35040
Operators Manual
or dielectric absorption. The start and stop thresholds are set independently, but the stop threshold must be less than the start threshold. The thresholds are settable in 0.1 pA increments from 0.0 pA to 9.9 pA. In some test setups, it may be necessary to raise the thresholds to reliably detect an exposure in the presence of a relatively large leakage current. Since the start threshold determines the minimum dose rate, it may be necessary to lower the thresholds for some exposures.
DoseRate = I [A] × ICCF [Units/C] × ADCF × RateMultiplier [s]
The minimum dose rate is
where I[A] is the start threshold setting.
When the start and stop thresholds are set to 0.0 pA, the automatic detection of start and end of exposure is disabled. This effectively means that the instrument is measuring as if an exposure has been running continuously. In this mode of operation, leakage current is not measured, the average current is measured from the last time RESET/MEASURE was pressed, and charge will accumulate due to leakage current.
NOTE
Thresholds other than those used by the calibration agency may render inaccurate measurements.
2.3.2 Exposure Time
Historically, the determination of exposure time for 60Co has involved multiple exposures to eliminate the effect of the shutter speed. By using the total charge and the average current for the exposure, the effective exposure time can be determined using a single exposure. If the average current varies by more than 10% during the exposure, this measurement is considered invalid and will be replaced by “INVALID.”
⎛ | Q2 [C] − Q1 [C] | ⎞ | ⎛ | Q [C] | ⎞ | |
IAvg = ⎜ | ⎟ | |||||
| ||||||
⎜ | T 2 [s] − T1[s] | ⎟ | Time = ⎜ |
| ⎟ | |
| ||||||
⎝ | ⎠ | ⎝⎜ IAvg [A] ⎠⎟ | ||||
The effective exposure time is derived from the total charge and the average current. In order to get enough data points to accurately calculate the time, the exposure must be a minimum of 3 seconds. To avoid artifacts at the start and end of exposure, the average current is calculated over the interval starting 1 1/3 seconds after the start is detected to 1 1/3 seconds before the end of exposure.
