Ludlum Measurements 334A, Руководство по эксплуатации

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Model 334A Operations Manual
Ludlum Measurements, Inc 36 January 2021
known, the calibration constant and flow volume are used to derive the concentration in the
specified measurement units.
An additional term is included in the concentration calculation to account for the expected
count rate losses due to activity that has decayed off during the last interval. For longer half-
lived isotopes the term Net
0
λ goes to zero. However, this term is very important for the
proper treatment of the short-lived radon daughters.
It should be noted that in situations where the counts under a peak are changing very slowly
or not at all, slight variations may occur in the determination of the net count rate, which can
result in small negative concentrations. These effects are minimized by the use of the two
different evaluation intervals, with a longer interval providing greater precision in low-level
measurements.
Alpha Concentration Calculation
The concentration equation used for the acute and chronic alpha calculation is:
   
Eff Cal K Vol
Yield
Net T Net Net ion Concentrat
DAC Sample
W A
  
   

0 0 1
/
Where,
T
WA
= Actual window time in seconds
Net
0
= Net counts: T – 2T
WA
to T – T
WA
Net
1
= Net counts: T –T
WA
to T
Cal = Calibration constant
Eff = Detector efficiency (4-Pi)
Yield = Isotope Alpha yield
Vol
Sample
= Air volume since filter change
K
DAC
= DAC conversion factor (equals
one if DAC not used)
Lambda = decay constant,
 
½
2 ln
T


where, T
½
= isotope half-life
The flow measurement is used to accurately determine the volume of air collected on the filter
paper during the count time.
The T
WA
actual window time in seconds may be shorter than the user-defined window time.
When a new spectrum is begun, following a spectrum reset or a filter change, a window time
equal to the elapsed time, is used so that a “best guess” concentration value can be
calculated. This actual window time will increase until it is equal to the user-defined time.
This occurs when the user-defined window time has elapsed since the filter change or
spectrum reset. After this point, the user-defined time is used.