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)
The initial activity of a sample is 640Bq. Calculate the final activity as a percentage of the initial activity after two half-lives
1 half life = 640 /2 =320
2 half-lives = 320/2 = 160
(160/640) x 100 =0.25 x 100 = 25%
this is the decline of activity or count-rate after a certain number of half-lives as a percentage of the original activity
A radioactive isotope sample has a half-life of 40s
The initial activity of the sample is 8000 Bq
The radioactive source is left until its activity falls to 100Bq
Calculate the final activity as a percentage of initial activity
100/8000 x 100
= 1.25%
Define radioactive contamination
Radioactive contamination is the unwanted presence of materials
containing radioactive atoms on other materials
What is the hazard from contamination caused by
The hazard from
contamination is due to the decay of the contaminating atoms
What affects the level of hazard (from contamination)
The
type of radiation emitted affects the level of hazard.
How to reduce irradiation
keeping sources in lead-lined boxes, standing behind barriers with lead-glass screens, being in a different rom and using remote-controlled arms when working with radioactive sources - ways of reducing irradiation
Define irradiation
Irradiation is the process of exposing an object to nuclear radiation (e.g. alpha, beta, gamma or neutrons)
The irradiated object does not become ______
An object does not become ______ after being irradiated
explain why?
The irradiated object does not become radioactive.
An object does not become radioactive after being irradiated
(since the object only comes into contact with the radiation - not the radioactive isotope itself)
Describe what is meant by radiation can be ionising
Radiation can be ionising - it can form charged atoms called ions
Risk of ionising radiation
Can increase the risk of cancer in humans
Describe the precaution we can take against ionising radiation (IRRADIATION/ radioactive contramination)
Shielding
wearing protectvie lead Gloves can protect against alpha radiation
Use tongs when handling
A lead apron can be used to protect against beta and gamma radiation
With high levels of radiation, lead walls and a lead-glass screen (glass containing lead) can be used
(beta and gamma radiation can be reduced using a lead apron)
Monitoring
With a radiation monitor, we can measure how
limit exposure time
keep the sample in a lead-lined box when not in use
Explain why radioactive contamination is hazardous
It is hazardous as the radioactive atoms decay and emit ionising radiation
could get a large dose of radiation
Explain the risk of alpha radiation
Strongly ionising but easily stoped by dead cells on the skin surface (easily blocked by a small air gap)
Alpha emitters can be dangerous if inhaled (e.g. on contaminated dust) or swallowed (or contaminated food). They can do all their damaged in a very localised area.
So contamination, rather than irradiation is the major concern when working with alpha sources
Explain the risk of beta radiation
Outside the body, beta and gamma are the most dangerous
Quite ionising and can penetrate skin into the body (where they can damage cells)
Less damaging inside the body since radiation is absorbed over a wider area, and some passes out of the body altogether
High levels of irradiation from all sources are dangerous, but especially from ones that emit beta and gamma
Explain the risk of gamma radiation
Outside the body, beta and gamma are the most dangerous -can penetrate body
Weakly ionising. Can penetrate body but likely to pass straight through
Gamma are the least dangerous inside the body
High levels of irradiation from all sources are dangerous, but especially from ones that emit beta and gamma
describe the importance of peer review
Over the years, scientists have explored the effects of radiation on humans
It is really important that these studies are published and then shared with other scientists
This allows the findings to be checked (PEER REVIEW)
