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35040
Operators Manual
2-6
Figure 2-2. Leakage Current and Start of Exposure
When automatic exposure detection is disabled (start and stop threshold set to 0.0 pA), there is no
correction for extrapolated charge due to leakage current. The dosimeter must be manually reset
between exposures and the leakage current kept as low as possible for best results.
Exposure Charge and Dose
The charge on the electrometer during an exposure is due to the incident radiation plus the charge that
was on the electrometer before the exposure plus the extrapolated charge due to any leakage currents in
the ion chamber, cables, or electrometer. To obtain only the charge due to the exposure, the instrument
employs the following equation:
The exposure charge variable Q
Exposure
is set to zero at the start of an exposure either manually by
pressing the RESET/MEASURE button or automatically if the start and stop thresholds are used. Over
the duration of the exposure, Q
Exposure
and Dose are updated about once a second. The displayed
measurement value (4-1/2 digit) is held following the exposure.
Timed Charge and Dose
This measurement accumulates charge only while the elapsed time is less than or equal to the timer
interval setting. The displayed value is held following the completion of the timer interval.
()()
[s] [s] [A] [C] [C] 12Leakage12Timed TTIQQQ
−
×−−=
ADCFICCFQDose TimedTimed ××= [Units/C] [C]
Time
0
- Charge samples in exposure.
- Extrapolated charge due to leakage current
QOffset
- Charge samples in Leakage.
Charge
- Samples discarded.
TOffset
Start of Exposure
TStart
X
X
X
X
ADCFICCFQDose Exposure ××= [Units/C] [C]
(
)()
[s] [s] [A] [C] [C] OffsetLeakageOffsetExposure TTIQQQ
−
×
−−=