Absorption, photoelectric

The energy level from which the electron is ejected depends upon the energy of the gamma-ray. The most likely to be ejected is a K electron. If sufficient energy is not available to eject a K electron, then L or M electrons will be ejected instead. This gives rise to the discontinuities in the photoelectric absorption curves. These 

The probability that a photon will undergo photoelectric absorption can be expressed as a cross section, t. This measure of the degree of absorption and attenuation varies with the atomic number, Z, of the absorber and the gamma-ray energy, E fmdi complicated manner  

The photoelectric attenuation coefficient, p-pg, can be derived from the related cross-section in the following manner  

It is normally assumed that photoelectric absorption results in the complete absorption of the gamma-ray. However, for those events near to the surface of the detector there is a reasonable probability that some fluorescent X-rays, most likely the K X-rays, might escape from the detector. The net energy absorbed in the detector would then be  

---

Energy Spectrometry (EDS) uses the photoelectric absorption of the X ray in a semiconductor crystal (silicon or germanium), with proportional conversion of the X-ray energy into charge through inelastic scattering of the photoelectron. The quantity of charge is measured by a sophisticated electronic circuit linked with a computer-based multichannel analyzer to collect the data. The EDS instrument is... 

Figures 2b and 3 demonstrate that X rays are produced over a range of depth into the sample. The X rays must propagate along a finite path through the specimen to reach the detector, and are subject to photoelectric absorption and scattering, which follows an exponential relation ...

The X-ray spectrum observed in PIXE depends on the occurrence of several processes in the specimen. An ion is slowed by small inelastic scatterings with the electrons of the material, and it s energy is continuously reduced as a frmction of depth (see also the articles on RBS and ERS, where this part of the process is identical). The probability of ionizii an atomic shell of an element at a given depth of the material is proportional to the product of the cross section for subshell ionization by the ion at the reduced energy, the fluorescence yield, and the concentration of the element at the depth. The probability for X-ray emission from the ionized subshell is given by the fluorescence yield. The escape of X rays from the specimen and their detection by the spectrometer are controlled by the photoelectric absorption processes in the material and the energy-dependent efficiency of the spectrometer. 

The basis of XRE analysis is the photoelectric absorption and the subsequent emission of X-ray photons characteristic of the fingerprints of analyte atoms in the sample. Element composition can be quantified by the relative intensities of the indivi-... 

Fig. 1-11. Fate of an x-ray beam. Two types of events occur as an x-ray beam is absorbed in matter—more precisely, as x-ray quanta disappear from the beam. These types of events are photoelectric absorption, measured by r, and scattering, measured by a. (See Eq. 1-8.)...

The values of r and a in Table 4-2 show that photoelectric absorption generally makes the greater contribution to the mass absorption coefficient. Absorption leading to scattering, which will be discussed in the next section, gains in relative importance as atomic number Z and wavelength X decrease. 

Wavelength of Primary Beam, A Exciting Potential, V Scattering Coefficient, a Photoelectric Absorption Coefficient, T cr/r... 

The most probable fate of a photon with an energy higher than the binding energy of an encountered electron is photoelectric absorption, in which the photon transfers its energy to the electron and photon existence ends. As with ionization from any process, secondary radiations initiated by the photoelectron produce additional excitation of orbital electrons. 

Far from the Laue condition the absorption shows the normal photoelectric absorption, as would be measured (with allowance for density) in a liquid or gas of the same atomic species. Close to the Laue condition, the absorption is quantified by the imaginary parts of the susceptibilities, leading to imaginary components of the wavevectors. These imaginary components are always normal to the crystal surface and hence the planes of constant attenuation are parallel to the surface. The attenuation coefficient (n) normal to the surface is given by (n)=-4 lm(K o) (4.28)... 

In this circumstance, it is more reliable to measure the absolute integrated intensity of the substrate peak and compare this with the integrated intensity from an equivalent crystal of the substrate material on which no layer has been grown. In the angular position for diffraction from the substrate, the layer will not diffract and the substrate peak intensity will be simply reduced by normal photoelectric absorption. For a symmetric reflection, it is easy to see that the integrated intensity 1 of the substrate peak with the layer of thickness t present is related to the integrated intensity of the bare substrate / o by... 

The high degree of X-ray polarisation in the electron orbit plane provides means of controlling both the signal/noise ratio and the penetration of the X-rays into the specimen. Depending on whether the incidence plane is chosen vertically or horizontally, sigma or pi polarisation may be selected. The strain sensitivity and the extinction distance can thus be varied while the normal photoelectric absorption conditions remain identical. 

Figure 5 X-ray mass attenuation coefficients for aluminum as a function of photon energy. At low energies, photoelectric absorption predominates. At higher energy, incoherent (Compton) scatter becomes almost the exclusive contributing mode. Eventually, pair production dominates at very high energies (above 10 MeV).

Photoeiectron, Auger-Electron (by Photoelectric Absorption) Compton Recoil Electron (by Compton Scattering)... 

As the column depth of the supernova ejecta decreases as t 2, 7-rays and hard X-rays suffer from less Compton scattering and photoelectric absorption and eventually emerge from the surface. The timing of their emergences and light curves are sensitive to E, Menv, and the distribution of elements. In other words, X-ray and 7-ray observations provide another good diagnosis of the supernova interior. 

To estimate the critical epoch for the emergence of the X-ray flux we need to balance the effects of energy degradation through multiple-scattering and photoelectric absorption. A photon of energy Eo has its energy reduced to... 

---