Calculating Decay Constant Without Initial Value

[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 don't know how to calculate.

 

[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? ​

 

[Flavia]

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? ​

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

 

[tiny-tim]

divide both sides by A !

 

[Flavia]

divide both sides by A !

tq!

Can i ask another question here? If it violate rules, I am sorry. If it given initial activity 10 mci, how to know the number of atom inside?

 

[tiny-tim]

If it given initial activity 10 mci, how to know the number of atom inside?

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.​

 

[Flavia]

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.​

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?

 

[tiny-tim]

Hi Flavia!

(just got up :zzz:)

2)What is seven half life mean? Is it 7T1/2 = value?

(try using the X₂ button just above the Reply box )

yes, seven half-lives are seven times one half-life (7t_1/2)

so the amount left will be 1/2⁷

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?

not really following you

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

 

[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

 

[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.

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

What is the complete question?

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