# Decay constant

1. Jun 27, 2012

### Flavia

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. Jun 27, 2012

### tiny-tim

Welcome to PF!

Hi Flavia! Welcome to PF!

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

what do you get?

3. Jun 27, 2012

### Flavia

Re: Welcome to PF!

It becomes A/5 = A exp -λ(2).. how can i get λ as the A is not given?

4. Jun 27, 2012

### tiny-tim

divide both sides by A !

5. Jun 27, 2012

### Flavia

tq!

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?

6. Jun 27, 2012

### tiny-tim

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.​

7. Jun 27, 2012

### Flavia

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?

8. Jun 28, 2012

### tiny-tim

Hi Flavia!

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

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

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

the decay rate depends on the radioactive material, and how much of it there is at any time

9. Jun 29, 2012

### Flavia

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

10. Jun 29, 2012

### tiny-tim

Hi Flavia!

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.
What is the complete question?

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

Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook