J.D. McGee

High Quality Phosphor Screens for Cascade Image Intensifies

Publisher Summary This chapter discuses improvements in the quality of the fluorescent screens used in the development of the three-stage cascade intensifier. This has led to improvements in the image quality of the completed device. The formation of the thin, fine-grain phosphor screen on a very thin mica sheet is of crucial importance in the development of cascade image intensifiers. The layer must be as thin as possible, compatible with high electron-energy conversion efficiency, in order to avoid loss of image definition. Methods for the deposition of continuous layer screens have been reported using either vacuum evaporation or vapor-phase formation. Two principal methods of screen preparation were employed: (a) gravity sedimentation or settling, a technique well known for its use in the manufacture of cathode-ray tube screens and (b) electrophoretic deposition, an old technique but one that has not been used much for the production of phosphor screens. At present, however, these techniques lead to the production of screens of rather low conversion efficiency. In order to ensure that the completed intensifier has the highest possible light gain, it is necessary to use phosphor materials of high intrinsic efficiency. The resolution of completed phosphor screens has been measured in single- and multi-stage intensifiers and also by optical methods.

Further Developments of the Spectracon

Publisher Summary This chapter discusses developments happening in the design of Spectracon. The normal tube has seen no major change, but many refinements in internal design, processing, encapsulation, and operation have led to considerable improvements in all aspects of its performance. One major development is an attempt to make a tube with a much larger effective area, with a mica window, 1 inch diameter and ∼4 μ thick, and to use it in the same manner for contact electronography. The dimensions of the Spectracon tube, 28-cm long by 5 cm diameter, have not been changed. The tube is mounted in a coaxial Perspex cylinder, which is closed at the front, or the photocathode, end with a high-quality fused-silica plate and at the back or mica-window end with a Perspex annulus, to which the applicator case can be attached. The potential divider resistor chain is attached to conducting bands on the external surface of the tube opposite the internal conducting-metal bands. This chapter briefly outlines different refinements in internal design, processing, encapsulation, and operation that led to considerable improvements in all aspects of its performance. Preliminary results of the experiments conducted on the tube are also reported.

The Spectracon—An Electronographic Image Recording Tube

Publisher Summary This chapter outlines the recent developments of the thin mica window electronographic image tube—the Spectracon—and gives an account of its present performance compared with that of the most efficient light sensitive photographic emulsion. This electronographic tube in which the photoelectrons from a photocathode are projected through a thin mica window, focuses on an electron image and records an electron-sensitive emulsion in contact with the outer surface of the window. The main operational characteristics of the tube are described under the following three headings: (1) Image quality, including resolution, contrast, geometry, and granularity. (2) Speed, or the rate at which a certain photographic density is reached. (3) Background, including both the fog background of the emulsion and the parasitic background from the tube.

The Evaluation of Cascade Phosphor-Photocathode Screens

Publisher Summary This chapter focuses on different screen characteristics such as efficiency, image resolution, freedom from pin-holes, screen texture, etc. with special reference to their suitability for cascade image intensifier screens for test of the prepared samples. The energy conversion efficiency of the phosphor screen may be defined as the ratio of the output light energy to the input electron energy. The results indicate that the relative efficiency of the phosphor screens with floated backings is 20-30% higher than that of the similar screens with conventional backings. The resolution of the electron image in each stage of a cascade image intensifier rail be very good, at least, 150 lp/mm, and the loss of resolution is due mainly to scattering of electrons and of the light they excite in the phosphor screen and in the spread of this light as it passes through the substrate from the phosphor to the photocathode film. Estimates of the image resolution of screens have been made, and an optical technique of image resolution measurement has been developed and assessed. The experimental results have been compared with the estimated values of image resolution.

Cascade Image Intensifier Developments

Publisher Summary This chapter discusses improvements made in the design and performance of the three-stage cascade image intensifier developed at Imperial College. It also elaborates the development and preliminary testing of a considerably modified design of tube especially suitable for spectroscopy. The phosphor screens used in the cascade intensifier are made by electrophoretic deposition, using a P.11 phosphor, E.M.I. type MA 214. Attempts were made to increase the resolution of screens by making the phosphor layer thinner. Intensifier tubes with a limiting resolution of ∼40 lp/mm were made with phosphor screens of 0.5 to 0.7 mg/cm 2 . The overall gain of the tube tends to saturate and does not continue to rise as the operating potential is increased. A considerable improvement in image resolution was obtained, at the cost of a slight loss in screen efficiency, by using an adaptation of the well-known organic film process to apply the aluminum backing. Efforts have also been made to increase the gain of the multiplying screens by using a photocathode with higher quantum efficiency in the emission band of the P.11 phosphor, than the simple S.9 (Sb–Cs) photocathode used.

