Radioactive decay

[Ice-cream]

Can any1 help me with this question?

The radioisotope cesium-137 has a half-life of 30 years. A sample decayed at the rate of 544 counts per minute (cpm) in the year 1985. In what year will the decay rate by 17cpm? (ans: 2135)

[Sayonara]

I'm sure someone will be able to help if you say which part of the question you are stuck on.

[swansont]

Can any1 help me with this question?

The radioisotope cesium-137 has a half-life of 30 years. A sample decayed at the rate of 544 counts per minute (cpm) in the year 1985. In what year will the decay rate by 17cpm? (ans: 2135)

 

[math]A = A_0 e^-^\lambda^t[/math]

 

^{[math]\lambda[/math]} = ln(2)/t_1/2

 

A is activity (decays per unit time)

 

Have at it...

[Gilded]

Talking about this sort of calculations, in my recent math test I had something like this: C-14 has a half-life of 5700 years (yeah, actually it's 5730). Then I had to make a function that has x as the amount of years passed, and with an outcome that tells how much of the C-14 is still left. Like if the x is 5700, then the outcome is 0.5, and if the x is 1, the outcome is something like 0.999 or so. I had NO idea how to do this sort of calculations (no matter how much we've practiced it before the test ). What's the correct function for this?

[swansont]

^{[math] A=\lambda N[/math]} Pop that into the equation for activity and you get # of atoms as a function of time.

 

You can also use N=N₀(1/2)ⁿ, where n is the number of half-lives that have passed. You should be able to convince yourself that the two equations are equivalent.

[Gilded]

Oh bugger. I would have earned 6 (of maximum 36) points if I knew it was that easy.

 

So... When a year has passed, the C-14 amount is approximately 99.9879% of the original amount (if we use 5730 years as the half-life time)?

[Ice-cream]

does any1 know of complex type questions of radioactive decay coz i need some practice but i'm always getting the same sort of questions...or does any1 know of any sites thats got some more complex sort of radioactive decay questions?

[swansont]

So... When a year has passed, the C-14 amount is approximately 99.9879% of the original amount (if we use 5730 years as the half-life time)?

 

Right.