O-LevelPhysicsNuclear Physics5054/21 · May/June 2017 · Q85 Marks

A doctor has the choice of two isotopes, ¹³¹₅₃I and ¹²⁵₅₃I, to inject into a patient. These isotopes emit radiation from deep inside the body. The radiation is detected outside the body and provides information to help the doctor find out what is wrong with the patient. The isotope ¹³¹₅₃I decays into a stable isotope of xenon by emitting a beta-particle (β) and a gamma ray (γ).

(a)State what is meant by a gamma ray.[1]
(b)Complete the equation to show the radioactive decay of ¹³¹₅₃I. ¹³¹₅₃I → …..Xe + …..₋₁β + γ[2]
(c)When a nucleus of ¹²⁵₅₃I decays, only a gamma ray is emitted. Suggest why it is better for the doctor to inject the isotope ¹²⁵₅₃I into the patient.[2]

Mark scheme

Marking points from the official mark scheme (May/June 2017). The mark or mark type is shown in brackets.

[1] (high frequency) electromagnetic wave / ray
[1] top line of beta 0
[1] Xe proton and neutron numbers both balance
[1] gamma rays pass out of/ are not stopped by body
[1] less chance of cell or gene damage / cancer / radiation sickness / mutation / ionisation (of body tissues)

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