R. G. Herb

Orbitron Ionization Gauge

An ion gauge with high ionization efficiency and low input power is described. In this gauge, called the orbitron, electrons are injected by a small tungsten filament into the electrostatic field between two concentric cylinders, with energies and angular momenta such that a relatively long mean free path is achieved. Only 4 μA electron current is needed to produce the same ion current obtained from 8 mA emission in a conventional Bayard‐Alpert gauge. X‐ray and photocurrents are held to low values by locating the ion collector remotely from the main source of light and x rays. The ion current is linear with pressure below 1×10−5 Torr and has been tested down to 5×10−11 Torr. The lower pressure limit has not yet been determined.

Orbitron Vacuum Pump

A pump was constructed in a casing 10 cm in diam in which electrons are orbited around a central anode consisting of a tungsten rod supporting a titanium cylinder. Electrons have mean free paths of several hundred centimeters, they ionize inert gas and they heat the titanium cylinder to give sublimation. Some values determined for pumping speeds when sublimation rate is sufficient are 900 liters/sec for hydrogen, 500 liters/sec for nitrogen, 300 liters/sec for air, and 7 liters/sec for argon. Sticking probabilities were measured for active gases and for inert gas ions as a function of the ratio of atoms of gas bound to atoms of titanium deposited. The pump is very stable in operation; it is simple and dependable.

Pelletron accelerators for very high voltage

Abstract The design features of the NEC Pelletron accelerators are presented and discussed. Particular attention is directed toward identification of those aspects of existing electrostatic accelerators which limit ultimate voltage performance and to progress which has been made toward eliminating or deferring these limitations.

Electrostatic Analyzer for Selection of Homogeneous Ion Beam

An electrostatic analyzer has been built for use with the high pressure electrostatic generator at the University of Wisconsin. The proton beam from the generator is deflected through a 90° arc of 40″ radius by means of a radial electric field between curved metal plates spaced 516′′ apart. Suitably disposed slits select a beam which has a very small energy spread. The apparatus has been operated with energy resolutions up to 5000, and preliminary tests have been made, using the 985‐ and 1020‐kv γ‐ray resonances in the reaction Al27+H1→Si28+hν.

Orbitron Pump of 30-cm Diameter

An Orbitron pump of 30-cm diameter was constructed and performance was studied as parameters were varied. The pump arrangement chosen gave pumping speed values of 105 liters/sec for argon, 4500 liters/sec for air, 6000 liters/sec for nitrogen, and 9000 liters/sec for hydrogen.

27Al(p, γ)28Si resonance widths with study of temperature and contaminant effects

Abstract A description is given of methods used to determine nuclear reaction widths from reaction yield curves. Limitations on the precision of measurment due to the Doppler width, contaminant effects, beam energy spread, and voltage instability are analysed. Experimental equipment was developed to reduce each of these spreading factors. Nuclear resonance half-widths were determined at the resonances near 992, 1262, 1317, 1328, 1364 and 1388 keV. The width of the resonance near 1388 keV, Γ = 550±110 eV, is at variance with the published estimate of 190±80 eV. The confusing effect of contaminants was reduced by attainment of a target chamber pressure during evaporation of the target of 3 × 10 −9 mm Hg in the best case, and 5 × 10 −11 mm Hg during proton bombardment. Yield curves taken from cold targets could not be fitted to calculated yield curves over the 27 Al(p, γ) 28 Si resonance near 774 keV ( Γ = 14±3 eV) if the Doppler width, Δ = 2(R g 3) 1 2 , was calculated in the Debye approximation. The observed variation with temperature of the Doppler width exceeds that predicted by Debye or Einstein theories for specific heats in solids. Favourable comparison of calculated and experimental yield curves was made if the classical approximation to the Doppler width was assumed: namely, Δ = 2 (RkT) 1 2 . For aluminium at T = 80° K this represents a difference in estimated values of the Doppler width of over 40%.

A Van de Graaff Electrostatic Generator Operating Under High Air Pressure

An electrostatic generator of the Van de Graaff type has been developed which operates in a large steel tank under an air pressure up to 45 lbs. The maximum steady voltage obtained was about 750 kv. In applying this voltage to an evacuated tube for acceleration of protons the usable voltage is limited by the dimensions of the present tube to about 400 kv. The apparatus has given satisfactory service during a year of use in nuclear disintegration work.

The Pelletron Accelerator

A review is given of early development work at the University of Wisconsin on electrostatic accelerators insulated by high pressure gas. This work started in 1933 and led in 1940 to a 4.5MeV accelerator. Starting in 1946 development work was initiated along three lines: 1) Bakeable high strength metal to ceramic bonding, 2) Organic-free pumping, 3) Metal charge carrying devices. In 1951 construction was started at the University of Wisconsin on an accelerator utilizing results of these three development programs. In 1965 much development work was transferred to the National Electrostatics Corp. where the charging chain was developed. It consists of metal cylinders joined by links of solid insulating material. A 1MV column support unit was perfected which is used to construct column modules. A bakeable all metal and ceramic tube, free of organic vapors was developed. It consists of short sections bolted together with three sections per column module. A three-stage accelerator was built and was tested with a beam up to energies of 22MeV. A two-stage accelerator to provide 28MeV protons (14MV on terminal) is under construction. A two-stage accelerator to give 40MeV protons (20MV on terminal) has been designed.

Sublimation and Ion Pumping in Getter‐Ion Pumps

A large variety of methods have been studied for evaporation and sublimation of titanium. For use in getter‐ion pumps, sublimation appears to be clearly superior to evaporation from the molten state. A number of getter‐ion pumps have been built and used for various applications. Each utilizes a plasma‐containing grid as a source of electrons for heating titanium rods. The grid also provides the necessary ionization for the pumping of inert gases. Consideration is also given to effects of geometry, scaling, pressure, and recombination rates on ionization and pumping by the grid.

New Electrostatic Accelerator

A new electrostatic accelerator has been constructed and tested. The entire vacuum system including the accelerating tube and ion source is of bakable construction. The accelerating tube, corona tube, and support column are constructed of thin metal disks hard‐soldered to alumina ceramic rings and the insulation length of each is 5 ft. Without the accelerating tube the generator has been operated at potentials in excess of 7 Mv corresponding to gradients of over 1400 kv/ft. With flat disk molybdenum electrodes usable voltage was limited to 2.1 Mv. When these electrodes were replaced by flat disk steel electrodes having smaller beam apertures and provided with side pumpout holes, the accelerator was operated for short times at potentials in excess of 4 Mv corresponding to a tube gradient of over 800 kv/ft. There was no evidence of small discharges or electron loading in the tube. During the course of these tests three tube ceramics were cracked—probably by sparking—and it is now difficult to operate the ma...