X photon

If an intensifier, such as the 85 mm presented here, is now replacing the screen, a relative gain of the order of x50 is obtained which results in a conversion factor of 1 to 7.5 (1 incident X photon --> 7.5 electrons). This conversion efficiency not only resolves the quantum sink problem but also increases the light level significantly to compensate for the low gamma fluxes obtained from radioactive sources. 

It is well known that y or X photons have energies suitable for excitation of inner electrons. We can use ultraviolet and visible radiation to initiate chemical reactions (photochemistry). Infrared radiation excites bond vibrations only whereas hyperfrequencies excite molecular rotation. In Tab. 1.1 the energies associated with chemical bonds and Brownian motion are compared with the microwave photon corresponding to the frequency used in microwave heating systems such as domestic and industrial ovens (2.45 GHz, 12.22 cm). 

Photopotential as a fimction of wave length of photons for zinc oxide and cadmium selenide electrodes in aqueous solutions X. = photon wave length. [From Williams, I960.]... 

X photon/pulse. (Reproduced with permission from... 

In addition to the coherent scattering just discussed, X photons can scattered incoherently, that is, with a change in wavelength. This effect can be described with classical mechanics by considering the incident photons and the electrons as particles and by describing their interactions as collisions, as shown in Figure 1.2. 

The X-ray sources we have just described in the previous sections emit X photons by de-exdtation of atoms that underwent electronic transitions. The origin of synchrotron radiation is different. 

The chamber s sensitivity increases with the fraction of absotbed radiation, so a heavy gas is generally used. Argon at atmospheric pressure absoibs the X photons of copper s peak almost completely. The sensitivity depends on the wavelength and therefore the intensities of two beams with different wavelengths can be compared. 

This type of detector enables the user to measure the number of X photons and also to know where the ionization center of the gas molecules is along the anode [CHA 70]. This makes it a one-dimensional (or two-dimensional) detector [GAB 78, SUL 94] that can measure the function 1 = f(x) (or I = f (x,y)), where x is the position. The position is determined by measuring the time difference for the signal to travel along the anode between the impact point and one of its ends. A multichannel analyzer memorizes the number of hits per channel. 

In these detectors, as for the previous ones, the X photons are transformed into light photons. However, the measurement is then achieved by transforming these light photons into electrons and into an electrical signal by way of a photocathode. A diagram explaining how such a device works is shown in Figure 2.22. 

The scintillator that transforms X photons into light photons is made of crystals, most often Nal doped with thallium, Csl doped with thallium, Cal2 doped with europium and ZnS doped with silver. 

X-rays are absorbed by a diode made from a semiconductor single crystal of silicon or germanium doped with lithium. The X photons produce electrons that induce electronic transitions in the valence and conduction bands of the silicon atoms. 

A voltage applied between the two faces of the semiconductor enables the user to measure the electric charge, which is proportional to the energy of the incident X photons. A diagram of such a device is shown in Figure 2.24. 

The sample is a fine cylinder usually comprised of a capillary filled with the powder we wish to study. It can also be made of a thin wire, particularly when the material we wish to study is a metal. In any case, this sample is placed in the center of a curved, position sensitive gas detector used to simultaneously detect all of the diffracted beams. We saw before that when these gas detectors are irradiated with X photons, the gas is ionized and a local avalanche effect takes place which leads to the ionization of the neighboring atoms. The size of this ionized zone depends on... 

We have already said that traditional soiuces produce divergent beams. Therefore, the production of parallel beams requires the use of adequate optical elements which will simultaneously modify the trajectory of the X photons and... 

Figure 3.2. Description of the path of the X photons after taking into account the axial and equatorial divergences of the incident and diffracted beams [MAS 01]...

The number of photons absorbed by a molecule A in a wavelength region X to X + dX is the product of its absorption cross section oA(X) (cm2 molecule-1), the spectral actinic flux I(X) (photons cm-2 s-1 nm-1), and the number concentration of A (molecules cm-3) ... 

Molecules of AB reacting by pathway x Photons of wavelength. A, absorbed... 

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