Nuclear Physics

The nuclear atom

An atom is almost entirely empty space. Its mass and positive charge are crammed into a central nucleus thousands of times smaller than the atom itself, with electrons in orbit far beyond.

Nucleus and electrons

An atom has a small, positively charged nucleus surrounded by negatively charged electrons in orbit. A neutral atom has equal numbers of protons and electrons. An atom becomes a positive ion if it loses electrons, or a negative ion if it gains them.

ExtendedThe scattering experiment

Firing alpha particles at a thin metal foil provides the evidence for the nuclear model. Most pass straight through — so the atom is mostly empty space. A few are deflected, and a very few bounce back — so the nucleus is tiny, holds most of the mass, and is positively charged (it repels the positive alpha particles).

The nucleus, nuclides and isotopes

The nucleus is built from two particles — protons and neutrons. How many of each there are fixes which element it is, and which version, or isotope, of that element.

Protons, neutrons and notation

The nucleus contains positively charged protons and uncharged neutrons. Their relative charges are +1, 0 and −1 for proton, neutron and electron. In nuclide notation the nucleon number A is written above and the proton number Z below the element symbol. The proton number Z sets the element; the nucleon number A is the total of protons and neutrons; so the number of neutrons is A − Z.

Isotopes are atoms of the same element — the same Z — with different numbers of neutrons. For example, carbon-12 and carbon-14 both have 6 protons but 6 and 8 neutrons.

Fission and fusion

This whole section is Extended — Papers 2 and 4 only. Two opposite nuclear processes release enormous energy: splitting a heavy nucleus apart, and joining light nuclei together. Both are described qualitatively — the syllabus asks for the idea and a balanced nuclide equation, not energy values.

ExtendedFission: splitting nuclei

Nuclear fission is the splitting of a large, unstable nucleus into two smaller nuclei, usually triggered when it absorbs a neutron. Extra neutrons and energy are released; the mass of the products is fractionally less than the original, the lost mass appearing as energy. This is the process used in nuclear power stations.

ExtendedFusion: joining nuclei

Nuclear fusion is the joining of two light nuclei to form a heavier one, again with a tiny loss of mass released as energy. It needs extremely high temperature and pressure, which is why it powers stars but is so hard to reproduce on Earth.

Detecting radioactivity

A low level of radiation is around us at all times. Before measuring any source, this background must be accounted for — otherwise every reading is too high.

Background radiation

Background radiation comes from natural sources: radon gas in the air, rocks and buildings, food and drink, and cosmic rays from space. Radon gas is typically the largest single source. Ionising radiation is measured with a detector (such as a Geiger-Müller tube) connected to a counter, giving a count rate in counts per second or per minute.

Alpha, beta and gamma radiation

An unstable nucleus emits radiation spontaneously and in a random direction. There are three kinds — alpha, beta and gamma — and they differ sharply in what they are, how strongly they ionise, and how far they penetrate.

Nature, ionising and penetrating power

Alpha is the most ionising but least penetrating — stopped by a sheet of paper or skin. Beta is moderately ionising and penetrating — stopped by a few millimetres of aluminium. Gamma is the least ionising but most penetrating — only reduced by thick lead or concrete.

ExtendedDeflection and ionising effect

In an electric or magnetic field, alpha (+) and beta (−) are deflected in opposite directions, with beta deflected more because it is much lighter; gamma is undeflected. Alpha is the most ionising because it has the greatest charge and kinetic energy, so it interacts most strongly with the atoms it passes.

Radioactive decay

When an unstable nucleus emits radiation, it changes. Alpha and beta decay leave behind the nucleus of a different element entirely — a transformation written down as a balanced decay equation.

A change of element

Radioactive decay is a spontaneous, random change in an unstable nucleus that emits alpha or beta particles and/or gamma radiation. After alpha or beta decay the nucleus becomes a different element.

In alpha decay the nucleus loses 2 protons and 2 neutrons, so A falls by 4 and Z by 2. In beta decay a neutron becomes a proton plus an electron (the beta particle), so A is unchanged and Z rises by 1. Gamma emission changes neither.

Worked example: radium-226 (Z = 88) emits an alpha particle. Alpha decay removes 4 nucleons, so A = 226 − 4 = 222, and 2 protons, so Z = 88 − 2 = 86 (radon).

Half-life

Decay is random, so we cannot say when a single nucleus will decay. But across a large sample the pattern is utterly reliable: in each half-life, half of what remains decays away.

Halving, and halving again

The half-life is the time for half the nuclei in a sample to decay. After one half-life half remain, after two a quarter, after three an eighth, and so on — a steadily falling decay curve.

Worked example: a source’s count rate falls from 800 to 100 counts/s and its half-life is 5 hours. The halvings 800 → 400 → 200 → 100 are 3 half-lives, so the time taken is 3 × 5 = 15 hours.

Uses of radioactivity and safety

Ionising radiation damages living cells — it can cause cell death, mutations and cancer. Its uses, and the way it is handled safely, both come down to matching and controlling the radiation.

ExtendedCommon uses

Smoke alarms use an alpha source to ionise the air, so smoke disrupts the current and triggers the alarm. Sterilising equipment and irradiating food to kill bacteria use gamma, which penetrates sealed packaging. Thickness control of paper or metal uses beta, whose transmission through the sheet changes with its thickness. Diagnosis and treatment of cancer use gamma, directed to image or destroy affected tissue.

Dangers and handling

Ionising nuclear radiation harms living things — it can cause cell death, mutations and cancer. Radioactive materials are therefore moved with tongs or handling tools, used at arm’s length, and stored in lead-lined containers well away from people.

Safety precautions for all ionising radiation reduce the dose received by: minimising the time of exposure, maximising the distance between the source and living tissue, and placing shielding (such as lead or concrete) to absorb the radiation.