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Physics (14 - 16) Nuclear Radiation - 2 Types of Radiation © SSER Ltd. Radiation The nucleus of most atoms is stable, however there are three causes of instability of a nucleus: A nucleus with more than 82 protons A nucleus with an atomic mass of more than 209 An unstable ratio of neutrons to protons - this can be caused by too many or too few neutrons in the nucleus Radioactive decay is a process by which the nucleus of an atom becomes more stable. There are three types of radiation: Alpha () Beta () Gamma () Alpha Radiation () Alpha Radiation () Alpha radiation is formed when an unstable nucleus decays, emitting an alpha particle. The particle emitted is a helium nucleus (a helium atom minus its two orbiting electrons) composed of two protons and two neutrons. Alpha particles have an atomic mass of 4 and a relative charge of +2. This can be written in either of two ways: 4 or 2 He 4 2 Alpha Radiation () Alpha particles have about 8000 times the mass of an electron. Alpha radiation is strongly ionising as the large, slow moving alpha particles are very likely to collide with atoms as they pass through a substance. (An ion is a charged atom that has fewer or more electrons than normal, so that the positive charge from the protons in the nucleus is not exactly cancelled out.) As an alpha particle collides with atoms it loses some of its energy. After many thousands of collisions the energy of the alpha particle is low enough for electrons to attach themselves. When two electrons have attached themselves to an alpha particle, one atom of helium is created. Alpha Radiation () Alpha particles are commonly emitted from heavy nuclei which have a large number of protons and neutrons, e.g. 226 88 Ra Radium - unstable metal 222 86 Rn + Radon - more stable gas Checking the atomic mass figures... Checking the atomic number figures... 4 2 He Alpha particle 226 = 222 + 4 88 = 86 + 2 When a radioactive atom emits an alpha particle, its mass number decreases by four and its atomic number decreases by two. Alpha Radiation () Sometimes, a decay reaction produces another radioisotope, which itself decays... Thorium becomes radium: 228 90 Th 224 88 Ra + 4 2 He Radium becomes radon... 224 88 Ra 220 86 Rn + 4 2 He Each individual step is known as an alpha decay. The changing of one radioactive element progressively into others, is called a decay series. The series stops when a stable nucleus is formed. Beta Radiation () Beta Radiation () Beta radiation consists of free, high energy electrons. There are protons and neutrons in the nucleus of an atom, but no electrons. So it may seem strange that beta particles are emitted from the nuclei of atoms. Beta particles are made when a neutron changes into a proton and an electron. The proton stays in the nucleus and the electron is emitted as a beta particle. The stability of a nucleus is determined by the relative proportion of protons and neutrons. Radioactive decay changes this relative proportion, to make the nucleus more stable. Beta Radiation () The mass of a beta particle is negligible, so when a beta particle is emitted, it does not alter the atomic mass of the nucleus. It does however change the charge of the nucleus, because a beta particle carries a relative charge of -1. Beta particles have an atomic mass of 0 and a relative charge of -1. This can be written in either of the two forms below: 0 e -1 -1 0 Beta Radiation () Beta radiation is moderately ionising as beta particles are less likely than alpha particles to collide with the atoms in a substance. Beta radiation ionises by transferring energy to outer electrons which then leave their orbits. When a radioactive atom emits a beta particle, its mass number remains unchanged and its atomic number increases by one (as a neutron changes into a proton). Example: Carbon-14 becomes nitrogen-14. 14 C 6 14 7 N This decay reaction is known as beta decay. + 0 -1 e Gamma Rays () Gamma Rays () Gamma rays are one of the seven types of electromagnetic wave which form the electromagnetic spectrum. Gamma rays have the highest frequency and shortest wavelength of all the seven types of electromagnetic wave. Wavelength Gamma Rays () As gamma rays are electromagnetic waves, they have zero mass and zero charge. A gamma ray is written as: 0 0 When a gamma ray is emitted from an atom, it has no effect on either the atomic mass or atomic number. It does however increase the stability of the nucleus, as it reduces its energy level. The gamma rays are weakly ionising, as they tend to pass through a substance rather than collide with its atoms. When gamma rays do collide with an atom, energy is transferred to an electron which leaves its orbit. Gamma Rays () Gamma rays mostly occur as a result of alpha or beta radiation, although there are some elements that only emit gamma radiation, such as protactinium... 234 Pa 91 234 Pa 91 Checking the atomic mass figures... 234 = 234 + 0 Checking the atomic number figures... 91 = 91 + 0 + 0 0 Gamma Rays () When a radioactive nucleus emits an alpha or beta particle, the protons and neutrons in the newly formed nucleus are often in an excited, unstable state. In this case, a gamma ray is emitted from the newly formed nucleus, reducing the energy level and increasing the stability of the nucleus... 60 Co 27 60 Ni 28 + 0 e -1 + 0 0 Cobalt-60 is commonly used as a gamma ray source for food irradiation, radiotherapy and for general use in laboratories. Penetration of Radiation Press the button to see how different types of radiation penetrate... Penetration of Radiation - Alpha () As alpha particles have a relatively high mass and low speed, they collide easily with atoms in a substance. When they collide with atoms, it prevents them from passing through materials, so alpha particles have a very short range and low penetration. They only travel around 6cm in air. They are stopped by skin or a piece of paper. Penetration of Radiation - Beta () Beta particles have a very low mass and high speed, so they collide less easily than alpha particles with atoms in a substance. Beta particles have a greater range than alpha particles, and are considerably more penetrating. They can travel up to 10m through the air, and through some obstacles, such as pieces of paper. They are stopped by a 3mm thick layer of aluminium. Penetration of Radiation - Gamma () Gamma radiation has no mass and travels at the speed of light, so gamma rays collide infrequently with atoms in a substance. Gamma rays have a greater range, and are considerably more penetrating, than alpha or beta particles. They can travel long distances through the air, through pieces of paper and aluminium foil. Several centimetres of lead or several metres of concrete are needed to stop the passage of gamma rays. The Effect of a Magnetic Field on Radiation Press a button to see the effect of a magnetic field on different types of radiation... The Effect of an Electric Field on Radiation Press a button to see the effect of an electric field on different types of radiation... Effects of Electric & Magnetic Fields - Summary The effects of an electric field can be summarised as: Alpha particles are bent slightly towards the negative terminal of an electric field. Beta particles are bent strongly to the positive terminal of an electric field. Gamma rays are not affected by an electric field. The effects of a magnetic field can be summarised as: Both Alpha and Beta particles are deflected at right angles to a magnetic field. Alpha and Beta particles will move in opposite directions according to Fleming’s left hand rule. Gamma rays are not affected by a magnetic field. End of Show Copyright © 2006 SSER Ltd. and its licensors. Images are for viewing purposes only. All rights reserved.