Thesis (Ph. D.)--University of Chicago, 1911. / "Reprinted from the Physical review, vol. XXXIII, no. 2, August, 1911." Includes bibliographical references. Also available on the Internet.

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The preionization of a shock tube's gas before the shock passes through it is called the precursor effect. An experimental and theoretical study has been carried out on precursor ionization in an electromagnetic shock tube. The precursor ionization was detected with different types of electric probes and also with photoraaltipliers. Extensive experiments indicated that the ionization was not due to diffusion of particles from the discharge in the shock tube driver. The ionization is primarily caused by radiation from the discharge of wavelengths less than 2000 Å. Radiation from the shock front makes a negligible contribution to the ionization. Langmuir double probe measurements indicated that the gas was about 0.1% ionized and that the electrons in the precursor were not in thermal equilibrium with the gas atoms and ions. The time interval between detection of ionization at two stations was independent of the shock tube gas (air, argon, helium), and corresponded to a propagation speed greater than 1/20 the speed of light. The precursor had a main component lasting about 50 microseconds with ionization proportional to the square of the discharge current. There was also a weaker component which lasted for about 500 microseconds. The experimental results can be understood in terms of a theoretical model based on black body radiation. Considering the driver to act as an infinite slab radiator, whose temperature is a function of the discharge parameters, an expression is derived for the number of photons emitted in some frequency interval. Assuming the electron density to be proportional to radiation absorption from such a radiator, the electron density variation with distance from the driver can be adequately understood. When the shock tube is considered to behave as a transmission line, whose resistance per unit length is proportional to the electron density, one can explain the variation of the shock tube's gas potential both with distance from the driver and with time. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

Part I An efficient ionizer designed for the ionization of a low intensity molecular beam has been developed. The ionizer will have an efficiency for an argon beam at room temperature and 0.8 amperes plate electron current of greater than 5.3% and for an helium 4 beam at room temperature and 0.8 amperes plate electron current of greater than 0.12%. The electron bombardment ionization occurs between two flat plates both of which are at 200 volts potential with respect to the cathode. The two plates are at a 2.5° angle to each other so that the ions once formed experience a field gradient produced by this angle which accelerates them out the open side of the ionizer 90° to the direction of the incident neutral beam. The electrons were emitted from 6, 0.030 x 0.004 inches, thoriated tungsten ribbon filaments heated by d.c. power. Successful activation of the filaments, however, was not achieved and so they were used as pure tungsten filaments. The ionizer ran at between 1900° Kelvin and 2600°Kelvin for about 8 hours without significant distortion, filament sag, or appreciable outgassing. Part II The design of a source of doubly charged helium ions was carried out to provide an helium beam of twice the terminal energy of the Van de Graafe Accelerator. The unit, to be installed in the top terminal of the Van de Graaffe, employs a radio frequency ion source followed by a double focusing magnet. The magnet has plane parallel pole pieces with a fringing field that produces focusing in a vertical plane. The unit is designed so that at 2.5 kilovolts extraction voltage a magnetic field of 3060 gauss is required to bend the He⁺⁺ through an angle of 90° and bring them to a focus at 5.8 cm. from the exit face of the magnet. The beam acceptance angle of the magnet is 10° at a source distance from the entrance face of 5 cm. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

The threshold energy for the reaction Br⁸¹ (p,n)Kr⁸¹ was measured and found to be 1.133±.020 Mev. The threshold for Cd¹¹⁶(p,n)ln¹¹⁶ was searched for up to 1.8 Mev but was not found. The density of cosmic neutrons was measured at the earth's surface. It was found to be 4.2±.7x10⁻⁹ n/cm³ over land with 66% in thermal equilibrium. Over the sea, this density was 3.72±.98x10⁻⁹ n/cm³ with 64% in the thermal peak. The earth's albedo or reflectivity to neutrons impinging on it from the atmosphere was estimated to be .22. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

