Atomic and Nuclear Physics

Atomic and nuclear physics looks inside the atom — at the structure of the nucleus, at unstable atoms that give out radiation, and at the enormous energy locked in the nucleus. It links closely to the chemistry of radioactivity but focuses on the physics of the radiations and their effects.

Structure of the atom

An atom has a tiny, dense, positively charged nucleus containing protons (charge +1) and neutrons (no charge), surrounded by electrons (charge −1) orbiting in shells. The nucleus is extremely small compared with the whole atom — most of the atom is empty space, a model supported by the famous alpha-particle scattering experiment (most alpha particles passed straight through gold foil, while a few bounced back off the concentrated nucleus).

• Atomic (proton) number, Z = number of protons.
• Mass (nucleon) number, A = protons + neutrons.
• An atom is neutral because the number of protons equals the number of electrons.

Isotopes are atoms of the same element (same proton number) with different numbers of neutrons, written for example as carbon-12 and carbon-14.

Radioactivity

Some nuclei are unstable and break down, emitting radiation to become more stable. This radioactive decay is spontaneous (it happens on its own) and random (you cannot predict which nucleus will decay next, or speed it up with heat, pressure or chemical change).

There are three main types of radiation:

Note the inverse relationship: alpha is the most ionising but least penetrating; gamma is the least ionising but most penetrating. Because they are charged, alpha and beta are deflected by electric and magnetic fields (in opposite directions); gamma, being uncharged, is not.

Half-life

The activity of a radioactive source falls over time. The half-life is the time taken for half of the radioactive nuclei (or for the activity) to decay. Each isotope has its own fixed half-life. For example, if a source has a half-life of 3 hours and an activity of 800 counts/s, then after 3 hours it is 400, after 6 hours 200, after 9 hours 100 — halving each half-life. Half-life questions usually ask you to read or calculate values from this halving pattern or from a decay graph.

Nuclear energy: fission and fusion

The nucleus stores huge amounts of energy.

• Nuclear fission is the splitting of a large nucleus (such as uranium-235) into smaller nuclei when it absorbs a neutron, releasing energy and more neutrons — which can cause a chain reaction. This is the energy source in nuclear power stations.
• Nuclear fusion is the joining of small nuclei (such as hydrogen) to form a larger one, releasing even more energy. Fusion powers the Sun and stars but is very hard to achieve on Earth because the nuclei must be forced together at enormous temperatures.

Detecting radiation and background count

Radiation cannot be seen, so it is detected with instruments such as a Geiger–Müller (GM) tube connected to a counter, which clicks or counts each time radiation enters it. Before measuring a source you must record the background count — the small amount of radiation always present from rocks, soil, cosmic rays and traces in the air — and subtract it from your readings to find the true activity of the source. Forgetting to allow for background radiation is a common error in calculations.

A worked half-life calculation

Half-life questions are very common. Suppose a radioactive source has an initial activity of 2400 counts per second and a half-life of 4 hours; find the activity after 12 hours. Twelve hours is three half-lives (12 ÷ 4 = 3), so the activity halves three times:

2400 → 1200 (after 4 h) → 600 (after 8 h) → 300 (after 12 h)

So the activity is 300 counts per second. You may also be asked the reverse — how long for the activity to fall from 2400 to 300 — which is the same three half-lives, or 12 hours. Reading these values from a decay graph uses the same idea: find the time for the curve to drop to half of any starting value.

Uses and dangers

Uses: generating electricity, sterilising medical equipment, treating cancer, medical and industrial tracers, smoke detectors (alpha source), and carbon dating.

Dangers: radiation is ionising, so it can damage or kill living cells, cause mutations and cancer, and in large doses cause radiation sickness. Safety measures include shielding (lead/concrete), keeping a distance, limiting exposure time, wearing film badges to monitor dose, and careful disposal of radioactive waste.

Common exam mistakes

• Reversing penetration and ionisation — alpha is least penetrating but most ionising.
• Saying decay can be sped up by heating — it is spontaneous and random.
• Confusing fission (splitting large nuclei) with fusion (joining small nuclei).
• Treating half-life as the time for the whole sample to decay — it is for half.

Key terms to remember

• Nucleus — the small, dense, positively charged centre of an atom (protons + neutrons).
• Proton number (Z) — the number of protons; mass number (A) — protons + neutrons.
• Isotopes — atoms of the same element with different numbers of neutrons.
• Radioactive decay — the spontaneous, random breakdown of unstable nuclei.
• Alpha (α) — a helium nucleus; beta (β) — a fast electron; gamma (γ) — high-energy EM radiation.
• Ionising radiation — radiation that can damage living cells.
• Half-life — the time for half the nuclei (or activity) to decay.
• Background radiation — the low-level radiation always present, which must be subtracted from readings.
• Fission — splitting a large nucleus; fusion — joining small nuclei (powers the Sun).

Quick recap

• Atoms have a small dense nucleus (protons + neutrons) with electrons around it; isotopes differ in neutron number.
• Radioactive decay is spontaneous and random; alpha (paper, most ionising), beta (aluminium), gamma (lead, most penetrating).
• Half-life is the time for half the nuclei/activity to decay.
• Fission splits large nuclei (power stations); fusion joins small nuclei (the Sun).
• Radiation is useful but ionising — protect with shielding, distance, time and monitoring.