John G. Trump

Back‐Scattering of Megavolt Electrons from Thick Targets

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.

Irradiation of Biological Materials by High Energy Roentgen Rays and Cathode Rays

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 varie...

Insulation of High Voltage Across Solid Insulators in Vacuum

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.

Distribution of Ionization in Materials Irradiated by Two and Three Million‐Volt Cathode Rays

Measurements are reported on the distribution of ionization in depth of aluminum produced by steady beams of two and three million‐volt electrons. The variation of cathode‐ray current density in a plane transverse to the beam, the effect on this transverse distribution of additional aluminum scattering foils, and a practical method of cathode‐ray dosage computation are given.

Electron Emission from Metals under High‐Energy Hydrogen Ion Bombardment

Measurements of the electron emission from Mg, Al, Fe, Ni, Au, and Pb surfaces bombarded by atomic (H1+) and molecular (H2+) hydrogen ions with energies from 0.7 to 2.0 Mev showed a diminishing emission with ion energy and little dependence on either atomic number or on the field intensity at the metal surface. Measurements of this type are of interest in connection with the mechanism of high voltage discharges in vacuum. The expected variation of secondary emission with angle of incidence and with conditioning was found. The emission ratios are compared with measurements by others at lower ion energies.

Post-Type Support Spacers for Compressed Gas-Insulated Cables

The flashover performance of post-type support spacers for concentric compressed gas-insulated transmission lines was investigated. The experiments were made with SF6 gas at 4.4 atm. abs. and low frequency AC voltage in the EHV range. Nine cast epoxy spacers with metal inserts at each end shaped to produce different electric field distributions along the solid-gas interface were tested. The results show that flashover voltage is controlled by the electric field distribution and is inversely proportional to the stress enhancement in the gas. This conclusion is shown to be consistent with ionization-initiated breakdown at a certain stress limit about 300 kV/cm in these studies. The better designed spacers were limited by sparkover of the radial gas-gap at 1300 kV peak and not by surface flashover at the insulator.

Synchronous Field Shaping and Protection in 2-Million-Volt Rotational Therapy

This presentation is concerned primarily with certain irradiation technics evolved over the past eleven years of clinical radiotherapeutic experience with the 2-million-volt Van de Graaff electrostatic x-ray generator. Field shaping may be considered to have two objectives: that of conforming the shape of the beam of radiation to the known tumor volume and its possible extension and that of protecting adjacent normal areas by exclusion from the beam. In the absence of motion between patient and beam, the field may be given any desired shape depending upon the clinical problem and the philosophy of the radiotherapist. Accessory protective devices in such stationary-field therapy are usually modifications in the outline or dose distribution of the original field. Applied to 360° rotation of the patient, a fixed field is usually considered as necessarily symmetrical about the axis of rotation. The use of an accessory protective device in this situation requires in general that it also be shaped and rotate. T...

Role of Positive Ions in High‐Voltage Breakdown in Vacuum

Measurements were made of the electron emission from targets of magnesium, aluminum, steel, copper, gold, and lead when bombarded by ions from hydrogen, helium, nitrogen, xenon, and mercury. The ions, whose energy was varied over the range from 10 to 140 kilovolts, were produced by ionization of the residual gas in the anode electrode and emerged through a small opening into an essentially uniform accelerating field. An initial rapid rise of electron emission with ion energy was followed in all cases by a slow linear increase. The emission ratio varied from 2 to 20 and was maximum for nitrogen ions on steel. The effect of extractive gradient at the bombarded metal surface was slight and no simple dependence on the nature of the ion or of the bombarded metal was observed. The significance of these measurements on the electronion interaction theory of high‐voltage breakdown in vacuum is discussed.

Electrostatic sources of electric power

Because they are inherent in the structure of matter, electrostatic phenomena were noted and recorded even in ancient times. However, neglect of their practical application to the generation and utilization of electric power has resulted from the slow development of insulating media. From this viewpoint, the author examines the voltageinsulating qualities of compressed gases and high vacuum and the prospects for obtaining compact machinery so insulated.

Generating Voltmeter for Pressure‐Insulated High Voltage Sources

A description is given of a generating voltmeter suitable for a voltage source insulated by compressed gas. The calibration is linear and the design simple in that neither amplification nor mechanical commutation is required. The paper discusses the principle of operation, the construction, calibration, and sensitivity, and tests indicating that the influence of corona on the voltage readings is negligible.