Photon backscatter

Scanning electron microscopy (SEM) scans over a sample surface with a probe of electrons (5-SO kV). Electrons (and photons), backscattered or emitted, produce an image on a cathode-ray tube, scanned synchronously with the beam. Magnification of 20-50,000 are possible with a resolution of about 5 nm. There is a very high depth of field and highly irregular structures are revealed with a three-dimensional effect. 

Photon backscatter Scans the ground with a pencil-thin beam of X-rays. X-rays produce scattered returns from objects that are collected by detectors on either side of the vehicle and processed. Early in development. Has large power needs, slow speed, and a small footprint. Can change soil characteristics and harm life forms and equipment. Has a high data processing requirement. 

Advanced sensor technologies with application to detection and clearance can be grouped as follows infrared sensors, ground-penetrating radars, microwave, photon backscatter, nuclear or thermal neutron analysis, and lasers. Their characteristics were summarized in Table 12.1. 

Burchanowski C., Moler R., and Shope S., Scanned-beam x-ray source technology for photon backscatter imaging technique of mine detection Advanced technology research, SPIEProc., 2496, 368-373, 1995. 

Extended X-ray absorption fine structure (EXAFS) measurements based on the photoeffect caused by collision of an inner shell electron with an X-ray photon of sufficient energy may also be used. The spectrum, starting from the absorption edge, exhibits a sinusoidal fine structure caused by interferences between the outgoing and the backscattered waves of the photoelectron which is the product of the collision. Since the intensity of the backscattering decreases rapidly over the distances to the next neighbor atoms, information about the chemical surroundings of the excited atom can be deduced. 

Radiative Saturation. Higher levels of radiation create a larger population in the excited state, allowing stimulated emission to become a competing process. In this process, atoms in the excited state absorb photons, which re-emit coherently that is with the same frequency, phase and direction as the incident photon. Thus stimulated emission does not produce backscat-tered photons. As the incident energy increases, a greater proportion of the excited atoms absorb a photon and produce stimulated emission before they decay naturally. The net result is that the population of atoms available to produce backscatter decreases, i.e., the medium saturates. 

Figure 7. Depiction of origin of EXAFS. An X-ray photon is absorbed by A, resulting in the photoionization of a core-level electron represented as an outgoing ( + ) photoelectron wave which is backscattered (<- ) by a near neighbor, B.

The ejected photoelectron may be approximated by a spherical wave, which is backscattered by the neighboring atoms. The interference between the outgoing forward scattered, or ejected, photoelectron wave and the backscattered wave gives rise to an oscillation in the absorbance as a function of the energy of the incident photon. These oscillations, which may extend up to 1000 eV above the absorption edge, are called the EXAFS, extended X-ray absorption fine structure. Analysis of the EXAFS provides information regarding the identity of, distance to, and number of near neighboring atoms. 

In transmission electron microscopy (TEM), a beam of highly focused and highly energetic electrons is directed toward a thin sample (< 200 nm) which might be prepared from solution as thin film (often cast on water) or by cryocutting of a solid sample. The incident electrons interact with the atoms in the sample, producing characteristic radiation. Information is obtained from both deflected and nondeflected transmitted electrons, backscattered and secondary electrons, and emitted photons. 

The surface sensitivity is ensured by detecting the decay products of the photoabsorption process instead of the direct optical response of the medium (transmission, reflection). In particular one can measure the photoelectrons, Au r electrons, secondary electrons, fluorescence photons, photodesorbed ions and neutrals which are ejected as a consequence of the relaxation of the system after the photoionization event. No matter which detection mode is chosen, the observable of the experiment is the interference processes of the primary photoelectron with the backscattered amplitude. 

Calibration of the intensities of the radiation flelds is traceable to the NIST. The ionization chambers and electrometers used by the service laboratories to quantify the intensity of the radiation fields must be calibrated by the NIST or an accredited secondary standards laboratory. The intensity of the field is assessed in terms of air kerma or exposure (free-in-air), with the field collimated to minimize unwanted scatter. Conversion coefficients relate the air kerma or exposure (free-in-air) to the dose equivalent at a specified depth in a material of specified geometry and composition when the material is placed in the radiation field. The conversion coefficients vary as a function of photon energy, angle of incidence, and size and shape of backscatter mediiun. 

BARTLETT, D.T., FRANCIS, T.M. and DIMBYLOW, P.J. (1989). Methodol-ogy for the cahbration of photon personal dosemeters Calculation of phantom backscatter and depth dose distributions, Radiat. Prot. Dosim. 27, 231-244. 

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