X-ray effect

The discovery of the x-ray effect spurred the development of a new generation of hot-cathode gauges designed to minimise this effect. One of the earhest, and commercially the most successful, the Bayard-Alpert gauge shown in Eigure 14b, was developed in 1950 (13). A fine wire, the ion collector, is... 

X-ray diffraction profile, 26 417 X-ray diffractometers commonly used, 26 422 X-ray effect, 20 661 X-ray electromagnetic spectrum,... 

At low pressures the measuring range is limited by two effects by the X-ray effect and by the ion desorption effect. These effects results in toss of the strict proportionality between the pressure and the ion current and produce a tow pressure threshold that apparently cannot be crossed (see Fig. 3.14). 

Fig. 3.14 Apparent iow pressure iimit due to X-ray effect in a normai ionization vacuum gauge...

The electrons emitted from the cathode impinge on the anode, releasing photons (soft X-rays). These photons, in turn, trigger photoelectrons from surfaces they strike. The photoelectrons released from the ion collector flow to the anode, i.e. the ion collector emits an electron current, which is indicated in the same manner as a positive ion current flowing to the ion collector. This photocurrent simulates a pressure. This effect is called the positive X-ray effect, and it depends on the anode voltage as well as on the size of the surface of the ion collector. 

Fig. 3.15 Explanation of the X-ray effect in a conventional ionization gauge. The electrons e emitted by the cathode C cottide with anode A and trigger a soft X-ray radiation (photons) there. This radiation strikes, in part the ion collector and generates...

The pg share of the X-ray effect is the same in both cases. After determining the difference between (3.4) and (3.5), we obtain the equation for the gas pressure p ... 

The Bayard-Alpert system with modulator (see Fig. 3.16 d), introduced by Redhead, offers pressure measurement in which errors due to X-ray and ion desorption effects can be quantitatively taken into account. In this arrangement there is a second thin wire, the modulator, near the anode in addition to the ion collector inside the anode. If this modulator is set at the anode potential, it does not influence the measurement. If, on the other hand, the same potential is applied to the modulator as that on the ion collector, part of the ion current formed flows to the modulator and the current that flows to the ion collector becomes smaller. The indicated pressure p, of the ionization gauge with modulator set to the anode potential consists of the portion due to the gas pressure pg and that due to the X-ray effect pg ... 

Exposures may be shortened by putting a fluorescent screen behind and in contact with the film, so that the optical fluorescence reinforces the direct X-ray effect on the photographic emulsion or two screens might be used, one in front and one behind the film. (Note that the intensity relations are much changed by the optical fluorescence weak reflections come out relatively much too weak.)... 

Particularly at low pressures, errors may arise with hot cathode ionisation gauges because of two effects the X-ray effect and gas-ion desorption. 

Six months later Szilard wrote another paper in thermodynamics, On the decrease of entropy in a thermodynamic system by the intervention of intelligent beings, that eventually would be recognized as one of the important foundation documents of modem information theory. By then he had his advanced degree he was Dr. Leo Szilard now. He experimented with X-ray effects in crystals, von Laue s field, at the Kaiser Wilhelm Institute for Chemistry in Dahlem until 1925 that year the University of Berlin accepted his entropy paper as his Habilitationsschrift, his inaugural dissertation, and he was thereupon appointed a Privatdozent, a position he held until he left for England in 1933. 

Keller et al. [86, 89,90] proposed the existence of two families of X-ray effects in stretched commercial poly(vinyl chloride) A-crystals with an orientation of the a-axis (i.e. in the direction of the polymer backbone) in the stretch direction and B-crystals with an orientation of the c-axis (i.e. perpendicular to the polymer backbone) in the stretch direction. The B-crystals are smaller and are supposed to form the network junctions (crosslinks) of fringed micellar crystallites, whereas the large A-crystals are supposed to be well developed... 

Chest x-ray effective dose Radon in the home Man-made (medical x-rays, etc.)... 

Huang, S. S. S., Gee, N., and Freeman, G. R., Ionization of liquid argon by X-rays effect of density on electron thermalization and free ion yields, Radiat, Phys, Chem, 37, 417, 1991. 

Bayard-Alpert gauge (vacuum technology) A hot cathode ionization gauge using a fine-wire ion collector to minimize X-ray effects in the gauge. 

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