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atomic structure and radioactivity
half life
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uthman Afzal
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Cards (9)
Half-life
The time it takes for the number of
nuclei
of a
radioactive
isotope in a sample to
halve
Determining half-life
1. Observe the time it takes for the number of
undecayed
nuclei to
halve
2. Observe the time it takes for the
count rate
to
halve
if Radioactive isotope has a long half-life
Nuclei decay relatively slowly
if Radioactive isotope has a short half-life
Nuclei
decay
relatively
quickly
Scientists cannot predict when a specific nucleus will decay, as
radioactive decay
is a
random
process
Determining half-life from a graph
Start with
1000 undecayed
nuclei, time taken for
1000
to become
500
is the half-life
Calculating decrease in count rate after a given number of half-lives
1. Count rate
halves
after each
half-life
2. Calculate the
count rate
after the given number of
half-lives
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.
count rate = 25
counts
/
s
After 45 days, which is 3 half-lives, the count rate will have
halved 3
times, going from 200 to
100
to
50
to
25
counts
per
second.