The conductance of a small electric probe has been determined for radial distances (2 - 15 cm) from a recovering spark gap as a function of time after discharge initiation. The times investigated were from 0.2 to 15 ms and the gas pressure was varied from 22 mmHg down to 0.1 mmHg. The voltage applied to the probes was a sawtooth pulse which rose to about 80 volts in 10 μs. It is shown theoretically that the probe conductance should be proportional to the three-halves power of the gas temperature. Prom a known value of the temperature, deduced from recovery measurements, and the known probe conductance the constant of proportionality was deduced. Hence it was possible to determine the temperature from the probe conductance. The probe conductance measurements show that at 200 mmHg pressure the spark channel is only 2 cm in radius and that there is no detectable ionization left 2 ms after the discharge. As the gas pressure decreases the spark channel increases in size and takes longer to deionize, until at 1 mmHg pressure the channel fills the whole spark chamber (spark channel radius is 15 cm) and requires almost 15 ms to deionize. At 1 mmHg gas pressure there is a radial temperature gradient, while at 0.1 mmHg pressure the gas everywhere in the channel recovers at the same rate. In this experiment it is theoretically predicted that volume recombination should be the dominant recovery method and this is experimentally verified. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

This thesis is concerned with a theoretical and experimental investigation of Alfvén waves in an ionized medium, and magnetic interaction effects between a moving plasma and a magnetic coil external to the plasma. Methods for generating Alfvén disturbances for varying conditions of gas density and magnetic fields are considered and various means for measuring any effects that may be produced. It will be seen that for propagation of m.h.d. waves, extremely strong coupling between the plasma and field is necessary with consequent necessary high fields and Alfvén speeds. The effect of an axial magnetic field modifying the shock speed in the plasma is investigated and also the effect of the field on incident and reflected shock speeds by placing a plain obstruction in the shock tube which blocks the plasma flow. A further study of magnetic interaction effects between a moving plasma and a localized radial field was undertaken with the desire of correlating mechanical momentum transfer with varying conditions of applied field and gas pressures in the plasma (hence conductivity, density, and shock speed variations). Mechanical and electrical measurements of momentum transfer are compared with theory, and it will be seen that the mechanical method offers a fairly reliable means of measurement. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

Experiments are described on the possibility of developing a gas luminescence chamber for the observation of particle tracks. A brief review of devices currently available for obtaining ionizing particle tracks, particularly those capable of time resolutions in the microsecond region, is included. Attempts were made to detect scintillations in several gas and vapour mixtures. For some gas mixtures the possibility of using ionizing radiation to initiate a light producing chain reaction was investigated. These experiments were performed in the temperature range from 25 to 220° centigrade. Results obtained were not of such a nature to encourage further work towards the development of a gas luminescence chamber. As a background of non-radiation induced photon emission was observed for several reactions, it is possible that the methods employed in the experiment may be adopted to measure the onset and rate of some chemical reactions. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

In the work described in this thesis attempts have been made to obtain information on three aspects of the behaviour of liquid argon as an ionization counter. Ionization was produced by alpha particles from a source deposited on the negative electrode of a small parallel-plate chamber in which pure argon was liquefied. The current pulses resulting from movement of the liberated electrons in the field applied to the plates were analysed electronically. Firstly, it was desired to determine the time taken by the electrons originating from distinct ionization events to traverse measured electrode separations and be collected by the positive electrode. These transit times depend upon the electron mobility in liquid argon, defined as the velocity of the electrons per unit field. From this mobility, the mean free path and collision cross-section of the electrons with respect to argon atoms can be calculated. Some estimates of transit times and mobilities, and resulting values of mean free path and cross-section are given. However, it appears that limitations of the electrode spacing and the applied field cause the transit time to be so short as to necessitate the use of wide band amplifiers with, consequently, high noise levels. Secondly, the causes of variation in size of current pulses with angle of emission of the initiating alpha particles were investigated. The effect of greatest interest was that, due to the geometry of the chamber, as from it a determination was made of the range of alpha particles in liquid argon. For 5.3 MeV alpha particles, the weighted centre of ionization was found to be 0.006 cms. from the source, implying a particle range of about 0.009 cms. The number of electrons contributing to current pulses was also found to be a function of the angle of emission, on account of a variable degree of recombination with the positive ion column. This number was determined, and even under the most advantageous conditions fell considerably short of the probable total number liberated. This fact imposes a serious limitation on the potentialities of liquid argon as a useful counter. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

