no abstract provided by author / Master of Science

M.S.

M.S.

An r-f ion source has been constructed for the V.P.I. electrostatic generator. The ion source has been in operation for approximately one thousand hours with an average beam current of one micro-ampere. This beam has been sufficient to test and calibrate the accelerator. Experiments which require beam currents in the order of 100 micro-ampere can now be conducted with the ion source operating at its designed capacity. / Master of Science

Lithium, lithium oxide, and the group V elements, phosphorus, arsenic, antimony, and bismuth, when present in silicon as impurities, act as "shallow" donors (2, 7, 11). These donors have electrons which are bound so loosely that they may be thermally excited into the conduction band at less than room temperature. The excited bound states and, less precisely, the ground states of these electrons are understood within the framework of the shallow donor theory. A transition from the 1S(E+T₁) state of lithium oxide, which lies 7.7± .1 mev above its ground state, to the 2P^± state was observed. When phosphorus and lithium were both present as donors in silicon, the phosphorus peaks normally occurring at 34.4 mev, 2P_O and 39.2 mev, 2P^±, seemed to be split by .8 ± .1 mev. An upper limit of 10¹⁴/ cc was observed for the oxygen content of zone refined silicon using Pell's (15) value for the dissociation constant, C. / Master of Science

Ballistic measurements were made of the critical magnetic fields of superconducting. cylinders of cerium-doped lead. Within the accuracy of our measurements, the introduction of the paramagnetic impurity, cerium, in lead has no effect other than a slight broadening of the transition. The samples, containing 0.00, 0.10, 1.0, and 1.5 atomic percent cerium in lead had been vacuum cast in pyrex tubes. The relative fractions of the cerium in these samples and their final purities were tested by the techniques of neutron activation analysis. / Master of Science

The reflectivity, transmissivity, and absorptivity of freshly evaporated silver and copper films were measured in vacuo and then in the presence of several gases. These measurements were performed, using the photoelectric method, with two photomultiplier tubes (931-A and 1P21), and were made at an angle of incidence of zero degrees for two different wavelengths (4047 A and 4358 A). The gases used were air, argon, nitrogen and oxygen. The use of oxygen required several changes in the evaporating system in order that the diffusion pump oil would not oxidize too rapidly and cause an explosion. Air was found to increase the transmissivity in two steps, first an initial change, and second a gradual change. The reflectivity decreased in two similar changes. Nitrogen and oxygen revealed to a greater or lesser extent the same changes as were observed for air. Argon produced only a gradual change in both reflectivity and transmissivity. The initial change in oxygen was attributed to oxidation, and the initial change in nitrogen attributed to chemisorption. The gradual change in all gases was due to physical adsorption. The possibility that chemisorption might have caused the initial change in oxygen was pointed out, but this experiment did not allow a definite choice to be made. / Master of Science

Copper and silver films of various thicknesses were deposited at a rapid rate, from four to twenty-five seconds, on glass substrates in ultra high vacua at pressures of 10⁻⁸ torr or lower. Their optical properties, reflectivity, transmissivity and absorptivity, were determined in vacua and in air by direct measurement using photomultiplier tubes. All the measurements were made at nearly normal incidence with a monochromatic light beam of wavelength 4358 A. It was found that the reflectivity of films of both copper and silver decreased as they were exposed to air. The change took place in equal steps. Under the same circumstances, transmissivity increased but in a less regular way. Absorption changed in a most irregular fashion; no pattern could be established. From this study of optical properties, the layer formation on fresh, clean surfaces of copper and silver caused by the air molecules was quite evident. In the case of copper, there may be one or two layers depending on the thickness of the film. They are formed primarily by cuprous oxide. In the case of silver the layer was attributed to chemisorption. Even though there was some indication that more than one layer was formed, the evidence was not conclusive. / Master of Science

The buckling constant for neutron moderators in the shape of an elliptic cylinder, hemisphere, and"piggy-back" cylinder has been determined by experimental measurements using paraffin as a moderator. The pulsed neutron sources technique was used throughout the work; a fast neutron burst of short duration was injected into regular paraffin shapes (cylinders with diameter and height ratio equal approximately to one). An empirical curve of decay constant of the neutron population versus the buckling was obtained. The measured decay constant, λ, was fitted by the method of least square to a parabola in B² of the form: λ=Σ_av+ B²D - CB⁴ where λ= the decay constant B² = the buckling constant Σ_a = the macroscopic absorption cross section v = the neutron velocity D = diffusion coefficient C = diffusion cooling coefficient The resulting values of the diffusion parameters are: Σ_a v = 4858 ± 162 sec⁻¹ D = 25911 ± 202 sec⁻¹ cm² C = 1186 ± 2558 sec⁻¹ cm⁴ By measuring the decay constant, the buckling of the moderators with irregular shapes were determined from the above parameters. The result was in general accord with the theoretical approximations for such shapes. / Master of Science

M.S.