Calculating activity using half life

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SUMMARY

The discussion focuses on calculating the activity of potassium-40 (40K) in the human body, specifically for an adult weighing 75 kg. The participant initially calculated the decay constant using the formula 0.693 divided by the half-life of 40K, which is 1.28 billion years, resulting in a value of 6.18x10^-14. However, the participant's calculation of the number of potassium-40 atoms and subsequent activity resulted in a discrepancy, as the expected activity is 4.39x10^-6, prompting questions about the mass of potassium-40 and the appropriate units for the final answer.

PREREQUISITES
  • Understanding of radioactive decay and half-life concepts.
  • Familiarity with the decay constant calculation.
  • Knowledge of Avogadro's number and its application in chemistry.
  • Basic proficiency in unit conversions and dimensional analysis.
NEXT STEPS
  • Review the calculation of decay constants in radioactive decay.
  • Study the concept of natural abundance and its implications in isotopic calculations.
  • Learn about the application of Avogadro's number in determining the number of atoms in a sample.
  • Investigate the units of radioactivity, specifically becquerels (Bq) and their relevance in activity calculations.
USEFUL FOR

Students in nuclear physics, health physicists, and anyone involved in radiological calculations or studying the effects of radioactive isotopes in biological systems.

Kira127
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The question is that the human body contains 0.2% potassium by weight and the natural abundance of 40K is 0.0118%. The half life of 40K is 1.28x10^9 years and I have to calculate the activity in an adult weighing 75 kg. What I tried was to find the decay constant by 0.693/1.12x10^13 hours = 6.18x10^-14. Then I took the mass and divided it by the 40K 14.11kg/40 * 6.02x10^23 = 2.12 x10^23. Then I tried Activity=lambda*N =6.18x10^-14*2.12x10^23=1.31x10^10. The answer is supposed to be 4.39x10^-6 and I don't know what I'm doing wrong if someone could help me that would be very much appreciated.
 
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A couple of questions:

(1) If you weigh 75kg, and 0.2% of that is K, and 0.0113% of THAT is K40, do you really think there are 14kg of K40 in your body?

(2) Do you really want the time constant in 1/hr?

(3) If you think the final answer is 4.39E-6, what are the units of that?
 

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