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Count rate

What would the count rate be after for 24 seconds for source A

Count rate.JPG

I don't know how to get the answer help.
Reply 1
Avatar for 05sykesd
05sykesd
Find the half life of source 1 from the graph (4 seconds) ans then find the decay constant by doing ln2/Half life

Then use this in the equation A = A0 * e^-kt where K=decay constant, t=24 and A0=origional count rate = 80

A=80*e^-(ln2/T)24
A=1.3ish
Reply 2
Avatar for zed963
zed963
OP
16
Original post by 05sykesd
Find the half life of source 1 from the graph (4 seconds) ans then find the decay constant by doing ln2/Half life

Then use this in the equation A = A0 * e^-kt where K=decay constant, t=24 and A0=origional count rate = 80

A=80*e^-(ln2/T)24
A=1.3ish


The answer is 15 but I don't know how to get to that answer.
Original post by zed963
What would the count rate be after for 24 seconds for source A

Count rate.JPG

I don't know how to get the answer help.


How long does it take for the activity of A to fall to half? (120 to 60)

Spoiler



24 seconds is a whole number of this half-lives.

Spoiler


Each half life means the count rate is halved.
So what is it after that many half lives.
Reply 4
Avatar for 05sykesd
05sykesd
Damn! did it for the wrong source -_________- one day ill learn to read!!!
Reply 5
Avatar for zed963
zed963
OP
16
Original post by Stonebridge
How long does it take for the activity of A to fall to half? (120 to 60)

Spoiler



24 seconds is a whole number of this half-lives.

Spoiler


Each half life means the count rate is halved.
So what is it after that many half lives.


i'm not following.
Original post by zed963
i'm not following.


Look at the graph for A.
The activity falls from 120 to 60 in 8 seconds.
That means 8 seconds is one half life because that's how long it takes the activity to fall to half the original value.
That means, every 8 seconds the activity halves.
So after 8 seconds its activity is 60
After another 8 seconds (=16 seconds) the activity will halve again. (to 30)
What will it be after 24 seconds?
Reply 7
Avatar for zed963
zed963
OP
16
Original post by Stonebridge
Look at the graph for A.
The activity falls from 120 to 60 in 8 seconds.
That means 8 seconds is one half life because that's how long it takes the activity to fall to half the original value.
That means, every 8 seconds the activity halves.
So after 8 seconds its activity is 60
After another 8 seconds (=16 seconds) the activity will halve again. (to 30)
What will it be after 24 seconds?


15.

I'm getting confused with Becquerel and count rate please can u define it.
Original post by zed963
15.

I'm getting confused with Becquerel and count rate please can u define it.


Becquerel is a measure of activity and is the number of disintegrations per second.
Count rate (from a detector) gives a measure of how the activity of a sample is varying.

Count rate does not equal activity in becquerel because the detector will not detect all the disintegrations.
However, count rate is a measure of activity.
If the count rate drops to half then the activity of the sample has dropped to half.
That's what is happening in this question. The count rate tells us the activity has fallen to half after 8 seconds.
It does not tell us the activity in becquerel, though.

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