Backscattered particles

Fig. 11. Energy distribution of the positive, negative and neutral atoms backscat-tered from a stainless steel (304) surface for 10 keV D bombardment. The ion beam was at normal incidence and the backscattered particles had been observed at an angle of 45 deg. to the normal70 ...

In contrast to the previous section where we considered infrared radiation that passed through a thin film and was partially absorbed, we now consider emission from a thin film when we bombard it with radiation. Therefore, we now consider a variety of excitation sources and an equally large number of emitted or backscattered particles or photons. For excitation sources, we have ... 

Because RBS is rather insensitive to light elements and unable to detect hydrogen, one can make use of the complementary technique Elastic Recoil Detection (ERD) when sensitivity for light elements is required. In this case, recoiled particles are detected instead of the back scattered particles. The incident beam usually consists of heavier ions, e.g. 2 Si (4He is sufficient when one is interested in H and D only), and a stopper foil prevents backscattered particles from entering the detector, whereas the lighter recoiled particles are transmitted [32]. 

As a result one gets the intensity of the backscattered particles as a function of their energy. By applying the formula for the energy loss of ions in matter one converts this into a depth below the surface scale for a homogeiieous target. If the target contains more than one element, the situation is more complicated because of... 

Eq. (1) shows that the energy of the backscattered particle is a function of the incident particle and target atom masses, the scattering angle, and incident energy. 

We illustrate the analysis of the formation of Ni-silicide in Fig. 5, which plots backscattering yield versus the kinetic energy of the backscattered particles whose incident energy was 2.0 MeV. The solid line is the spectrum from a 200 nm layer of Ni deposited on Si. The thickness is indicated by the width of the energy signal. 

The layered samples were analyzed with RBS/channeling method using 3 MV single stage accelerator at TLARA, JAERI/Takasaki. The analyzing beams of He ions with energy of 1.5 to 2.7 MeV were incident on samples. The size of the beam was about 1 mm in diameter and beam current was about 10 nA typically. Backscattered particles were detected by standard surface barrier detectors at 160° and 110° to the incident beam. 

A source cannot be placed in midair. It is always deposited on a material that is called source backing or source support. The source backing is usually a very thin material, but no matter how thin, it may backscatter particles emitted in a direction away from the detector (Fig. 8.15). To understand the effect of backscattering, assume that the solid angle in Fig. 8.15 is ft = 10. Also assume that all the particles entering the detector are counted, self-absorption is zero, and there is no other medium that might absorb or scatter the particles except the source backing. 

Figure 8,15 The source backing material backscatters particles and necessitates the use of a backscattering factor /. ...

RBS can provide absolute quantitative analysis of elemental composition with an accuracy of about 5%. It can provide depth-profile information from surface layers and thin films to a thickness of about 1 pm. In some cases, however, the high-energy beam can damage the surface. This is particularly a problem with insulating materials, such as polymers, alkali halides, and oxides. The Mars Pathfinder mission in 1997 contained an alpha proton X-ray spectrometer (APXS). In its RBS mode, the spectrometer bombarded samples with alpha particles and determined elemental composition via energy analysis of the backscattered particles. In addition to RBS, the APXS instrument was designed to carry out proton emission and particle-induced X-ray emission (PIXE) experiments. Soil and rock compositions were measured and compared to those from the earlier Viking mission. 

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