The isotope carbon-14 decays by giving out beta minus particles.
(a)(i) Pick the unit used for the activity of a radioactive sample. Tick one box.
Our own questions on this topic, marked straight away, so you see what to revise. Free now.
Show model answer for 09.1Hide model answer for 09.1
Bq, the becquerel, which is one decay per second. Pa is the unit of pressure, N m is the unit of a moment, and Gs is a billion seconds.
(a)(ii) Pick the order of the three radiations from least to most penetrating. Tick one box.
See how you would score on Radioactive DecayShow model answer for 09.2Hide model answer for 09.2
Alpha, then beta, then gamma. Alpha is stopped by paper, beta by a thin sheet of aluminium, and gamma only by thick lead or concrete.
How the marks are earned
- Tick alpha, beta, gamma, with alpha the least penetrating.
(a)(iii) A beta minus particle of mass 9.1 × 10-31 kg moves at 3.8 × 106 m/s. Find its kinetic energy in J.
See how you would score on Kinetic EnergyShow model answer for 09.3Hide model answer for 09.3
Use the kinetic energy equation:. kinetic energy = ½ × m × v2. The values are:. m = 9.1 × 10-31 kg. v = 3.8 × 106 m/s. Square v before anything else.
(3.8 × 106)2 = 1.444 × 1013. kinetic energy = ½ × 9.1 × 10-31 × 1.444 × 1013. = 6.6 × 10-18 J
How the marks are earned
- One mark: select kinetic energy = ½ × mass × speed squared.
- One mark: the kinetic energy, 6.6 × 10-18 J. Any value that rounds to this, such as 6.5702 × 10-18, is accepted.
- The right final answer on its own earns full marks.
Why it scores
Choosing the equation earns the first mark. The speed is squared before it is multiplied by the mass.
The evaluation earns the second mark. The tiny answer fits a particle with a tiny mass.
Revise Kinetic Energy(b) Describe the change inside a nucleus when it emits a beta plus particle.
See how you would score on Radioactive DecayShow model answer for 09.4Hide model answer for 09.4
A proton in the nucleus turns into a neutron, and a positron, the beta plus particle, is emitted.
How the marks are earned
- One mark: a proton in the nucleus changes.
- One mark: it becomes a neutron. Writing p → n earns both marks. Saying the charge, or the atomic number, goes down by 1, the mass number is unchanged, or the neutron number goes up by 1 is also accepted. If nothing else scores, the reverse change, n → p, earns one mark.
Why it scores
One mark is for the proton and the other is for it becoming a neutron. The atomic number falls by 1 and the mass number stays the same.
Revise Radioactive Decay(c) Compare alpha and gamma radiation: what each is, and how each kind of decay affects the original nucleus.
See how you would score on Radioactive DecayShow model answer for 09.5Hide model answer for 09.5
An alpha particle is a helium nucleus: 2 protons and 2 neutrons stuck together, with a charge of plus 2. When a nucleus emits one, its proton number falls by 2 and its mass number falls by 4, so it becomes a different element.
Gamma radiation is a high frequency, short wavelength electromagnetic wave with no charge and no mass. Gamma emission does not change the proton number or the mass number, so the element is the same, but the nucleus loses energy and becomes more stable.
In comparison, alpha is strongly ionising but stopped by paper, while gamma is weakly ionising but needs thick lead to stop it.
How the marks are earned
- This is level marked. For 5 to 6 marks, give a detailed comparison covering what one radiation is and how its decay changes the nucleus, plus either what or how for the other. For 3 to 4 marks, give a limited comparison: what both are, or the effect on the nucleus of both, or what and how for just one, or what one is together with the effect of the other. For 1 to 2 marks, give isolated facts about alpha or gamma.
- Alpha: a helium nucleus, made of 2 protons and 2 neutrons, with charge plus 2. Its decay lowers the proton number by 2 and the mass number by 4, so the element changes.
- Gamma: an electromagnetic wave with very high frequency and short wavelength, carrying no charge and no mass. Its decay leaves the proton and mass numbers alone, so the element stays the same, though the nucleus rearranges, loses energy and becomes more stable.
- Ionising and penetrating: alpha ionises heavily but barely penetrates, and is blocked by paper or a few centimetres of air; gamma ionises weakly but penetrates deeply, and needs thick lead. A statement about ionising or penetrating can replace one about nature or effect, though only for a lower mark within a level.
Why it scores
This gives both the nature of alpha and its effect on the nucleus, accurately and with numbers. That alone builds a strong base for the top level.
This covers the nature of gamma and its effect, and it sets up the contrast. Alpha changes the nucleus. Gamma does not.
These can stand in for a nature or an effect, but only at the lower end of a level. This answer does not need them.
Revise Radioactive Decay