Chemistry Reference
In-Depth Information
the beam axis runs horizontally. On the outside, the focus is
observed
under the
small angle of 6
°
and
appears
as a line of 25
μ
m
×
10 mm.
Fine-focus X-ray tubes are available with several different target materials:
Au, W, Ag, Mo, Cu, Co, Fe, and Cr (listed with decreasing atomic number).
The maximum permissible power is about 2 kW for high-
Z
anodes and about
1 kW for the low-
Z
ones. The exit window is made of beryllium, which is highly
transparent for X-rays. Foils of 0.2-1 mm thickness are used, while the high-
Z
anode tubes require the thicker windows. The water consumption is 4 l/min at a
pressure of 3-5 bar and a temperature of 20-30
°
C. The tubes'life span is some
3000-6000 operational hours.
In order to achieve such a long life, some rules have to be observed. Loading
of the tube as well as switching it off should never be performed suddenly but
only as a careful step-by-step operation. The warm-up should take between 15
and 45 min, depending on the period of interruption, and the cooling-down
phase should last for 10 min. The maximum power should never be exceeded.
Voltage and current should be chosen so that their product is always less than a
maximum rating. All this should be performed under automatic control. Tube
changes, however, have to be carried out manually with utmost care.
Modern X-ray tubes are perfectly shielded and permit operation without
radiation hazard. Nevertheless, participation in a monitoring program is
recommended.
3.2.2RotatingAnodeTubes
Conventional X-ray tubes have a fixed anode and are sealed off under vacuum.
In contrast to these tubes, a new class of tubes with a rotating anode as
demonstrated in Figure 3.3 is also available. The anode is bombarded by
Figure3.3.
Rotating anode tube of Rigaku International Corporation [7] in two sectional views:
(1) cathode unit; (2) tungsten filament; (3) cylindrical anode with (3a) rotary shaft; (4) thin window;
(5) electrical connections; (6) cooling-water connection; (7) sealing gasket; (8) high-vacuum flange;
(a) line focus; (b) radiation cone. Figure from Ref. [2], reproduced with permission. Copyright
1996, John Wiley and Sons.
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