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Suppose I have a sample containing only 137Cs.

Now assume it shows 1000 cps counts for 137Cs, 662keV energy.

Now I added 60Co (1173, 1332 keV) source in the same sample. Compton scattering background shall increase. Each around 3000 cps.

What will happen to the 137Cs 662keV peak area? Shall it remain the same or decrease from 1000 cps?

Pl omit dead time effect fully.

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    $\begingroup$ Hello, if it is a homework question, you should add a tag for clarity. Despite a potential drawbacks. $\endgroup$
    – jaromrax
    Commented Feb 12, 2019 at 14:39
  • $\begingroup$ It is not a homework Q. $\endgroup$
    – ggs
    Commented Feb 12, 2019 at 15:20
  • $\begingroup$ Ok.............. $\endgroup$
    – jaromrax
    Commented Feb 12, 2019 at 15:26
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    $\begingroup$ Are you asking about the real peak area, or the count rate in the channels where the peak is located? $\endgroup$
    – Bill N
    Commented Feb 12, 2019 at 16:32
  • $\begingroup$ @BillN - yes, good point, there is a difference, now the question would make a different sense. $\endgroup$
    – jaromrax
    Commented Feb 12, 2019 at 16:52

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A practical answer:

If you added the $^{60}$Co the way you dont shield the Cs and the rate from Co itself is not huge (e.g. it is like 100 cps):

you will not be able to detect else than 100 cps in the 662 keV peak (the dead-time will not change in an important way).

2- If you added Co with a high countrate (several thousands or more), you will experience stronger dead-time effect and you will see less of 662 keV. Supposing you have an analog acquisition system and HPGe detector, 100 cps does not make an important increase in the dead-time.

3- If your system is especially slow or bad tuned (e.g. you have a high dead-time at low countrate), the results are not too much predictable.

A theoretical answer:

Every time you add more countrate to your system, you basically remove the time for the readout and conversion - you increase the dead-time. It is possible to imagine an experiment, where you would like to study this effect. For a case of conversion time 20$\mu$s and rate 100 cps, you would need to have more than $10^6$ pulses in the peak to be in the range (with statistical errors) that would allow to look for the dead-time effect. But real problems would only start here.

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  • $\begingroup$ Sir, assuming a situation with <5% dead time with 137Cs & 60Co, can you tell me what will happen to the peak area of 137Cs? Shall it be the same? $\endgroup$
    – ggs
    Commented Feb 12, 2019 at 15:17
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    $\begingroup$ I may be missing something - isn't it clear that if the rate remains 100 cps, the peak area over the same time remains the same? What precission do you speak of? $\endgroup$
    – jaromrax
    Commented Feb 12, 2019 at 15:28
  • $\begingroup$ @ggs I extended the answer, as it is not clear what is your exact situation... $\endgroup$
    – jaromrax
    Commented Feb 12, 2019 at 15:43
  • $\begingroup$ are you clear on what is the difference of the peak area and the number of counts in the channels - as @bill-n asks? $\endgroup$
    – jaromrax
    Commented Feb 12, 2019 at 16:53
  • $\begingroup$ Peak area is area under the curve. Peak area includes no. of channels. Each channel contains some counts. Peak area is sum of the total counts under a curve. $\endgroup$
    – ggs
    Commented Mar 23, 2019 at 16:52

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