Hannover, Universiẗat, Diss., 1999.

Bochum, Universiẗat, Diss., 2000.

Düsseldorf, University, Diss., 2002x.

Thesis (Ph. D.)--University of Chicago, 1922. / "Private edition, distributed by the University of Chicago libraries, Chicago, Illinois." "Reprinted from the Physical review, vol. 21., no. 6, June, 1923." Also available on the Internet.

The cross section for the photodisintegration of helium-3 has been measured at gamma ray energies of 8.06 and 9.17 mev. The He³ (ɣ,p)D reaction cross section at 8.06 and 9.17 mev was found to be 0.493± 0.066 and 0.723± 0.087 millibarns respectively. The He³ (ɣ,n)2p reaction cross section at 9.17 mev was found to be 0.25± 0.13 millibarns. These results are compared with other experimental work on the photodisintegration of helium-3 and tritium. The photodisintegration reaction was observed in a cylindrical gridded ionization chamber using a helium-3, methane, and argon gas mixture. The C¹³ (p,ɣ) N¹⁴ reactions at proton bombarding energies of 0.554 and 1.75 mev were used as the source of gamma rays of well defined energy. The preparation of carbon-13 targets is discussed in detail. Theoretical calculations on the photodisintegration of mass 3 nuclei are summarized. Photodisintegration and electron scattering measurements are compared as methods of determining the nature of the ground state wave function of the mass 3 system. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

This thesis describes experiments designed to test the prediction made by J. E. Robinson that adiabatic oscillations could be produced in liquid helium II. Oscillations have been observed in an experimental arrangement consisting of an adiabatic container placed in a helium II bath and connected with the bath by means of a "superleak". A "thermal pulse" introduced into the container caused the liquid level to oscillate. Containers of different geometries and employing two different types of superleak, were successfully used to observe oscillations. With one of these the temperature dependence of the frequency was measured between 1.38° and 2.065°K and found to be of the expected form and magnitude. The damping of the oscillations, the rate of fluid flow and the thermal relaxation of the container have also been investigated. Both quantitatively and in their general behaviour, the observed oscillations confirm the predictions of Robinson. An analogy is suggested between the oscillations in an adiabatic container in liquid helium II and those of a gas in a Helmholtz resonator. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

The development of low noise electronic equipment has facilitated the observation of ionic conduction pulses under alpha bombardment in liquid helium I and II, and a substantial extension of previous observations in liquid argon. Electron mobility in argon had been measured in fields between 2 kV/cm. and 200 kV/cm., the high field values agreeing with the order of magnitude previously reported. The scattering cross section for electrons is found to be of the order one hundredth that observed in gaseous argon. The low mobility of positive ions in liquid argon, observed for the first time, also shows that the liquid does not behave as a dense gas. Mobilities for both positive and negative ions in liquid helum I and II have been measured, also for the first time. The positive ions have low mobilities of the order of the positive ion mobility in argon, but the anomalously low mobility of the negative ions has not been explained. Ionic recombination, investigated in these liquids through direct current and pulse amplitude methods, is not described by the existing theory. Considerations are given which invalidates the model on which this theory is based, and an alternative model is suggested. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

The formation of positronium in helium gas under an applied electric field has been investigated in order to derive a value for the momentum-transfer cross section, (σ m, of positrons on helium atoms. The major increase in positronium formation was noticed at Ɛ/D's of 60 V cmˉ¹ amagatsˉ¹ . Ɛ/D values up to 100 V cm-¹ amagatsˉ¹ were obtained. A minor increase in positronium formation was found at 30 V cm-¹ amagatsˉ¹ and was attributed to impurities in the helium gas, specifically N₂, 0₂ and/or H₂. Using the theory of Teutsch and Hughes a value of σ m of (0.25±0.03) π a ₀² was found by fitting to the upper increase. This corresponds to positron energies near 17.7 eV. Although the theory is sensitive to the positronium formation cross section, the combined effect of impurities and counting statistics left this parameter undefined. A fit was made to the lower positronium formation increase under the assumption that this increase is due entirely to the impurity atoms, whereas the positron velocity distribution is due to the helium atoms. This gave a value for the positronium threshold for the impurities of 8 ±1 e V. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

Critical velocities have been measured in liquid helium at 1.40 K for counterflow, superflow and counterflow superimposed on superflow. The measurements were made in five pipes having rectangular cross-sections and ranging in size nominally from 0.2 x 1.2 x 10 cm to 1.2 x .1.2 x 10 cm. The turbulence was detected by means of negative ion currents perpendicular to the flow. The results are interpreted in terms of creation of quantized vortex lines. For superflow, the critical velocities were slightly higher but within the experimental range of those measured for counterflow. For the case of superflow-plus-counterflow a second critical point was observed, in addition to the one corresponding to superflow only. Considerable data was also collected regarding the nature of the ionic current attenuation effects for supercritical flow. The delay times between when the flow rate was raised to or above the critical velocity and when the current attenuations were first observed ranged from more than five minutes for the smallest pipe to three-quarters of a minute or less for the largest one. When the pipe width is greater than the ion source width the attenuations may be explained by Vinen's theory of vortex generation and decay but for smaller pipe widths the attenuation buildup and decay times are greatly increased. A method for increasing the sensitivity of the detection method for large pipes is also outlined. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

A study is made of the interaction between hydrogen atoms and the surface elementary excitations of superfluid "He. Calculations for the sticking probability and for the energy and effective mass of a hydrogen atom bound to the surface are presented. As a first step in the calculations, we formulate the Hamiltonian describing a hydrogen atom interacting with the surface of "He in its ground state together with the interaction coupling to the surface elementary excitations (the ripplons). The derivation is based on the association of the surface ground state with a flat surface and the excited states with a sinusoidally varying height of the surface. The interaction potential is derived by summing a *He-H atom-atom pair potential over helium atoms below the surface. The atom-atom pair potential is chosen so that the derived surface potential is a Morse potential with parameters which are fitted to the effective surface potential derived by Mantz and Edwards for a hydrogen atom interacting with N-1 helium atoms. The energy and angle dependent sticking probability, S(E,θ), and the thermally averaged sticking probability, S(T), are calculated. Results are compared with the experimental value of S(T), S(T)=0.035±0.005, measured for the temperature range 0.18<T<0.27K. The possibility that a hydrogen atom bound above the surface may exhibit polaronic behaviour is investigated. The energy of the hydrogen surface "polaron" and its effective mass are calculated using perturbation theory. The hydrogen atom is found to be weakly coupled to the surface elementary excitations, and therefore the polaronic effects are weak. The contribution to these results from virtual transition to free states is emphasized. / Science, Faculty of / Physics and Astronomy, Department of / Graduate