Nuclear physics

Question 1

Suggest, with a reason, which type of radiation is likely to be the most appropriate for the sterilisation of metallic surgical instruments.

Answer 1

γ radiation because it is very / the most penetrating
OR
γ radiation because it is penetrating enough to irradiate all sides of the instruments
OR
γ radiation is penetrating so instruments can be sterilised without removing the packaging

Question 2

Explain why the public need not worry that irradiated surgical instruments become radioactive once sterilised.

Answer 2

To become radioactive the nucleus has to be affected which (ionising) radiation does not do
OR
(Ionising) radiation only affects the outer electrons and not the nucleus
OR
The energy of the radiation is insufficient to induce radioactivity. (For this mark high energy is not the same as highly ionizing)
OR
(Ionising) radiation does not affect the nucleus ✔

Question 3

A nucleus of the stable isotope has more neutrons than protons.
Explain why there is this imbalance between proton and neutron numbers by referring to the forces that operate within the nucleus. Your explanation should include the range of the forces and which particles are affected by the forces.

Answer 3

Any 3 marking points from 1 to 5
1Strong nuclear force (SNF) affects nucleons or protons and neutrons. ✔
2SNF attraction extends up to 3 fm (allow 1–4 fm) ✔
3The SNF is repulsive below about 0.8 fm (allow 0.3 to 1 fm and prevents the nucleus totally collapsing) ✔
4Electromagnetic/electrostatic repulsive force (only) acts between protons ✔
5EM forces are long range/infinite/acts across whole nucleus/acts on all protons(so increases as proton number increases) ✔
PLUS one of following that explains the imbalance
More neutrons are needed to hold nucleus together / add to binding force/increase instability/reduce stability (owtte)
OR
Fewer protons are required so as to reduce the repulsion/reduce instability/increase stability (owtte) ✔

Question 4

The thallium nucleus is formed in an excited state. Electromagnetic radiation is emitted from the thallium atom following its formation.
Explain the origin and location of two sources of this radiation.

Answer 4

Orbiting electrons in the atom fall (to fill the positions vacated by inner orbiting electrons releasing their energy as em (gamma) radiation ✔
The excited nucleus emits gamma radiation (as it de-excites)✔