Half life Flashcards
Define half-life
The half-life of a radioactive isotope is the time it takes for the number of (radioactive) nuclei of the isotope in a sample to halve
the time it
takes for the count rate (or activity) from a sample containing the
isotope to fall to half its initial level.
What effect does nuclear radiation have on living cells?
(ii) damages them / changes DNA
damage tissue/kill cells
can cause cell mutations
How can you tell if an isotope has a relatively long half life
This isotope has a relatively long half-life
We can tell this because it is decaying relatively slowly - it will take a long time for the number of nuclei of the original isotope to halve
How can you tell if an isotope has a relatively short half life
This isotope has a relatively short half-life
We can tell this because it is decaying relatively quickly- it will take a short time for the number of nuclei of the original isotope to halve
A radioactive isotope can be called more stable if it has a ______
A radioactive isotope can be called more stable if it has a longer half life
A radioactive isotope can be called less stable if it has a ___
An isotope can be called less stable if it has a short half lie
https://filestore.aqa.org.uk/sample-papers-and-mark-schemes/2022/june/AQA-8464P1H-QP-JUN22.PDF
question 6.4
D B A C
explanation
a substance with a longer half-life has more stable nuclei
so answers are in order of
increasing half-life
https://bam.files.bbci.co.uk/bam/live/content/ztyxy4j/large
what is the half life of this isotope
Five days.
go from half initial value to the line, then go down
then half that value again, draw a line across, then go down
these time difference between these two points should be the same
https://bam.files.bbci.co.uk/bam/live/content/zsgpgdm/large
what is the half life of this isotope
1.3 billion years
go from half initial value to the line, then go down
then half that value again, draw a line across, then go down
these time difference between these two points should be the same
https://bam.files.bbci.co.uk/bam/live/content/zy68srd/large
estimate the half life of this isotope
between 6 and 7 hours
go from half initial value to the line, then go down
then half that value again, draw a line across, then go down
these time difference between these two points should be the same
A radioactive isotope has a half-life of 15 days and an initial count-rate of 200 counts per second
Determine the count-rate after 45 days
After each half-life, the count rate would have halved
The half life is 15 days so 45 days is 3 half lives
This means the count rate would have halved three times
45/15 = 3 half lives
Start = 200 countss/s
After 1 half life - 200/2 = 100 counts/s
After 2 half lives= 100/2 = 50 ocunts/s
After 3 half lives = 50/2 = 25 counts/s
Start Count rate = 200 counts/s
15 days Count rate = 100 counts/s
30 days Count rate = 50 counts/s
45 days Count rate = 25 counts/s
Uranium decays into lead
The half life of uranium is 4,000,000,000 years
A sample of radioactive rock contains 7 times as much lead as it does uranium
Calculate the age of the sample
The sample was originally completely uranium
8/8 of the sample was uranium
1 half life later —>
Now only 4/8 of the uranium remains - the other 4/8 is lead
1 half life later —->
Now only 2/8 of uranium remains - the other 6/8 is lead
1 half life later —–>
Now only 1/8 of uranium remains - the other 7/8 is lead
(7 times as much lead as it does uranium)
It takes 3 half lifes for the sample to decay until only 1/8 remained (which means that there is 7 times as much lead).
Each half life is 4,000,000,000 years so the sample is 12,000,000,000 years old
The half life of oxygen-15 is two mins.
What fraction of oxygen-15 will remain after 5 half-lives
100 % = initial amount
—> 1 half life later = 50%
——> 1 half life later = 25%
—> 1 half life later = 12.5%
——> 1 half life later = 6.25%
—> 1 half life later = 3.125%
3.125% = 0.03125 = 1/32
ANOTHER METHOD
1/2^5 = 1/32
It takes 35 days for a 512 gram sample of element X to decay to a final amount of 4 grams.
What is the half life of element X?
512
1 half life later —-> 256g remaining
1 half life later —-> 128g remaining
1 half life later —-> 64g remaining
1 half life later —-> 32g remaining
1 half life later —-> 16g remaining
1 half life later —-> 8g remaining
1 half life later —-> 4g remaining
7 half lives
If each half-life lasts the same amount of time then each half life = 35/7 = 5 days
ANOTHER METHOD
512/4 = 128
128 = 2^7
7 HALF LIVES
35/7 = 5 days
Use the table to determine the net decline, expressed as a ratio, after 6 half-lives.
Time (days)
Total number of half-lives
Count rate (counts/s)
0 0 8,000
4 1 4,000
8 2 2,000
12 3 1,000
16 4 500
20 5 250
24 6 125
count rate at start = 8,000 count/s
count rate after 6 half-lives = 125 counts/s
net decline = 125/8,000
= 1/64th
(Note that this is equal to: 1/2 x 1/2 x 1/2 x 1/2 x 1/2 x 1/2)
