Solutions: Radioactivity Questions
Q4.The radioisotope iodine-131
is used in medicine to treat over-active thyroid glands.
It decays into an isotope of xenon (Xe) by β– emission with a half-life of 8.1 days.
The xenon subsequently emits a γ ray.
(a) Explain what is meant by:
(i) isotope;
Isotopes are nuclides (or atoms) of the same element. They have the same number of protons
in the nucleus but a different number of neutrons.![](../../../../graphics/symbols/nuclides/ticksmall.png)
(2 marks)
(ii) half-life.
The half life of a radioactive nuclide is the time taken for half ofthe number of nuclei of that nuclide (not just 'atoms') in the sample to decay.
or
The half life of a radioactive nuclide is the time taken for the activity of the sample to fall to fall to half its initial value. ![](../../../../graphics/symbols/nuclides/ticksmall.png)
(1 mark)
(b) Write down the equation which represents the nuclear reaction.
![](../../../../graphics/equations/decay/iodine131.png)
(3 marks)
(c) Calculate the time (in days) for a sample of iodine to decay to 1% of its initial activity.
![](../../../../graphics/equations/dataSheet/radioactivity.png)
Thalf = ln 2/λ
λ = ln 2/Thalf
λ = ln 2/8.1 days-1
λ = 0.0856 days-1![](../../../../graphics/symbols/nuclides/ticksmall.png) |
N/No = A/Ao = 0.01
N = No e-λt
0.01 = e-λt![](../../../../graphics/symbols/nuclides/ticksmall.png)
ln 0.01 = -λt
t = (ln 0.01)/0.0856 days![](../../../../graphics/symbols/nuclides/ticksmall.png)
t = 53.8 days
time = 54 days ![](../../../../graphics/symbols/nuclides/ticksmall.png)
|
(4 marks)
(d) State and explain which decay product can be detected outside the body during treatment.
The γ radiation detected outside the body as its penetration power is great enough to penetrate tissue,
The β– cannot penetrate tissue. It would be absorbed by the body.![](../../../../graphics/symbols/nuclides/ticksmall.png)
(2 marks)
(Total 12 marks)