R. J. Van de Graaff

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...

Tandem Electrostatic Accelerators

Abstract In this type of accelerator a constant potential is applied to the acceleration of the ion beam, however, not as conventionally with just one voltage application, but instead with a number of applications made in succession by means of a tandem arrangement of high voltage tubes. This number of voltage applications, which is the number of the so-called “stages” of a tandem accelerator, may be two, three, or four, depending on the design chosen. The principles of this type of accelerator are described, and a brief account is given of the design and performance tests of a two-stage accelerator for an output rating of 10 MeV and one-half microampere of protons. Some possibilities for future development are discussed for reaching greater energies by means of three and four-stage tandem arrangements. Preliminary experimental results show the feasibility of operating these by means of the injection of a beam of neutral particles, thus retaining the use of an external ion source at ground potential, as in the present two-stage tandem accelerators.

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.

Secondary Electron Emission from Metals under Positive Ion Bombardment in High Extractive Fields

In the further study of breakdown mechanisms in high vacuum, experiments show that secondary electron emission from metals under positive hydrogen ion bombardment is small and increases only slowly with the electric field strength at the bombarded surface. Although more significant emission may yet be found with the heavier ions obtained in an actual vacuum gap, the particle interchange component of the breakdown process appears to be quantitatively inadequate on the basis of present measured electron and positive ion emission coefficients. The possible importance of negative ion emission deserves consideration in the particle exchange process.

Electrostatic Generator for Nuclear Research at M.I.T.

The electrostatic generator previously located at Round Hill has been transferred to the M.I.T. grounds in Cambridge. The present generator construction and also a high voltage accelerating tube mounted vertically in one of the columns are described. An account is given of experience with the acceleration of positive ion and electron beams and with the x‐ray bombardment of indium, beryllium, and deuterium.

The Production of Proton Beams

An account is given of further work on capillary ion sources of the diffusion type. Both metal and glass capillaries have been investigated. The performance of the metal sources is affected by surface conditions which are difficult to control. The average proton yield from these sources is about ten percent. It was found that the yield could be increased to about fifty percent by coating the surface with phosphorus pentoxide. Sources constructed of Pyrex have a proton yield of about sixty percent and also greater stability in operation, as well as certain other advantages. Various characteristics of the output of both types of source are described as functions of the source geometry, arc current, and other variables.

Electrostatic Accelerator for Electrons

An account is given of the design and operation of five electrostatic accelerators constructed to supply an urgent radiographic need of the Navy. Each generator is housed in a steel tank 4.5 feet in diameter and 10 feet long, the insulating medium being gas at 200 pounds gauge pressure. The x‐ray generators are rated at 2‐million volts but were frequently operated at somewhat higher potentials. Tests were made with various insulating media, including air, nitrogen, Freon, and sulfur hexafluoride. With a 3‐foot extension to tank and insulating column and without a vacuum tube, a potential of 5.6‐million volts was attained with sulfur hexafluoride. Both continuously pumped and sealed‐off x‐ray tubes were used. These were supplied by the Machlett Laboratories, Inc., following a cooperative development. The use of focal spots only .01 inch in diameter made possible radiographs of extremely high definition. Target currents up to 300 microamperes produced in this voltage range an x‐ray output of great intensity...

PROPOSED METHODS FOR PRODUCING INTENSE PULSED BEAMS OF MONO-ENERGETIC NEUTRONS

Abstract These methods utilize time-of-flight and extended targets to produce bunched beams of mono-energetic neutrons. It would appear that bunching factors of several hundred could be achievable by combinations of these techniques.

An Experimental Investigation of Ion Beam Focusing

An experimental investigation has been made of the initial focusing of ions from capillary sources of the diffusion type described in the previous paper. The lens system used consisted of a 90‐degree cone at the ion source followed by three coaxial cylinders which were arranged at various potentials up to 25 kilovolts. Since the beams are rendered slightly luminous by traces of gas in the high vacuum region, they have been studied by a photographic method. The beams have also been studied by an electrical scanning method. Both types of investigation were made as functions of the electrode potentials and arc variables and are in good agreement. Based on these observations a focusing and canal arrangement suitable for differential pumping has been tested. This arrangement has been embodied in a design for a projected ion gun assembly.