This data analysis exercise is to be completed on this worksheet. Complete the table
and staple your graph to these sheets.
AIM: to analyse a tabulated set of data and determine the half-life of cobalt 60. This
value is to be compared to the known value.
APPARATUS: sealed
60
27
Co source, clock, Geiger-Muller tube
THEORY:
The nuclei of all radioactive atoms decay or disintegrate in a random and spontaneous
manner.
The activity of a radionuclide is a scientific term used to describe the number of
nuclei in the sample that decay in a given unit of time.
The SI unit for activity is the becquerel (Bq) where 1 Bq is 1 decay per second.
The half-life of a radionuclide is the period of time during which the activity of a
sample is halved. In one half-life, the number of radioactive nuclei drops to half the
original value.
Each radioisotope has its own particular half-life. Determining this is one way in
which the isotope can be identified. The half-life cannot be altered by any chemical or
physical process.
The half-life of a given sample of a radionuclide could be found if the activity of the
sample could be measured over a period of time. The graph of this data would
typically be as shown below:
Activity
time
The half-life t1/2 can be found by determining the time that it takes for the activity to
halve.
Note that the curve approaches but does not actually reach the time axis. Eventually,
however, after a large number of half-lives, the number of ,  and  emissions will
be so low that the number actually counted and therefore the count rate may approach
zero.
Decay of 60Co
60
Co nuclei spontaneously decay to form nickel (Ni) nuclei. When first formed, these
nickel nuclei are in an excited state and immediately give up energy in the form of a
gamma ray. In this way, the nickel nuclei return to the more stable ground state.
RESULTS:
The table following gives data (collected over many years) of the activity of a cobalt60 source. A background count at each time has also been given.
This data is available on an EXCEL spreadsheet for you to analyse and graph.
Period of decay
(years)
0
0.53
1.05
1.59
2.11
2.37
2.58
2.95
3.69
4.43
5.06
7.27
8.75
13.18
14.85
17.03
18.42
Background count
(per minute)
20
20
21
20
21
21
20
22
20
19
21
18
22
23
17
22
19
Gross count
(per minute)
15360
14332
13021
12479
11647
11421
10942
10427
8820
8020
7907
5912
4922
2822
1520
1120
927
Net count
(per minute)
ANALYSIS OF RESULTS (This data is available on the student FTP site.)
1. Use EXCEL to plot a graph of the activity (net counts per minute) of 60Co against
time in years. Hand plot a line of best fit to the data.
2. Using the graph, determine the half-life of the cobalt-60 sample. t 1/2 =
3. The actual half-life of cobalt-60 is 5.27 years. Comment on the accuracy of your
result.
4.
(a)
(b)
(c)
What will happen to the half-life of the cobalt-60 sample as it ages?
It will increase
It will decrease
It will not change
(d) This depends on how the sample is stored
5. Cobalt-60 decays to form a stable isotope of nickel, a beta particle and a gamma ray.
Complete this nuclear equation by inserting the correct atomic and mass numbers.
Co Ni   
27
6. Discuss the origin of the beta particle and the gamma ray in the above decay. By what
process were they produced?
7. Use the Mega-table of radioisotopes website to determine the energy of the beta
particles. Express this value in MeV and joules.
8. Cobalt-60 is commonly used in industry for industrial radiography and in
radiotherapy for the irradiation of internal tumours. Why does its half-life make it
suitable for these applications?
9.
(a)
(b)
(c)
(d)
(e)
(f)
Which one or more of the following would alter the half-life of a cobalt-60 sample?
Incorporation in different chemical compounds
Subjecting the sample to high intensity sound
Heating the sample to extremely high temperatures
Cooling the sample to extremely low temperatures
Placing the sample in an intense magnetic field
None of the above
10. Cobalt-60 is manufactured by subjecting stable cobalt-59 to neutron irradiation in a
nuclear reactor. In this process, energy is emitted as gamma photons.
Write the balanced nuclear equation that describes this nuclear transmutation, showing
the atomic and mass numbers.
Finally, write a conclusion for this experiment.
No discussion of uncertainties is required in this experiment.