Zeiss INTRABEAM, Instructions For Use Manual

Page: 39 / 82

Version 6.2
G-30-1767-en
Page 39
INTRABEAM Water Phantom Operation
– Slowly move the probe tip along the X or Y axis while observing the
current value reading on the electrometer display. As long as the
current value keeps increasing, go on adjusting the probe tip in this
direction.
– When the current value starts to decrease, you have passed the exact
target point, i.e. the center. Set the probe tip back again slightly. Vary
your setting back and forth around this position until you have set the
maximum current value. On reaching the setting with the maximum
current value, you have reached the exact center (1).
– Lock the positioning of the probe tip using the clamping screw on the
corresponding micrometer screw.
• Search for the exact center position above the measuring window for the
probe tip in the other coordinate axis as well.
Precisely positioning the probe tip in the vertical (Z) direction
Using current measurements from the electrometer to check the Z position
The probe tip has been centered above the measuring window of the IC ion-
ization chamber:
• Release the locking lever on the Z direction adjustment knob.
• Move the probe tip downward until it almost touches the measuring
chamber (1).
• Watch the current reading on the electrometer display and move the
probe tip further downwards. You can see that the current reading
increases accordingly. The current value remains constant as soon as the
probe tip touches the measuring chamber.
• If you continue moving the probe tip downward and the current value re-
mains constant, you have exceeded the exact target point.
• Slightly move the probe tip upward again and vary its position until you
have obtained the maximum current value.
• The position where the maximum current value is displayed is the exact Z
position of the probe tip.
• Lock the probe tip in this Z position using the locking lever on the Z direc-
tion adjustment knob.
NOTE Incorrect determination of the probe tip position leads to a faulty depth dose
curve.
•
Determine the zero point by the method described here and calculate the
distance from the IC ionization chamber using the formulae specified be-
low.