. We have made 400 excellent pictures of tracks of protons from this accelerator. These track photographs are as easy to read as the best cloud chamber records and are about 10 times as accurate.
From Slowdown to Speedup
By Ada