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Homework Help: Measuring radioactivity

  1. May 6, 2005 #1
    It is necessary to determine the strength of a radioactive source. Counting equipment has been made available for only two hours. How should the time be schedule if:

    The net source counting rate is 7 times the background rate.

    I mean I would say that you would use relatively short counting periods since the count rate seems reasonably high.
    But it sounds like you have say more than that.
    Just to clarify as well the net source count rate is the
    (measured count rate from source - measured background rate) right?
    Any advice would be great
  2. jcsd
  3. May 6, 2005 #2


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    Do you know the source and its half-life, or are you trying to determine if there is a change in the activity during the two hours?
  4. May 6, 2005 #3
    no you don't know anything at all.
    All the information is there.
    You just have to write down how you would schedule the the time given that you have the counter for 2 hours.
    But all I can really think of is that since the number of counts seems high you would use short counting periods over the two hours.
    Although that isn't really a schedule so I don't really know what is expected of me as such.
  5. May 6, 2005 #4


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    Gross activity is net (source) activity + background. Seven times background is usually pretty low activity.

    What does your text book say about counting statistics?

    Also, are you supposed to take several time periods during the 2 hr period in order to determine the half-life, so as to project activity outside the 2 hr period? Of course, low activity could mean 1) long half life, 2) very few atoms of the radionuclide, or 3) well shielded. Is this a confined or distributed source?

    One would need at least 2 counting periods to determine half-life, but 3 would be better, and 4 even more so. If there would be a daughter radionuclide, more counting intervals might be needed.

    One could also do attentuation measurements during this period. Can you think of how you would differentiate between {alpha, beta, gamma}?

    Here is a nice little summary on counting in low activity with low background.
    http://www.detectors.saint-gobain.com/Media/Documents/S0000000000000000003/tinlow%20background.pdf [Broken]
    Last edited by a moderator: May 2, 2017
  6. May 6, 2005 #5
    yeah sorry what I meant when I said reasonably high was that it is high in relation to the other second situation which is the net source count rate is equal to the background rate.
    Yes I think it is suggesting that you would need to break your measurements up into certain time periods that span over the two hours.
    It doesn't actually say it is confined or distributed in the question.
    alpha, beta and gamma have different attenuation curves so you could differentiate between them that way by taking measurements taken at various ranges.
  7. May 7, 2005 #6
    oh and I think it's just parent decay because it doesn't mention anything about daughters at all.
  8. May 11, 2005 #7
    Well I've found an answer to a similar sort of question to the one I was trying to understand here.
    But it is just an answer and not a solution so I have no clue how they got it.
    It is the same question above but with this one the counting device is only available for 1 hour.
    (a) the net source count rate is about 5 times the background rate
    (b)the net source count rate is 10 times the background rate.

    ANS: Ts= source counting time in hours, Tb=background counting time in hours

    (a) Ts = 0.711 and Tb= 0.289
    (b) Ts = 0.586 and Tb = 0.414

    So at least now I know that I got that question wrong when I did it :( but if someone has an idea of how they calculated that I would appreciate any hints on understanding how to do those sorts of questions :)
  9. May 11, 2005 #8
    didn't you post a thread about books on this course in the gen disc. forum? if this an the other thread you posted here are the kind of problems you have to solve have a look at knoll: radiation detection and measurement.
  10. May 12, 2005 #9
    yes I did and thanks a lot for recommending that book.
    It really helps :)
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