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 Nuclear and Particle Physics Nuclear and Particle Physics Help Forum Mar 18th 2018, 06:00 PM #1 Junior Member   Join Date: Feb 2018 Posts: 8 Decay Rate of Cs Time Hello! Here is the question give to me by my teacher: A sample of 137Cs (half-life = 30.1 years) decays at a rate of 10^10 decays per second. How long will it take for the decay rate to decrease to 10^7 decays per second? The work I have done so far is: dN(t=0) = -λNo, where dN(t) = Noe^(-λt) (Although online a website as it as Noe -λt, so not sure if λt should be raised or not) Because I am trying to find the difference I can make this formula equal to itself: Noe^(-λt) = Noe^(-λt) Where: λ is my decay constant (10^10 and 10^7) e is e (2.71828) t is the time (guessing at 0 and x (what the question is asking for)) No is the amount - which I do not know Overall, the part I am confused with is if this is even correct, and if so how do I find the No of the amounts?   Mar 18th 2018, 08:31 PM #2 Senior Member   Join Date: Apr 2017 Posts: 499 I don't like using formulae and equations unless absolutely necessary, they tend to stop the thinking process ...Better derive the equation myself from first principles , The question is how long will it take for the decay rate to reduce a thousand fold ... When the Half life 30.1 years after one half life it's a half....2HL it's a 1/4 .....3 HL it's an 1/8 ..... 4 HL it's a 1/16th So if X is the number of half lifes then the radiation will be One over 2 to the power X 2 to the power 10 is 1024 ... but we want the number which will give a 1,000 ...it should be just less then 10. Using my calculator I get an exact value of 9.9658 half lives = Almost exactly 300 years ( 299.97) topsquark and studiot like this. Last edited by oz93666; Mar 18th 2018 at 08:36 PM.  Tags decay, rate, time Thread Tools Show Printable Version Email this Page Display Modes Linear Mode Switch to Hybrid Mode Switch to Threaded Mode Similar Physics Forum Discussions Thread Thread Starter Forum Replies Last Post ling233 Nuclear and Particle Physics 2 Oct 25th 2014 02:02 PM MikeReynolds Special and General Relativity 4 Feb 10th 2013 12:28 AM scottseptember1992 Nuclear and Particle Physics 0 Jun 3rd 2012 10:33 PM vyomictor Nuclear and Particle Physics 0 Mar 19th 2011 06:40 AM werehk Nuclear and Particle Physics 1 Jun 28th 2008 03:26 AM