Counting Times


Counting Times, Figure 1

Count Times

There are four count Time parameters which may be modified:

  1. The Default Background Counting Time,
  2. The Rapid Counting Rate Change, Sigma,
  3. The Default Subject Counting Time, and
  4. The Acquisition Testing Interval.
We shall now examine each of these parameters.
Default Background Time

The background counting time should always be at least as long as the subject counting time. Preferably, it should be muh longer, suh as 1000 seconds. If the background changes frequently by several standard deviations from the mean, then the background counting times should be kept relatively short with respect to the rate of change of the background. This condition might prevail during discharge of reactor off gases or during the movement of vehicles containing radioactive wastes. On the other hand, if the background is quite steady, then a long background counting time is most appropriate.

There are many organizations that employ a 60 minute bakground counting time.

Helgeson recommends that 240 seconds (four minutes) be used routinely for "Quicky" counters and 1000 seonds (16.7 minutes) for the "Do-It-Yourself" Lay Down Diagnostic Counter.

Rapid Counting Rate Change, Sigma

Because the background can change during a short period of time, Helgeson provides a means of minimizing false positive or false negative results. This is done by looking at the rate of change in the background counting rate over relatively short counting intervals, suh as a few seconds or perhaps 10 to 30 seconds.

It is possible for the counting rate to change by 4-sigma due to random chance only, but the probability is only 0.00013, or only 0.013 perent. If the counting rate changes by more than the sigma value set in this variable, the background count is aborted and started again.

Helgeson recommends that a value of 4 (4-sigma) be used routinely.

Subject Counting Time

As a general rule, the smaller the quantity of In Vivo radioativity you wish to detect, the longer should be the subject counting time. If you employ Sequential Analysis and if the Type 1 and Type 2 errors are 10% and 1%, respectively, then the values given below for 60-cobalt show typical counting times as a function of the nancourie level of the action point:

Action Point for 60-Cobalt
Minimum Subject Time
nCi
Seconds
50
10
40
30
30
60
20
120
10
240
Aquisition Testing Interval

Beause the background can change during a short period of time, Helgeson provides a means of minimizing false positive or false negative results. This is done by looking at the rate of change in the counting rate over relatively short counting intervals, such as a few seconds or perhaps 10 to 30 seconds. This time period is called the "Aquisition Testing Interval." Routine use the appropriate entry might be 10 seconds.

This is the end of the section on Counting Times