Voltage acceleration

Figure Bl.17.8. Iron oxide particles coated with 4 nm of Pt in an m-planar magnetron sputter coater (Hennann and Mtiller 1991). Micrographs were taken in a Hitachi S-900 in-lens field emission SEM at 30,000 primary magnification and an acceleration voltage of 30 kV. Image width is 2163 nm.

If only ions with a single charge (z = 1) are considered, then with a constant magnetic field strength and constant accelerating voltage, the radius of arc depends on mass and, from Equation 24.3, Equation 24.4 is obtained. 

V = accelerating voltage at the ion source E = electric-sector voltage B = magnetic-field strength. 

Therefore, for accurate mass measurement, a standard mass peak (M,) is selected, and the accelerating voltage (V) is changed until the sample ion peak (M ) exactly coincides with the position of Mj. This technique is called peak matching, and the ratio between the original and new voltages (VA ) multiplied by mass (Mj) gives the unknown mass, M . 

The accelerating voltage V for ions leaving the source and the electric-sector voltage E for energy focusing are the two electric ones. The field B is the magnetic one. 

Accelerating voltage (high voltage) scan. An alternative method of producing a momentum (mass) spectrum in magnetic-deflection instruments. This scan can also be used, in conjunction with a fixed radial electrical field, to produce an ion kinetic energy spectrum. 

It should be noted that for TEM at accelerating voltages of 100-400 keV the specimen thickness must be of the order of 10-100 nm which requires dedicated preparation techniques [2.173, 2.176, 2.178]. 

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