Anti-particles

Question 1

(2) Fluorine-18 has a half-life of 1.8 hours.
State one advantage and one disadvantage of using a substance with such a
short half-life.

Answer 1

1. advantage
(1)
minimises patient’s exposure to
radioactivity
2. disadvantage
(1)
(has to be produced)
 close to the {place / time} of
use

Question 2

(4) When a positron meets an electron they annihilate each other.
Explain how this enables the site of the tumour to be located.

Answer 2

gamma (rays emitted)
(1)
 two (gamma rays)
(1)
 in opposite directions
(1)
 (because) momentum is
conserved
(1)
 detectors / sensors placed
around the patient
(1)
 simultaneous detection
(1)

Question 3

(6) Describe, in terms of quarks, how ß+ particles are emitted from the nuclei of some atoms and ß– particles are emitted from the nuclei of others.

Answer 3

 protons and neutrons are made up of quarks
 quarks can change (flavour)
 proton made up of uud
 neutron made up of udd
 for β+ a u changes to a d
 for β-
 a d changes to a u
 for β+ a p changes to a n
 for β-
 a n changes to a p