- Consider counting a sample over a period of time and note the following points
- The longer you count a sample the greater your counts
- The greater your counts the smaller the SD
- So how many counts are enough to be confident the a given value?
- The inherent error of measurement of any count rate is determined by the following formula where
- c = courting rate
- t = time over which the counts are acquired
![sd2sd countingrate.jpg - 5538 Bytes](images/sd2sd%20countingrate.jpg)
- As an example let us say that you have acquired a total of 7345 counts in 5 minutes. What is the SD for this value at a 95% level of confidence?
![sd2sd countingrateex1.jpg - 6130 Bytes](images/sd2sd%20countingrateex1.jpg)
- Based on the calculation above there is a 95% certainty that the true counting rate will fall between 7268 to 7422
- One should note that based on the above data if you increase your counting time you increase your level of accuracy
- HW - From the above data what are your SDs for 68% and 99%?
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