Breakdown studies have been made between electrodes in high vacuum at constant voltages from 50 to 700 kv. These further demonstrate the inadequacy of the field emission theory to account generally for high voltage breakdown in vacuum. Experiments are described which investigate some of the ``total voltage'' breakdown mechanisms, including positive-ion emission by electron impact, electron emission by positive-ion impact and by photons. In the d.c. case these processes contribute to a steady interchange of charged particles between cathode and anode which increases with voltage until breakdown ensues. At higher breakdown voltages the cathode gradient has diminished far below the value for field emission. Measurements of electron emission by electrons with energies up to 300 kv for tungsten, steel, aluminum, and graphite are reported. The possibilities of predicting and of improving the insulating strength of electrode gaps in high vacuum by the study of the coefficients of the electrode materials are discussed.

Measurements are reported of the number and energy of the electrons which rebound from thick metal targets bombarded at normal incidence by monoenergetic electrons of from 1 to 3 Mev. The ratio of back-scattered to incident electron current, measured for twelve metal targets, was found to vary from 0.47 for 1-Mev electrons incident on U to 0.01 for 3-Mev electrons on Be. The total energy scattered backward from targets of Al, Cu, and Pb bombarded with 1- to 3.5-Mev electrons was determined by calorimetric methods. This energy diminished from 34% of the incident energy for Pb bombarded with 1-Mev electrons to 1% for Al bombarded with 3.5-Mev electrons. The dependence of the number and energy of the back-scattered electrons on the atomic number of the target and on the incident electron energy is discussed.

Surface conductivity and flashover in vacuum were studied using constant potentials up to 350 kV. Since discharges along the solid-vacuum interface appear to originate at or near the junction of the cathode electrode and the solid dielectric, methods of reducing the electric field at this critical region were explored. The more favorable approaches showed higher initial flashover values and could be conditioned to support more than 200 kV across borosilicate glass insulators in an interelectrode gap of 25 mm. The importance of intimate contact at the cathode-to-glass junction was confirmed.

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In this work roentgen rays and cathode rays of several million-volts energy have been applied to an investigation of their biological, photo-chemical, and germicidal effects, particularly as they are related to the processing of foods and biological materials. A constant-potential electrostatic generator, together with an acceleration tube, was used to produce continuous streams of electrons with homo-geneous and controllable energy. [R. J. Van de Graaff, K. T. Compton, and L. C. Van Atta, Phys. Rev. 43, 149 (1943).] These high energy electrons were utilized both for the production of penetrating roentgen rays and for the direct irradiation of materials. The mechanism of the biological action of both roentgen rays and cathode rays is discussed, as well as the energy considerations in their application to various absorbers. The companion paper [C. G. Dunn, W. L. Campbell, H. Fram, and A. Hutchins, J. App. Phys. 19, 605 (1948)] reports on measurements of the lethal action of these radiations on a wide variety of micro-organisms and also on the effect of the radiations on enzymes, vitamins, and certain whole food products. Both investigations have been conducted cooperatively by the Departments of Electrical Engineering and Food Technology at the Massachusetts Institute of Technology. The systematic and quantitative study of the effects of roentgen and cathode rays on various elementary forms of living matter is regarded as an essential prelude to their possible widespread application to the preservation of foods and the sterilization of various biological materials. It was realized that the mechanisms of biological action of high energy roentgen rays and cathode rays are similar and depend closely upon the ionization energy absorbed per gram of material. The studies were begun with roentgen rays since these were immediately available and permitted quantitative measurement of the energy absorbed in the biological materials. Later, the water-cooled gold x-ray target was replaced with an aluminum cathode-ray window, and corresponding studies were begun on the direct application of high energy electrons to these materials. During the period of these biological studies the x-ray source was also used in a daily clinical program of deep cancer therapy under the medical direction of Dr. Richard Dresser and also in the investigation of the physical properties of high energy radiations. [R. Dresser, Radiology, in publication.]