asha1494
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Hi everyone,

I would be grateful could help me out on this question that I've been stuck on for a while now on radioactivity in A2 level physics, or just a point in the right direction would be appreciated!

So here it is:

A radioactive sample, whose disintegration product is stable, was known to have an activity of 5x10^11 Bq at a certain time.

At that time an alpha particle detector showed a count rate of 32 per second.

Exactly 10 days later the rate had dropped to 8 per second.

Question:

a) Find the half life, the decay constant and the number of radioactive nuclei present in the sample at the start.

I thought you would start by finding the decay constant, but I just can't see the way to do it.

Thank you :-)
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Phichi
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(Original post by asha1494)
Hi everyone,

I would be grateful could help me out on this question that I've been stuck on for a while now on radioactivity in A2 level physics, or just a point in the right direction would be appreciated!

So here it is:

A radioactive sample, whose disintegration product is stable, was known to have an activity of 5x10^11 Bq at a certain time.

At that time an alpha particle detector showed a count rate of 32 per second.

Exactly 10 days later the rate had dropped to 8 per second.

Question:

a) Find the half life, the decay constant and the number of radioactive nuclei present in the sample at the start.

I thought you would start by finding the decay constant, but I just can't see the way to do it.

Thank you :-)
You have the count rate dropping by \dfrac{1}{4} in 10 days.

What does that tell you?

Formula for decay constant involving half life is?

Finally use the formula for activity and number of nuclei.
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MSB47
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(Original post by Phichi)
You have the count rate dropping by \dfrac{1}{4} in 10 days.

What does that tell you?

Formula for decay constant involving half life is?

Finally use the formula for activity and number of nuclei.
Could you use C=Co e-(decay constant)(t) and re arrange to find the decay constant?
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Phichi
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(Original post by MSB47)
Could you use C=Co e-(decay constant)(t) and re arrange to find the decay constant?
I would suggest the following:

Use the information about the count rate quartering over 10 days to find the half life. Then use T_{\frac{1}{2}} = \dfrac{ln2}{\lambda} to find the decay constant.

Then finally, A = \lambda N for the initial number of nuclei.

If you haven't seen these equations before, let me know.
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MSB47
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(Original post by Phichi)
I would suggest the following:

Use the information about the count rate quartering over 10 days to find the half life. Then use T_{\frac{1}{2}} = \dfrac{ln2}{\lambda} to find the decay constant.

Then finally, A = \lambda N for the initial number of nuclei.

If you haven't seen these equations before, let me know.
Oh ok that seems more straight forward but would the other way work too?
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