Abstract
The article intents to show that correction formula for gasfilled detectors has to contain the halflife constant of specimen, i.e. that characteristic of specimen may have influence on measurement in some rear situations. There is given a more accurate formula for correction of deadtime interval for gasfilled detectors and its deriving procedure:
(9)
Above formula is theoretically more acceptable than classic one.
DEAD TIME CORRECTION OF NON PARALYZING X RAY DETECTORS
In the following text the next symbol's definitions are adopted:
 t_{p}
  detector's time of paralyzing, i.e. dead time interval,
 λ
  constant of radioactive decay for nucleus,
λ = Ln(2) / T_{1/2} ,N_{0}   the initial number of non decayed nucleuses,
 N
  the number of decayed nucleuses,
 M
  the number of detected decays,
 k
  paralyzing factor.
The estimation of non detected particle's decays is based on the simple
relation of radioactive decay that compute the number of decayed nucleuses for the dead time of
detector paralysis t_{p}:
The value of paralyzing coefficient k is obtained by dividing the left and the right side of the above equation with the number of non decayed nucleuses for the time of paralysis:
(2)
Now it is possible to derive the difference equation of non paralyzing detector:
(3)
This equation has to have the next initial condition:
(4)
The variable M in difference equation is the number of
Mth detected decay.
On the left side of difference equation is the number of really decayed nucleuses.
On the right side is the number of really decayed nucleus before Mth decay, plus a just
detected one, plus the estimation of non detected nucleuses that have come on the just
detected decay. It is necessary to be known the number of non decayed nucleuses after
detection of Mth decay while the esteem of non detected decays is obtained via
multiplication of coefficient k with the number of non decayed nucleuses because the
paralysis begins after detection of that decay after that there is certainly a non
decayed nucleus less.
The solution of that difference equation is:
(5)
After substituting the value for k in previous equation, it is obtained:
(6)
Suppose that the speeds of counting and decay are known
(
(7)
There is also a valid formula:
(8)
(9)
If we assume that are
(10)
The relation between λ and T_{1/2} is given by next formula:
(11)
The classic correction formula for non paralyzing detectors could be simply derived from
this formula by introduction of the linear approximation of the exponential function
Absolute value of relative error of classic formula (13) related on the formula (9) is given by the next
expression:
(12)
The formula (9) shows that halflife constant of specimen may have influence on
measurement performed with the gasfilled detector especially when the decay's chain of specimen contains
extremely shortliving isotopes with the similar order of magnitude as the deadtime constant of
detector.
The classic formula
(13)
does not include the specimen halflife constant at all. If we assume that is:
(14)
Then the fist part of equation (10) that involves influence of decay constant of specimen to counting result will be transformed to:
(15)
The following table contains correction values for some characteristic ratios
between


0 
1 
¼ 
0.91585771041165242343 
½ 
0.83670266201424625056 
1 
0.69314718055994530941 
2 
0.46209812037329687294 
REFERENCES
G. F. Knoll, Radiation Detection And Measurement, (John Wiley & Sons, Inc., 2nd edition 1989).
N. Tsoulfanidis, Measurement And Detection Of Radiation, (McGraw Hill Book Company, 1983).
Author of this article and all
its formulas is Andrija S. Radovic´,
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Author:
Dipl. Ing. Andrija
S. Radović
Tel: +381 64 1404075