P4 Half-life (page 198)

Question 1

How quickly unstable nuclei decay is measured by?

Answer 1

using activity and half-life. Two very important terms

Question 2

Radioactiv substances give out what?

Answer 2

radiation from the nuclei of their atoms (no matter what).

Question 3

What is radiation from the nuclei measured with

Answer 3

a Geiger-Muller tube and counter.

Question 4

What do the Geiger-Muller tube and counter record?

Answer 4

the count-rate - the number of radiation counts reaching it per second.

Question 5

Radioactive decay is entirely random. What do this mean?

Answer 5

you can’t predict exactly which nucleus in a sample will decay next, or when any one of them will decay.

Question 6

Can you find out the time it takes for the amount of radiation emitted by a source to halve? Explain why?

Answer 6

Yes. This is known as the half=life. It can be used to make predictions about radioactive sources, even though their decays are random.

Question 7

What can Half-life be used to fine?

Answer 7

the rate at which a source decays - its ACTIVITY.

Question 8

Activity measured in what unit?

Answer 8

it is measured in Becquerels, Bq (where 1 Bq is 1 decay per second).

Question 9

Do the radioactivity of a source Increase or decrease over time?

Answer 9

Decrease.

Question 10

Each time a radioactive nucleus decays to become a stable nucleus, what happens to the activity?

Answer 10

as a whole it will decrease. (older sources emit less radiation).

Question 11

How long do Isotopes take before all the ustable nuclei have decayed?

Answer 11

For soe isotopes it takes just a few hours before nearly all the unstable nuclei have decayed, whist others last for millions of years.

Question 12

What is the problem with trying to measure isotopes decaying?

Answer 12

the problem is that the activity never reaches zero, which is why we have to use the idea of half-life to measure how quickly the activity drops off.

Question 13

What is meant by half-life?

Answer 13

it is the time taken for the number of radioactive nuclei in an isotope to halve.

Question 14

Half-life can also be described as what?

Answer 14

as the time taken for the activitty (and so count-rate) to fall to half of its initial value.

Question 15

The initial activity of a sample is 640 Bq. Calculate the final activity as a percentage of the initial activity after two half-lives?

Answer 15

1) Find the activity after each half-life

half-life: 640 ÷ 2 = 320.

2) Now divide the final activity by the initial activity, then multiply by 100 to make it a percentage.

half-lives: 320 ÷ 2 - 160

(160 ÷ 640) x 100 = 0.25 x 100 = 25%

(always double check what the question is asking for - it may want a fraction, ratio or a percentage).

Question 16

Can you measure Half-life using a Graph?

Answer 16

Yes

Question 17

If you plot a graph of activity against time, what will happen?

Answer 17

it will always be shaped like the one on page 198.

Question 18

How is the half-life found from the graph? (see graph on page 196)

Answer 18

by finding, the time interval on the bottom axis coresponding to the halving of the activity on the verticle axis.

Question 19

Half-life is…..

Answer 19

the time for the number of radioactive nuclei, the activity or the count-rate to halve.

Question 20

The initial count-rate of a sample is 40 cps. Show that the ratio of its final count rate to its initial count rate is 1:8 after three half-lives (3 marks).

Answer 20

After one half-life the count-rate will be
40 ÷ 2 = 20 cps (1 mark).

After a second: 20 ÷ 2 = 10 cps
After a third@ 10 ÷ 2 = 5 cps (1 mark)

so the ratio is 5:40 = 1.8 (1 mark).