Half-life Flashcards
What is half-life?
in terms of nuclei
The time it takes for the number of nuclei of the isotope in a sample to decrease by half
What is half-life?
in terms of count rate/activity
The time it takes for the count rate/activity
from a sample containing the isotope to decrease by half of its initial level
What is count rate?
The number of decays per second recorded by a detector (e.g. Geiger-muller tube)
What are the units of count rate?
Bq
What is activity?
The number of decays per second
What are the units for count rate?
Bq
A radioactive isotope has a half-life of 15 days and an initial count rate of 200Bq.
Determine the count rate after 45 days
- Calculate how many HALF-LIVES are involved
1 hl= 15
45/15 = 3
= 3 half-lives
- Do the count rate divided by 2 for how many half-lives it is
= 3 half lives
so…
1) 200/2= 100
2) 100/2=50
3) 50/2=25
=25 Bq
Answer = 25Bq
What is the half-life of a sample where the activity drops from 1,200 Bq to 300Bq in 10 days?
1.Find out how many half-lives are involved
Original activity = 1200
1)1200/2=600
2)600/2=300
=2 half lives
- Calculate the number of days using this information
If this occurs in 10 days, then 10 days = 2 half lifes
so…
= 5 days = 1 half life (10/2=5)
Answer = 5 days
What is the net decline expressed as a ratio?
The o.g number of nuclei: remaining number of nuclei
The half-life of cobalt-60 is 5 years.
If there are 100 grams of coalt-60 how many grams will remain in 15 years
So this is asking about the net decline (the fraction of radioactive nuclei remaining)
So to find the ratio (the factor you multiply it by) you 1/2 to the power of the number of half lives
= 1 half-life = 5 years
15/5=3
so it’s 3 half lives
(1/2)3= 1/8 of the cobalt-60 will be left
100 x 1/8 = 12.5g
= 12.5g of cobalt-60 remaining
(as ratio, thats 1:0.125)
How do you calculate the net decline in a radioactive emission after a given number of half-lives?
1) find the factor by doing 1/2 (the half-life) to the power of how many half-lives
2) Multiply the factor by the original number, this will get you the remaining number of radioactive nuclei left
3) Write the answer as a ratio of og:remaining
Radioactive decay is a [blank] process
fill in the blank
Radioactive decya is a random process
A sample of caesium-137 is initially 40kBq
How long will it take for the activity of the sample to fall BY 30kBq?
(1 half life of the isotope= 30 years)
The question is NOT asking how long it will take for it to fall TO 30kBq but how long it willtake for it to fall BY 30kBq- it’s trying to CATCH YOU OUT
It’s really asking how long it will take for the activity to get to 40-30= 10 SO 10kBq
so it’s
1) 40/2= 20
2) 20/2=10
= 2 half lives
1 h-l = 30
so 2 h-l = 60
= 60 years
Answer = 60 years
Americium-241 is an alpha-emitting isotope with a half-life of 432 years
Calculate the fraction of americim-241 remaining after 1,296 years
This question is asking about the ✨net-decline✨!!
we know how to do this 😉
1 half-life = 432 years
1296/432 = 3 half-lives
= (1/2)3= 1/8
since they only want the fraction of what’s remaining, we don’t need to multply it by the o.g. (even though there is no o.g. given)
=1/8th of Americium-241 will be remaining
Answer = 1/8 remaining
Why aren’t the activity and countrate of an isotope the saame?
The count-rate is the decays per second recorded by a detector so italso detects decays produced by background radiation
