Robert K. Swank

Absorption and noise in x‐ray phosphors

When the signal in an x‐ray image system is formed by integrating the scintillation pulses rather than by counting them, the signal‐to‐noise ratio is reduced by a factor which depends on the shape of the pulse‐height distribution. The signal‐to‐noise ratio cannot be related directly to either quantum absorption or energy absorption, and a new quantity called noise‐equivalent absorption is defined which bears a simple relationship to the signal‐to‐noise ratio. Quantum, energy, and noise‐equivalent absorption are calculated as a function of thickness and x‐ray energy for CsI, Gd2O2S, LaOBr, Zn0.6Cd0.4S, and CaWO4.

Calculation of Modulation Transfer Functions of X-Ray Fluorescent Screens

Formulas have been derived for calculating the modulation transfer function of x-ray intensifying screens. Emphasis has been placed on simplified models where results can be obtained in closed form and a physical understanding more easily realized. For selected cases, the MTFs of transparent screens are calculated by direct integration. The MTFs of diffusing screens are determined from suitable approximate solutions of the Boltzmann equation. The intermediate case and the transition from scattering to nonscattering are discussed. As a byproduct, formulas for light output and signal-to-noise ratio are obtained.

Measurement of absorption and noise in an x‐ray image intensifier

Three experimental methods for measuring the noise‐equivalent absorption of an x‐ray image intensifier are described. In all cases, the image intensifier is irradiated with a source of x rays or γ rays, and the output is monitored with a photomultiplier. In one method the rms value of the photomultiplier current is observed as a function of the bandwidth of the measuring circuit. In the second method, the scintillation pulse spectrum is recorded, and its moments computed to give the required information. In the third method, the response to a series of short bursts of radiation is measured. The noise‐equivalent absorption is determined from the standard deviation of a large number of observations, automatically recorded. The three methods are described and compared, and typical results are shown.

Barrier Heights and Contact Properties of n‐Type ZnSe Crystals

Barrier‐height measurements performed on ultrahigh‐vacuum‐cleaved and on chemically etched ZnSe crystals have shown that for most metals the barrier height increases linearly with its electronegativity. The highly reactive metals Mg, Ca, and Ba were found to be an exception, showing a decrease in barrier height with increasing electronegativity. Aluminum, when diffused into ZnSe from a film deposited onto a vacuum‐cleaved surface, was found to produce a low‐resistivity surface layer, followed by a highly compensated region in the crystal. This effect is suggested to arise from copious generation of Zn vacancies at the Al‐doped crystal surface, and their influx, ahead of the Al, into the crystal.

X-Ray Image Intensifiers

Publisher Summary This chapter describes that an X-ray image intensifier (XRII) is a device that converts an incident X-ray pattern to yield a visible-light image of brightness, substantially higher than a simple phosphor screen. In most XRIIs, the final image is considerably smaller than the incident X-ray pattern that facilitates the coupling of optical lenses to transfer the image to the final image receptor such as video pickup, eye, cine film, and many more. The brightness gain is achieved in two ways: first, by increasing the number of light quanta generated from a given region of the X-ray pattern and second, by reducing the size of the final image so that the quanta is emitted from a smaller area. The chapter also discusses that in almost all XRIIs, the X-ray pattern is converted to a light pattern using a phosphor and the brightness of the light pattern is then intensified. Many systems employ electron-optical processes to provide the quantum gain, even though this requires additional stages of quantum conversion; the light from the input phosphor is converted to an electron image by a photoemitter that forms the cathode of an electron-optical system. After gaining energy by passing through an electrostatic field, the electrons are converted back into a light image in another phosphor.

Characteristics of a ZnS:Pd:Cs2O Cold Cathode

Schottky‐barrier cold cathodes have been fabricated using Pd on n‐type ZnS. The Pd was deposited in the form of a film 30–50 A thick on the freshly‐cleaved surface of the ZnS in an ultrahigh‐vacuum system, and cesiation was carried out without removal from the system. Cold emission currents up to 0.01 A/cm2 were obtained with an efficiency as high as 1.5%. Barrier height measurements indicate that the electrons enter the junction at an energy level 0.5 eV above the vacuum level, but current characteristics reveal a loss process not yet identified.

The development of a self‐contained instant‐display erasable electrophoretic x‐ray imager

We report the construction and operation of a self‐contained electrophoretic x‐ray image cell which, between two transparent‐electrode coated glass plates, has an electrophoretic liquid in direct contact with radioconductive amorphous selenium. The image cell has a built‐in memory which holds the x‐ray image after a transitory x‐ray exposure. The x‐ray image can be developed and erased, without handling the liquid toner or radioconductive material. Potential applications include radiography, mammography, and x‐ray inspection of baggages, mail, industrial products, and machines.