• Support PF! Buy your school textbooks, materials and every day products Here!

Certain isotope tell you that the decay rate decreases

  • Thread starter xinlan
  • Start date
61
0
1. Homework Statement

Measurements of a certain isotope tell you that the decay rate decreases from 8337 decays/minute to 3174 decays/minute over a period of 5.00 days.

2. Homework Equations

R=Ro.e^-lamda/t
then T1/2 = ln(2)/lamda

3. The Attempt at a Solution
I converted all the units..
t = 5 days = 432000 seconds
R = 8337 decays/minute = 52.9 decays/s
Ro=3174 decays/minute = 138.95 decays/s
I did:
R=Ro.e^-lamda/t
52.9=138.95 . e^-lamda*432000
lamda = -1.86*10^-6

I am confused.. the lamda should not be a negative number right?

because half-life should be a positive number..
please help me..
 
Last edited by a moderator:

Answers and Replies

Astronuc
Staff Emeritus
Science Advisor
18,542
1,684
Well, if one take R0 to be the lesser activity and the later activity to be greater, then one will get a negative lamdba since the activity is increasing.

Usually R(t) < R0, i.e. R(t) is decreasing from R0.


If one takes the ln of the decay equation, one can make it linear.

Start with R0 = 8337 decays/minute, then R(t=5days) = 3174 decays/minute, but make sure units/time are on the same basis.
 
61
0
I should convert all the units right?

then if the lamda is negative, the half-life will be negative, and should I just plug in the minus sign as well on the computer?
 
61
0
I got it.. :)
 

Related Threads for: Certain isotope tell you that the decay rate decreases

  • Last Post
Replies
5
Views
1K
  • Last Post
Replies
5
Views
1K
Replies
2
Views
1K
Replies
5
Views
2K
Replies
1
Views
1K
Replies
5
Views
1K
Top