Radioactivity

The basic structure of an atom is a small central nucleus, made up of protons and neutrons, surrounded by electrons. The nuclei of radioactive substances are unstable. They become stable by radioactive decay. In this they emit radiation and turn into other elements. So the nucleus will emit the radiation. The three types of radiation emitted are alpha, beta and gamma.

We can't predict when an unstable nucleus will decay, as it is a random process and is not affected by external conditions.

Background radiation is around us all the time. This is radiation from radioactive substances in the environment, from space, devices like x ray tubes.

Scientists used to think that atoms consisted of spheres of positive charge with electrons stuck to them, like plums in a pudding. So it was known as the plum pudding model of the atom. Then Rutherford, Geiger and Marsden devised an alpha particle scattering experiment, when they fired alpha particles at thin gold foil. Most passed straight through, meaning that most of the atom was just empty space. Some of the particles deflected through small angles. Suggesting the alpha particle has a positive charge. A few rebound through large angles, suggesting the nucleus has a large mass and very large positive charge.

Alpha decay: the nucleus loses 2 protons and 2 neutrons. They are emitted as an alpha particle.

Beta decay: a neutron in the nucleus changes into a proton and an electron. The electron created is instantly emitted.

Isotopes are atoms of the same element with different numbers of neutrons.

When a nucleus emits an alpha particle the atomic number goes down by 2 and the mass number by 4. An alpha particle has a relative mass of 4 and relative charge of +2. A beta particle is a high speed-electron from the nucleus, emitted when the nucleus changes. It has a relative mass of 0 and relative charge of -1. When a nucleus emits gamma radiation, there is no change in atomic or mass number. It is an electromagnetic wave released from the nucleus, it has no change or mass.

When nuclear radiation travels through a material, it will collide with the atoms of the material. This knocks electrons off them, creating ions. This is called ionisation. This in a living cell can damage or kill the cell. Alpha particles are relatively large, so they have lots of collision, so are strongly ionising. Because of these collisions, alpha particles don't penetrate far into the material. The can be stop by a thin sheet of Parker and human skin. They are defected by electric and magnetic fields. Beta particles are much smaller and faster, so they are less ionising and penetrate further. They are blocked by a thin sheet of aluminium. They are negative charge, and are deflected by electric and magnetic fields in the opposite direction to alpha. Gamma rays are electromagnetic waves so they will travel a long way through the material before colliding with the atom. They are weakly ionising but very penetrating. Several cms of lead are needed to absorb most of the radiation. They are not deflected by electric and magnetic fields.

We can measure the radioactivity of a sample of a radioactive material by measuring the count rate from it. The radioactivity decreases over time. How quickly the rate falls to 0 depends on the isotope. Stem take a few minutes, others millions of years.

We use the idea of half-life to measure how quickly the radioactivity decreases. It is the time taken for the count rate from the original isotope to fall to half it's original value. Or the time taken for the number of unstable nuclei in a sample to halve. Every half-life the number of atoms of a radioactive isotope decrease by half.

Alpha sources are used in smoke alarms. They are not dangerous as they are poorly penetrating. The source needs a half life of several years.

Beta sources are dues for thickness monitoring. They would be stopped by a thin sheet of paper and all gamma rays would pass through. The source needs a half life of many years so that decreases in count rate are due to changes in thickness of paper.

Gamma and beta sources are used as tracers in medicine. It is injected or swallowed by the patient, it's progress is then monitored around the body by a detector outside. The source needs a half life of a few hours so the patient isn't exposed to unnecessary radioactivity.

Radioactive dating is used to find out the age of ancient material. Carbon dating is used to find the age of wood and other organic material. Uranium dating is used to find the age of igneous rocks.