Half Life. Measuring Radioactivity

1. Half Life

2. Measuring Radioactivity

› We can measure the amount of radiation given
off by a radioactive source by measuring the
number of clicks or counts on a Geiger-Muller
tube in a certain amount of time.
› The is called the activity or the count rate of a
radioactive sample.
› It is measured in units of:
– counts per minute (cpm); or
– counts per second (cps)
also known as becquerels (bq)
› In practice is is necessary to subtract any counts
coming from background radiation.

3. Losing Atoms

› If we have a sample of radioactive material,
atoms inside the sample will be decaying at
random so we are left with fewer and fewer of the
original parent atoms as they change into
daughter atoms of a different type.
› After a certain amount of time we will be left with
½ the original number of parent atoms
› In the same amount of time again we will be left
with ½ the number again. Or ¼ of the number of
parent atoms.

4. Half life

› The half life is the time taken for the number of
parent atoms in a radioactive sample to halve.
• During this time the mass of parent atoms will
fall by half and the activity, or count rate, from
the sample will fall by a half;
• We can also say that
• The half life is the time taken for the activity
of the original substance to fall to half its
initial level.

5.

Example
› Imagine starting with 400
Time
radioactive atoms
(Hours)
› Over time the number of
0
parent atoms decreases
1
› In 2 hours the number of
2
parent atoms falls to 200.
3
› Half the original number.
4
› The half life is 2 hours
5
› In 2 more hours the
number of atoms has fallen 6
to 100.
› Half again.
Number of
Parent Atoms
400
280
200
140
100
70
50

6. Decay Curve

Number Of Parent Atoms
Decay
Curve
400
300
200
100
0
0
1
2
3
4
5
6
Time (Hours)
Half Life = 2 hours

7. Measuring Half Life

› Scientists measure the number of counts
recorded in a Geiger counter placed near the
radioactive sample.
› Over time the number of counts decreases.
› The scientist can draw a graph of the number of
counts per minute against time.
› They use the graph to work out the half life.

8.

Activity (bq)
20
15
10
5
0
0
2
4
6
8
10
12
Time (minutes)
Half life = 5 minutes

9.

10.

11.

12. Draw decay curve (Half-Life 3 days)

13. Alpha,beta and gamma radiation

14. Blooket game

› https://dashboard.blooket.com/set/645b73a8bade9ac1446
b41d2

15. Quizizz game

› https://quizizz.com/admin/quiz/5a0a4561c28d6211006312
b7/4-balancing-nuclear-equations?source=search-resultpage&page=search-result-page&arid=d3a29b56-cb4e4bb9-b68b7a1c6f6849a3&apos=0&term=Alpha+beta+gamma+and+n
uclear+equations