Atomic structure and radiation

Revision notes

Atoms contain a tiny nucleus of protons and neutrons surrounded by electrons. Isotopes are atoms of the same element with the same proton number but different neutron numbers. Unstable nuclei can decay randomly, emitting ionising radiation. Alpha particles are helium nuclei, beta-minus radiation consists of fast electrons emitted in nuclear decay, and gamma radiation is high-frequency electromagnetic radiation.

Activity is the decay rate in becquerels (Bq). Half-life is the time for the number of undecayed nuclei, or the activity/count rate after background correction, to fall to half its value. Individual decay is unpredictable, but large samples show statistical behaviour. Radiation risk depends on ionising ability, penetration, exposure and whether radioactive material is inside or outside the body.

Atomic and nuclear structure

Atoms contain a tiny positively charged nucleus surrounded by electrons. The nucleus contains protons and neutrons. Proton number identifies the element; nucleon number is protons plus neutrons. Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons.

Nuclear equations must conserve nucleon number and charge. In alpha decay, the nucleus emits two protons and two neutrons, so nucleon number falls by 4 and proton number by 2. In beta-minus decay, a neutron changes into a proton and an electron is emitted, so nucleon number stays the same while proton number rises by 1.

Alpha, beta and gamma radiation

Alpha particles are strongly ionising and have short range and low penetration. Beta radiation is moderately ionising and moderately penetrating. Gamma radiation is weakly ionising, because it rarely interacts with atoms along its path, and highly penetrating. The best shielding depends on the type and situation; 'lead stops all radiation' is not a sufficient explanation.

Ionising radiation can damage biological tissue by altering atoms and molecules. Risk depends on activity, radiation type, exposure time, distance, shielding and whether radioactive material is outside or inside the body.

Half-life and activity

Radioactive decay is random for an individual unstable nucleus but predictable statistically for a large sample. Half-life is the time taken for the number of undecayed nuclei, or a quantity proportional to it such as activity, to fall to half its value. After n half-lives, the remaining fraction is \(\dfrac{1}{2}\)ⁿ.

Measured count rate includes background radiation. If a question gives background count, subtract it before analysing the source decay and add it back only if the question asks for the detector's total observed count.

Irradiation and contamination

Irradiation means exposure to radiation from an external source. Contamination means radioactive material is deposited on or inside an object or person. Contamination can continue to irradiate tissue after the original source has been removed because the radioactive material itself remains present.

Fission and fusion

Nuclear fission is the splitting of a heavy nucleus, usually after neutron absorption, releasing energy and more neutrons. A chain reaction occurs if emitted neutrons cause further fissions. Fusion joins light nuclei and releases energy but requires extremely high temperatures and suitable confinement because positively charged nuclei repel.

Exam technique

When comparing radiations, link property to consequence: penetration determines shielding and external hazard; ionising ability affects biological damage over a short path. For half-life, count the number of complete halvings rather than trying to treat decay as linear.

Models of the atom

Ideas about the atom changed as new evidence was found.

• Dalton described atoms as tiny solid spheres that could not be divided.
• Thomson discovered the electron and proposed the plum pudding model: a ball of positive charge with negative electrons embedded in it.
• Rutherford, Geiger and Marsden fired alpha particles at thin gold foil. Most went straight through, so the atom is mostly empty space. A few were deflected through large angles, and a very small number bounced back, so the positive charge and most of the mass must be concentrated in a tiny nucleus. This nuclear model replaced the plum pudding model.
• Bohr suggested that electrons orbit the nucleus at fixed distances (energy levels or shells). His calculations agreed with experimental observations of the light emitted by atoms.
• Later experiments showed the positive charge of the nucleus is carried by protons. Chadwick provided evidence for neutrons in 1932, about 20 years after the nucleus was accepted.

Energy levels and ions

Electrons can move to a higher energy level (further from the nucleus) by absorbing electromagnetic radiation, and move back to a lower level by emitting electromagnetic radiation. If an atom loses one or more outer electrons it becomes a positive ion. Radiation that can knock electrons out of atoms is called ionising radiation.

Background radiation and dose

Background radiation is around us all the time. Natural sources include radon gas from rocks, rocks and building materials, cosmic rays from space, and food and drink. Artificial sources include medical X-rays and fallout from nuclear weapons testing and nuclear accidents. The level varies with location and occupation. Radiation dose, measured in sieverts (Sv), describes the risk of harm to the body; everyday doses are usually given in millisieverts (1 mSv = 0.001 Sv).