Page 318 - Biomedical Engineering and Design Handbook Volume 2, Applications

296 DIAGNOSTIC EQUIPMENT DESIGN

FIGURE 10.33 Electromagnetic lens performance.

The chromatic aberration coefficients C and C are referred to the object side and to the image side
ci
co
respectively and ±dV is the variation about the beam voltage V . The focal spot size d is found by
0
f
0
adding the components d , d , and d in quadrature according to
m s c
2
2
d = d + d + d c 2 (10.98)
f
m
s
For good focal quality, low values of C , C , and M are sought. For weak lenses, these parameters
si ci L
are dependent on the excitation parameter NI/V , where NI is the ampere-turns (excitation) of the lens.
r
In this respect, they are seen to decrease with increasing excitation parameter (Fig. 10.33b and c).
However, to seek a high value of z will result in a low value of excitation parameter, which is not the
i
desired outcome (Fig. 10.33a). Consequently, a balance must be struck between the need for close
access to the x-ray source and the need for a small focal spot size. An important lens parameter in
this respect is the pole bore-to-gap ratio B /G . If this is chosen for optimum performance, a large
L L
value of B will allow an increase in z , without a corresponding increase in aberrations. However,
L i
for a given excitation NI, a larger lens bore leads to a reduction in the value of the axial flux density.
To compensate, the size of the lens must be increased. A further compromise must be made in the
choice of the diameter d of the beam-defining aperture. This determines the value of beam current
B
I and also fixes the beam semiangle a referred to the object side, where I 0 d B 2 and a d . An
0 0 o B
increase in d produces a corresponding increase in x-ray flux Φ at the cost of an increase in focal
B
spot size d .
f
A Practical Design. The technology involved in the development of the electron beam column
conforms to the principles and practices associated with electron microscopes (Fig. 10.34). To pro-
vide versatility with optimum performance, the electron gun accommodates lanthanum hexaboride
cathodes, as well as the more conventional tungsten hairpin cathodes. Therefore, the construction
complies with standard ultrahigh vacuum (UHV) practice to avoid cathode poisoning. In this respect,
the interior vacuum envelope is fabricated from UHV-grade stainless steel, the vacuum seals are flat
copper rings (conflat), moving components are sealed with stainless steel bellows, and electrical
insulators are metal-ceramic. To maintain the required vacuum < 10 −9 mbar, the system is turbo-
pumped and all resins, rubber o rings, greases, etc., are rigorously excluded.
The electron chamber has a demountable triode electron gun with a very compact metal-ceramic
insulator profile that can support voltages up to 100 kV. Cathode replacement is a routine bench