Ludlum Measurements 334A, Руководство по эксплуатации

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Model 334A Operations Manual
Ludlum Measurements, Inc 37 January 2021
Alpha Dose Calculation
The Model 334AB program calculates dose according to the activity collected on the filter.
Because the Model 334AB is typically used to continuously monitor the air in a facility, the
concept of “worker dose” is dependent upon the portion of time the worker spends in the
facility in comparison to the time the Model 334AB filter has been accumulating activity.
For instance, if a unit has been sampling air at 1 CFM for 40 hours, the worker dose is
dependent on how many of the 40 hours the worker spent inside the room being monitored.
Therefore, the determination of worker dose must be accomplished administratively by
noting the dose readings when the worker enters and leaves the area and calculating the
increase in dose.
Eff Cal T
Yield Net
Activity
W A
 


1
 
60
 

Sample DAC
Flow K
Activity
Dose
Where,
T
WA
= Actual window time in seconds
Net
1
= Net counts from T – T
WA
to T
Cal = Calibration constant
Eff = Alpha efficiency
Yield = Isotope Alpha yield
Flow
Sample
= Flow rate
K
DAC
= DAC conversion factor
(equals one if DAC not used)
Minimum Detectable Measurement Calculations
Uncertainties in Peak Counts
In most ROI-based Alpha-CAM’s, the minimum detectible concentration is based on the
statistical uncertainty of the background counts that fall within the region, in combination with
the uncertainty of any counts in excess of the background. In the Model 334A, the minimum
detectible concentration (MDC) is not directly related to the spectrum counts, but to the
uncertainty in the peak fit.
An examination of the peak area variances confirms that when the counts due to an interfering
isotope greatly outnumber the counts due to the isotope of interest, the variance of the isotope of
interest increases. Likewise, when there are very few interfering counts from other isotopes, the
variance begins to approach a value equal to the peak area counts.
When spectrum counts are zero, there is still a minimum variance of the peak areas. This
minimum variance affects the minimum detectable levels of dose, concentration and activity. At
the start of operation, the program calculates and saves the minimum variance for the Isotope of
Interest for use later in the calculation of the minimum detectable levels.