atomic physics - half life

Question 1

what is the best way to describe decay

Answer 1

it is a random event

Question 2

what is a half life used for

Answer 2

measuring the rate of decay

Question 3

rules of half-lives

Answer 3

the shorter the half-life, the faster the decay
isotopes with longer half-lives are more dangerous as they stay radioactive for longer

Question 4

what is a half-life (2)

Answer 4

the time taken for the number of radioactive nuclei in a sample to halve

the time taken for the count rate of a sample to halve

Question 5

what is count rate measured in

Answer 5

counts per minute (cpm)
counts per second (cps)
Becquerels (Bq)

Question 6

EXAMPLE
a radioactive isotope has an activity of 328cpm
three hours later, the activity had reduced to 41cpm
calculate the half life of this isotope

Answer 6

METHOD 1
1) find how many times the activity halves to reach its new value
2) divide the total time by the number of half-lives

1 HL - 164
2 HL - 82
3 HL - 41
–> 3 half lives in 3 hours
3/3 = 1 half life per hour

METHOD 2
1) express the initial and final count rates as a fraction (initial as denominator)
2) simplify where you can –> the denominator is then put in the format bⁿ where b = 2 and
n = the power to make the format become the denominator
3) n = how many half-lives have passed
4) divide the total time by n to determine the length of one half-life

41/328 = 1/8
1/8 = 2^3
3 is how many half-lives have passed
3/3 = 1 half life per hour