Coolidge x-ray tube

X-rays from a Coolidge X-ray tube with a tungsten target were directed on to a slab of paraffin A. By a suitable arrangement of apertures... 

Fig. 1-2. Schematic diagram of the Coolidge (high-vacuum) x-ray tube. Coolidge tubes are widely used because they are stable and long-lived and permit tube current and voltage to be controlled independently.

The design and development of the Autrometer, as sketched by Behr,6 give us an excellent opportunity to indicate by reference a, number of items that were considered by its designers. (1) The need for better Coolidge tubes (9.4). The FA-100 series of x-ray tubes designed for the Autrometer has a peak of 100 kv, and a top rating of 3.5 kw with full-wave rectification, or 2.5 kw at constant potential,... 

In X-ray tubes the electrons are produced either by ionization of air at a moderately low pressure (in 4gas tubes ) or by emission from a heated filament at a much lower pressure (in hot cathode or Coolidge tubes). In most commercially obtainable X-ray tubes, one of which is... 

For most analytical applications, primary x-rays are produced by bombarding a suitable target with 10- to 100-keV electrons. This is a Coolidge-type x-ray tube and is illustrated in Figure 14.3. The spectrum resulting from electron excitation consists of a broad band of energies (the continuum or Bremsstrahlung) plus photons of dis-... 

There are several types of X-ray tubes but since Coolidge introduced the (side-window) X-ray tube in 1913, a variety of modifications to his basic design have been proposed. The ideal X-ray tube produces sufficient photon flux over a wide spectral range with a stability better than 0.1%. Via a switchable tube potential (usually in the 10-100 kV range), it is possible to optimize the shape of the tube spectrum for the intended application. 

The spectrum from a Coolidge tube often contains lines traceable to impurities, those of copper, nickel, and iron being the most common. The impurities may be present in the target of the new tube, but they are more likely to be deposited on the target during operation. It is consequently desirable that the analytical chemist maintain current acquaintance with the spectrum of his x-ray source. 

In most ordinary cases, the disadvantages of x-ray excitation are more than compensated by the absence of the disadvantages peculiar to electron excitation, by the great convenience of Coolidge tubes (1.3), and by the absence of the large background count to which the continuous x-ray spectrum excited by electrons gives rise (1.5). 

P-10 gas, 45, 219 Pair production, 290 Palladium, determination by x-ray emission spectrography, 328 Particle size, effect of variations of, in mineral analysis, 200 Philips Autrometer, 252-256, 280 Philips Electronics gas analyzer, 135 Philips Electronics improved Coolidge tubes, 248, 252, 253... 

X-ray intensity, see Intensity X-ray irradiance, definition, 6 X-ray methods of analysis, comparative, statistics, 254-256, 280 use of Coolidge tubes in, 6 X-ray Microscope, General Electric, 291-296... 

The intensity of the discharge current in a tube of this kind is highly dependent on the gas pressure, hence the need to be able to control it precisely and maintain it a constant level. This feature constitutes the main weakness of this kind of tube. The anticathode s lifetime is short (roughly 100 hours) but, on the other hand, its surface is not polluted, unlike in Coolidge tubes, and therefore the X-rays emitted are pure. Currently these tubes ate in practice no longer used. 

In 1913, Coolidge [COO 13] imagined another kind of X-ray source. The cathode is comprised of a tungsten filament heated by the Joule effect. According to the Edison effect, this filament emits electrons that are accelerated by an electrical field and bombard the anticathode which then emits X-rays. The entire device is placed in a sealed tube inside which the pressure must be as low as possible. A schematic view of such a tube is shown in Figure 2.3. 

This was the device used in 1916 by Debye and Scherrer [DEB 16] in Germany, and later in 1917 by Hull [HUL 17a, HUL 17b] in the USA, to conduct the first diffraction experiments on polycrystalline samples. A schematic view of the geometrical arrangement of this device is shown in Figure 2.25. The sample is irradiated with an X-ray beam produced by a Coolidge tube. The diffracted beams are collected by using a cylindrical detector placed so as to have the sample on its axis. 

The X-rays emitted by the Coolidge tube penetrate the cylindrical chamber on the axis of which is placed the sample. An imprint of the diffracted beams is produced using a film placed on the inside of the chamber. The intersection of the diffraction cones and the cylinder gives diffraction arcs with a curvature that depends on the angle. These arcs become hnes when 20 = 180° and their curvature is reversed beyond that. 

There are several sources of X-rays such as a Coolidge tube, vacuum sparks, hot-dense fusion plasmas, synchrotron, pinch devices, muonic atoms, beam-foil interaction, stellar X-ray emitters, solar flares, etc. The X-rays originating from all these sources can be broadly categorized into main types (1) atomic inner shell transitions, (2) emission by free electrons, (3) X-rays from few electron systems. The basic spectroscopic aspects of the various types of X-rays are discussed in this article. 

---