(b) The lowest tube current station available
on the control panel exceeds the maximum
continuous rated current for the maximum
rated tube potential, and
(c) Dental x-ray tubes may have a specified
duty cycle in addition to the maximum
continuous rated tube current.
If the ionization chamber cannot be positioned one meter
from the source in certain radial directions the readings
can be corrected using the inverse square law.
Radiographic x-ray tubes normally have a maximum
continuous rated tube current of 3-5
mA
at maximum kVp.
This value must be obtained from the unit's tube rating
chart. If, for example, the lowest
mA
station on the unit
is 50
mA
and the maximum "continuous"
tube current is 5
mA, an exposure rate leakage reading must be scaled by
l/10 (5 mA/50mA) to obtain the leakage rate at the maximum
continuous rated tube current.
If the
mA
station and tube
ratings remain the same, but an integrated leakage reading
using 100 mAs is measured, multiply the measured reading
by 180/hr to obtain the correct leakage rate.
[2]
An integrated leakage reading is preferred on radiographic
x-ray tubes to prevent long exposures
and excessive heat
loading
to the tube.
I
f
the lowest
mA
station available 'exceeds the
maximum continuous rated tube current on a dental x-ray
tube, the correction factor is calculated as described in
the previous paragraph.
However, many dental x-ray tubes
also have a specified duty cycle, eg. 6 seconds/minute.
The leakage reading in this case must also be multiplied
by this duty cycle, 1/10, to obtain the leakage radiation
rate at the tube's maximum continuous tube current.
The following
recommendations
should be followed
during leakage radiation measurements:
2)
3)
4)
5)
1) Select the lowest tube current station.
Select an exposure time that is appropriate
for the ionization survey meter's response
time.
(Most survey meters have a response
time of 4 to 5 seconds.)
Select the highest tube potential allowable.
(Consult the tube rating chart.)
Do not exceed the total heat capacity of
the anode and the x-ray tube housing
during the survey.
Close-the collimator blades and block
the collimator port with at least 10
HVL equivalent of lead.
49
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