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Decay constant

  1. Jun 27, 2012 #1
    Radioactive sample activity is said decreases by factor 5 during 2-h interval. How to find the decay constant? If the given initial value is not given? I dont know how to calculate.
     
  2. jcsd
  3. Jun 27, 2012 #2

    tiny-tim

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    Welcome to PF!

    Hi Flavia! Welcome to PF! :wink:

    Call the initial value "A", and write out an equation to show when it reaches A/5 …

    what do you get? :smile:
     
  4. Jun 27, 2012 #3
    Re: Welcome to PF!

    It becomes A/5 = A exp -λ(2).. how can i get λ as the A is not given?
     
  5. Jun 27, 2012 #4

    tiny-tim

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    divide both sides by A ! :smile:
     
  6. Jun 27, 2012 #5
    tq!:smile:

    Can i ask another question here? If it violate rules, im sorry. If it given initial activity 10 mci, how to know the number of atom inside?
     
  7. Jun 27, 2012 #6

    tiny-tim

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    you mean mCi ?

    see http://en.wikipedia.org/wiki/Curie
    The curie (symbol Ci) is a non-SI unit of radioactivity, named after Marie and Pierre Curie. It is defined as
    1 Ci = 3.7 × 1010 decays per second.​
    Its continued use is discouraged.

    Curies are occasionally used to express a quantity of radioactive material rather than a decay rate, such as when one refers to 1 Ci of cesium-137.

    This may be interpreted as the number of atoms that would produce 1 Ci of radiation. The rules of radioactive decay may be used convert this to an actual number of atoms. They state that 1 Ci of radioactive atoms would follow the expression:
    N (atoms) * λ (1/s) = 1 Ci = 3.7 × 1010 (Bq)​
    and so,
    N = 3.7 × 1010 / λ​
    where λ is the decay constant in s-1.​
     
  8. Jun 27, 2012 #7
    Really helps. tq! another question

    1)If given half life. how to get initial decay rate?

    From half life, i can get the λ.
    Ro = λNo. how to get No?

    2)What is seven half life mean? Is it 7T1/2 = value?
     
  9. Jun 28, 2012 #8

    tiny-tim

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    Hi Flavia! :smile:

    (just got up :zzz:)
    (try using the X2 button just above the Reply box :wink:)

    yes, seven half-lives are seven times one half-life (7t1/2)

    so the amount left will be 1/27
    not really following you :confused:

    the decay rate depends on the radioactive material, and how much of it there is at any time
     
  10. Jun 29, 2012 #9
    Hi! i dint find the the X2 button just above the Reply box.

    1)If given half life. how to get initial decay rate?
    -the question is, the half life of Ga-67 is 78 hours. Calculate initial decay rate
     
  11. Jun 29, 2012 #10

    tiny-tim

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    Hi Flavia! :smile:

    If you click "QUOTE" or "NEW REPLY" or "Go Advanced", you get to a page with buttons above the Reply box, and symbols to the right. :wink:
    What is the complete question? :confused:

    (in other words: what is the meaning of "initial"?)
     
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