Fluke Biomedical 35080B - Manual (Page 24)

35080B
Operators Manual
5-2
5.2 Specifications
Intended Use
Beryllium window, molybdenum anode mammographic generators
Range
27.5 kVp to 29.5 kVp
Accuracy
±0.5 kV
Sensitivity
0.1 volt per kV
Zero
27.0 kV
Cal
0.75 V
Minimum Anode Current
50 mA
Target to kVp Divider Distance
12 in (30.5 cm)
Effect of Molybdenum Filtration
Negligible, 12 to 35 microns
5.3 Theory of Operation
The Cadmium K-edge Mammo Filter Pack (37355) contains two different filters side-by-side. One is
constructed of cadmium, an element whose x-ray attenuation increases in a step-fashion at its K-edge
(26.7 kV). The other filter is made of an element like aluminum that has no K-edge in the mammographic
region. These two filters are selected to have identical attenuation characteristics for photon energies
less than 26.7 kV.
The Model 35080B kVp Divider takes the ratio of the x-ray signals passing through the two filters. As long
as the applied voltage to the x-ray generator is less than 26.7 kV, the output of the kVp Divider is constant
(approximately 3 volts). When the applied voltage exceeds 26.7 kV, the ratio and thus the output voltage
increases rapidly. The filters are carefully manufactured so that the voltage increases at 0.1 V/kV above
27.0 kV. For molybdenum anode, beryllium window generators, there is very little effect on the calibration
for changes in molybdenum filtration from 12 to 35 microns. As shown by Figure 5-2, the addition of
aluminum filtration does influence the sensitivity. Adding 0.25 mm of aluminum can increase the kVp
divider's reading by up to 0.5 kVp.