A Study of Pre-breakdown Phenomena in n-Hexanc Using an Image Intensifier Tube

Publisher Summary This chapter focuses on study of the behavior of n-hexane pre-breakdown at room temperature and at atmospheric pressure using an image intensifier tube. Currently, using Schlieren optical techniques, some investigators have found that the refractive index of the liquid in the region near the electrodes changes at field strengths close to breakdown stress. This pre-breakdown phenomenon may have an important relationship with the effect of pressure on electric strength and warrants further investigation. Other useful information concerning pre-breakdown processes may be found by seeking to detect and study pre-breakdown light generation. The light emission in n-hexane at high electric fields commences in the cathode region regardless of the geometry of electrode systems used. For sphere-sphere and negative point-sphere electrode systems the plasma always commences in the cathode region and expands toward the anode until breakdown occurs, while for point-point and positive point-sphere electrode systems, the plasma still commences in the cathode region, but after its extension for a certain distance from the cathode, light emission also starts in the anode region. This can be accounted for by the fact that some free electrons, when reaching the intense field region near the positive point electrode, may gain enough energy to cause ionization and then form a separate volume of plasma near the anode. In this case, breakdown occurs when the plasmas in the two regions extend until they bridge the gap.

A Cascade Image Intensifier

Publisher Summary This chapter describes various methods and techniques that developed to achieve that degree of excellence, as regards gain, resolution, and background and image quality that is necessary if the tube is to be useful in many important applications. The tube described is a three-stage cascade image intensifier utilizing two phosphor-photocathode multiplying dynodes. There follows a description of the main features of its design, construction and processing, followed by an account of its characteristics and performance. In the cascade tubes described here, there are several potential sources of background: signal induced background produced by spurious electrons liberated by light scattered back to the photocathode from surfaces inside the tube; secondary electrons emitted from the glass walls; spurious electrons from field emission at sharp points; thermal dark current of the photocathodes; secondary electrons from the photocathodes caused by the impact of positive ions. The figures obtained are only approximate, since no two tubes have the same gain, and the measurements of attenuation were not very accurate. However, the quality of such an image is very bad, since every photoelectron from the primary photocathode produces a large number of developed grains in the emulsion, giving a very granular image. The image quality can be improved by stopping down the recording lenses or reducing the gain of the tube, or both.

Electron Transmission through Mica and the Recording Efficiency of the Spectracon

Publisher Summary The percentage transmission of electrons of energies from 0 to 40 keV through a number of mica foils was found by direct current measurements which are described in the chapter. It has recently been found possible to construct and operate tubes with windows 30 mm x 10 mm using mica 4 ± 0.5 μm thick where previously ∼ 6 μm was the minimum thickness that had been used. It has the great advantage that it is easier to construct the tube to operate at this lower potential without spurious “dark current” which limits the practical length of exposure. The proportion of electrons of energies between 20 and 45 keV that penetrate through mica foils of surface densities 1.0-2.5 mg/cm 2 (i.e. 4 to 10 μm thick) has been investigated by direct current measurement. These results were checked by determining the efficiency of recording of individual electrons by a typical tube by counting the number of tracks of electrons recorded in the emulsion after a known electron charge had passed through the tube. This method is described in the chapter.

An Image Tube with Lenard Window

Publisher Summary This chapter discusses development of an image tube with Lenard window. In this tube, the electron image emitted from a photoemissive cathode is accelerated to sufficient energy to penetrate a thin mica window which is capable of withstanding atmospheric pressure. The electron image is recorded on an electron sensitive emulsion, which is held in close contact with the outside of the window. This device has the following advantages: (1) because the tube is sealed off, it should have a long life; (2) no special preparation is required before the tube is put into operation; (3) there is no predetermined limit to the number of exposures that can be made; (4) the tube does not require any auxiliary pumping equipment. This tube has a satisfactory performance as regards gain, geometry and resolution, however is characterized by an extremely high background even at operating voltages as low as 2 kV. It was observed, however, that this background did not occur when the magnetic field was removed.

A Photon-counting Detector for Stellar Spectrophotometry

Publisher Summary This chapter describes a photon-counting detector for stellar spectrophotometry. The online digitization of two-dimensional star-field photometry leads to serious problems in data handling. These problems are considerably reduced if the input signal is one-dimensional in space as is the case with stellar spectra. One way of generating a real-time electrical signal from an image without losing positional information is to use an image dissector. The image dissector is scanned synchronously with the cascade tube at 90° to the dispersion, as well as in the direction of the dispersion, so that it reads the signal displayed at the cascade tube output. The choice of dwell time is governed by the variation of the most probable scintillation amplitude as the dissector progresses from channel to channel. The two scintillations provide the brightest and weakest signals read by the dissector respectively. The amplitude of the brightest signal in the first channel varies linearly with dwell time as the delay before this signal is read is independent of the dwell time. The first detector that has been tested has a resolution of about 500 channels, and this should be significantly increased in the second detector.