Cloud chamber experiments

Beginning in 1937, both Kamen and Kurie became associated with E. O. Lawrence s newly organized radiation laboratory. At that time, recoil tracks were used to calibrate cloud chamber experiments, but little was known about the isotope s physical characteristics. It was assumed that it was radioactive with a half-life of a few hours or, at most, a few days based on an analogy with the 0.8-sec half-life of He (J2). In view of its assumed short half-life, no determined eflFort to isolate it was undertaken immediately since the Lawrence laboratory between 1937 and... 

The cloud chamber experiments of Wilson (summarized in his 1927 Nobel Lecture) qualitatively demonstrated two nudeation mechanisms (1) condensation on ions at relatively low saturation ratios and (2) condensation on uncharged molecular dusters at much higher saturation ratios. Wilson s studies of condensation on ions are discussed briefly in this chapter. His resulLs on nudeation by molecular dusters which served as a starting point for development of the theory of homogeneous nudeation are discussed in the next chapter. Wil.son .s principal interest was in condensation on ions and its application to the measurement of high-energy nuclear particles,... 

Cloud chamber experiments of the type carried out by Wilson at the end of the nineteenth century (summarized in his Nobel Lecture. 1927) demonstrate the nature of the condensation process at various saturation ratios with and without foreign particles. The air in a chumberis first saturated with water vapor. By rapid expansion of the chamber contents, both pressure and temperature fall, carrying the sy.stem into a supersaturated state. At first, condensation talces place on small particles initially present in the air. Concentration.s of. such panicles in urban atmospheres range from 10 to 10 cm . By repeatedly expanding the chamber contents and allowing the drops to settle, the vapor-air mixture can be cleared of these particles. 

On the other hand, if we are to understand radiation chemistry and discharge chemistry at all we must understand them in terms of elementary process, instead of in terms of over-all effects which can be so readily observed. We establish the reality of such elementary processes for the most part on an essentially intuitive basis. We are of the opinion that we know that certain elementary processes occur in radiation chemistry or in discharge chemistry because of observations made, for example, in cloud-chamber experiments or in mass spectroscopy. We conclude... 

The distribution of the number of ionizations in the groups in the tracks of high-energy electrons in the gas phase has been determined from cloud-chamber experiments as well as by calculation. Results are shown in Table 2. We see that about 23% of the energy is... 

Obtained from cloud-chamber experiments " , Calculated . 

Since all Rutherford could know from his scintillation experiments was that alpha particles infrequently caused nitrogen nuclei to emit protons—he could not see the actual interaction—he had assumed it was a disintegration process. Only the cloud chamber could provide a visual representation of the transmutation process itself and give physicists the chance to discover the intricacies of the exchange. 

Before Anderson began his experiments he constructed a cloud chamber (a device that allowed physicists to see the paths that particles followed) with the most intense magnetic field that had ever been achieved. Magnetic fields cause charged particles to follow curved paths. By studying the paths, physicists can draw conclusions about their mass. 

However, in the context of the everyday laboratory the question is moot. It may be that the immutable laws of a deterministic universe dictated that in the middle of a star at the edge of the universe millions of years ago an atom was stripped of its electrons and sent our way at just less than the speed of light. But when that "cosmic ray" crashes through out cloud chamber, ruining our experiment, we have no choice but to regard it as a chance event. Even if we had an infinite capacity to store facts about the present state of the universe and had them all in place (universal data base) and if we had an infinite processing rate (the ultimate computer), we still would need an exact model of the universe... 

Scientists in the United States and elsewhere quickly confirmed the idea of uranium fission, using other experimental procedures. For example, a cloud chamber is a device in which vapor trails of moving nuclear particles can be seen and photographed. In one experiment, a thin sheet of uranium was placed inside a cloud chamber. When it was irradiated by neutrons, photographs showed a pair of tracks going in opposite directions from a common starting point in the uranium. Clearly, a nucleus had been photographed in the act of fission. 

While studying the passage of high-speed alpha particles (helium nuclei) through water vapor in a cloud chamber, Ernest Rutherford observed some long, thin particle tracks. These tracks matched the ones caused by protons in experiments performed earlier by other scientists. 

Condensation can take place on ions as well as on aerosol particles. In his classic cloud chamber studies. Wilson (1927) found that a rain of relatively large droplets at low concenuation formed at a. saturation ratio of about 4.2 compared with a dense fog of smaller droplets at saturation ratio.s above 7.9. Wilson hypothesized that ions continuously generated in the air by natural processes served as nuclei at the lower saturation ratio he verified this hypothesis using ions produced by an x-ray source. In later experiments he showed that condensation took place on negative ions at saturation ratios near 4 at about —6°C and on positive ions at a saturation ratio near 6 at a slightly lower temperature. Similar results were obtained by later investigators. 

The ideas which we have arrived at in the preceding chapters with regard to the structure of matter all rest on the possibility of demonstrating the existence of fast-moving particles by direct experiment, and indeed of making their tracks immediately visible, as in the Wilson cloud chamber. These experiments put it beyond doubt that matter is composed of corpuscles. We are now to learn of experiments which just as definitely seem to be only reconcilable with the idea that a molecular or electronic beam is a wave train. Before we enter upon this, however, we shall briefly recall the main facts of wave motion in general, using the phenomena of optical dift raction as a concrete example. 

Example 7.2 The Wilson cloud chamber is a device that allows one to rapidly expand and cool a sample of gas adiabatically. This can be used to induce supersaturation in the gas, which in turn causes aerosol droplet formation by nucleation on particles in the chamber. The paths of ionized particles travelling in a gas are made visible since the particles, such as alpha or beta particles, leave a noticeable trail of the larger aerosol mist droplets as they move through the chamber. When this experiment is done using regular air, the ions are those that result naturally in the air from interaction with cosmic rays and from the decay of radioactive gases from soil emissions. The Wilson cloud chamber is named for Charles T.R. Wilson, who developed it in the late 1800s, and received the Nobel Prize for physics in 1927 (see Reference [65]). 

A comparison of emulsions and various chambers. In planning experiments with track recording devices some of their properties and characteristics are of importance. Table 3 is an attempt to provide a rapid basis for comparing emulsions, various cloud chambers and bubble chambers. The particular measurement that one is trying to perform determines which properties and characteristics are desirable and undesirable it is a rare experiment which does not demand some compromise. 

These results were important in establishing the validity of the assumption of the reaction being (50.3). In studying the stopping of cosmic ray -mesons they employed cloud chambers in some experiments, a large neutron detector in other experiments, and the combination of the two techniques in one case. 

A variety of experiments employing photographic emulsions and cloud chambers and counters have been used to study the scattering of //-mesons at small angles. These experiments are summarized by Rochester and Wolfendale and associates, by Kan Angara and Shrikantia and by George . 

The capture rate is the collision rate of a cluster of size i with a monomer times the probability that the monomer will stick. If the concentration of monomers does not change significantly during the experiment, Ci will be independent of time. A typical nucleation experiment in a cloud chamber or diffusion chamber is carried out in such a way that the pressure in the region observed remains essentially constant during the time of observation. Therefore we can assume that rii is nearly constant and hence that Ci is a constant. 

To sum up the argument we find a ubiquitous role played by electrons in chemical theory. Electrons-as-particles phenomena can be obtained from atoms by means of certain procedures such as the cloud chamber. Electrons as wave phenomena can be obtained from atoms by distinctive and independent procedures such as double slit experiments. But it does not follow that the electrons obtained... 

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