Some of the evidence supporting the olefin-like character of cyclopropyl groups, the double-bond assisted solvolysis of 7-norbornenyl derivatives and the ability of the cyclopropyl group to anchimerically assist ionization is reviewed. With regard to the high rate of solvolysis of 7-norbornenyl esters, the similarity of olefins and cyclopropyl groups and the steric features of the tricyclo [3.2.1.0 ²'⁴] octane ring system, a solvolytic study of tricyclo [3.2.1.0²'⁴] oct-8-yl derivatives was undertaken to determine whether a suitably oriented cyclopropyl group can also provide a large anchimeric assistance to ionization. The synthesis of the four isomeric tricyclo [3.2.1.0²'⁴] octan- 8-ols (II-OH, VIII-OH, XVI-OH and XVII-OH) is described. The cuprous chloride-catalyzed addition of diazomethane to anti-7-norbornenol gave pure exo-anti-tricyclo [3.2.1.0²'⁴] octan-8-ol (II-OH). Addition of diazomethane to 7,7-dimethoxynorbornene gave exo-8,8-dimethoxytricyclo [3.2.1.0²'⁴] octane (Vl) which was hydrolyzed to the corresponding ketone (VII) with acid. Reduction of the exo ketone afforded pure exo-syn-tricyclo [3.2.1.0²'⁴] octan-8-ol (VIII-OH). Attempts to prepare endo methylene adducts from norbornenes failed, although they were obtained from norbornadienes. The endo-tricyclo [3.2.1.0²'⁴] octan-8-ols were therefore prepared by a different route. Reaction' of 5,5-dimethoxy-l,2,3,'f-tetrachlorocyclopentadiene with cyclo- propene gave the endo Diels-Alder adduct (Xii). Dechlorination of this compound with metallic sodium, followed by catalytic hydrogenation, gave endo-8,8-dimethoxytricyclo [3.2.1.0²'⁴] octane (XIV). This ketal, upon acid hydrolysis, afforded the endo ketone (XV) which was reduced by a variety of reagents to give mixtures of the two endo alcohols. Preparative gas-liquid phase chromatography (glpc) gave pure samples of endo-syn and endo-anti-tricyclo [3.2.1.0²'⁴] octan-8-ols (XVI-OH and XVII-OH). Acetolysis of the p-bromobenzenesulfonates (brosylates) of the endo-syn and exo-syn alcohols in acetic acid and hydrolysis of the p-nitro- benzoates of the endo-anti alcohol and of anti-T-norbornenol in 70% aqueous dioxane gave relative rates of ionization of 37, 10⁴, 10¹² and 10⁹, respectively, compared to the solvolytic rate of a 7-norbornyl derivative at 100°. The exo-anti derivative (II-OBs) had previously been shown to have a relative solvolytic rate of 1.65 under these conditions. Acetolysis of the exo-syn and endo-syn brosylates gave rise to mixtures of entirely rearranged products which were not identified. Hydrolysis of the endo-anti p-nitrobenzoate in 70% aqueous dioxane gave rise only to endo-tricyclo [5-1.0.0⁴'⁸] oct-3-yi (XVIIl) products. The kinetic and product study data are consistent with the proposal that the endo-anti derivatives ionize to the 2,4-ethanotrishomocyclopropenyl cation. The stereochemistry of the reactions by which the tricycle [3.2.1.0²'⁴] octane ring system is formed, the effect of the stereochemistry of this ring system on the spectral characteristics of its derivatives and the mechanistic implications of the kinetic and product study data are discussed. [Symbols and illustrations omitted]. / Science, Faculty of / Chemistry, Department of / Graduate