4a. The Properties of the three types of Radioactive Emission and symbols IONISING RADIATIONS emitted when unstable atomic nuclei undergo radioactive decay REMINDER: Experiment to show there are at least three types of emissions from radioactive substances. Left to right - alpha particles, gamma rays and beta particles Detection systems include photographic plate, electronic screen from their ionisation effect generating an electronic signal and a cloud chamber. The radioactive emissions become separated in a strong electromagnetic field because alpha particles (+2) and beta particles (-1) have different charges, so go in opposite directions in electric or magnetic fields. Alpha particles have a positive charge and bend one way in the electric/magnetic field. Gamma photons (rays of electromagnetic radiation) have no charge (0) and go straight on to the detector The beta particles are deflected more because they have a much smaller mass than alpha particles (for more details see table below). Being of opposite charge to an alpha particle, they deflect in the opposite direction. The PROPERTIES of ALPHA, BETA, GAMMA and neutron RADIATIONS The three radiations highlighted are the one you most likely need to know , but maybe the other two as well. Absorption is all about the probability of the radiation hitting atoms. These collisions can slow down or absorb the radiation. The range of penetration depends on the type of radiation and the material it is passing through i.e. how easily is it absorbed depends on these two factors.

Type of radiation emitted & symbol Nature of the radiation formation, structure, relative mass, electric charge Other nuclear Symbols Penetrating power (and speed), and what will block it (more dense material, more radiation is absorbed BUT smaller mass or charge of particle, more penetrating). Ionising power - the ability to remove electrons from atoms to form positive ions, the process is called ionisation

Alpha particle radiation a helium nucleus of 2 protons and 2 neutrons, mass = 4, charge = +2, is expelled at high speed from the nucleus Low penetration , slowest speed (but still ~10% speed of light!), biggest mass and charge, stopped by a few cm of air or thin sheet of paper, so obviously will be stopped by a few cm layer of concrete, sheets of aluminium or lead. Very high ionising power , the biggest mass and charge of the three radiation's, the biggest 'punch' in ripping off electrons from molecules, other ions are formed

e – beta minus particle radiation high kinetic energy electrons , mass = 1/1850, charge = -1, expelled when a neutron changes to a proton in the nucleus beta minus, beta – Moderate penetration (~90% speed of light), 'middle' values of charge and mass, most stopped by a few mm of metals like aluminium, will travel quite a few metre in air, will be stopped by a few cm layer of concrete, sheets of lead. Moderate ionising power , with a smaller mass and charge than the alpha particle, but still quite good at knocking off electrons from molecules - moderate ionisation

Gamma radiation very high frequency electromagnetic radiation , mass = 0, charge = 0, gamma emission often accompanies alpha and beta decay Very highly penetrating (100% speed of light !), smallest mass and charge and greatest speed, most stopped by a thick layer of steel or a very thick layer of concrete, but even a few cm of dense lead doesn't stop all of it! Gamma rays can pass through many m of air. It takes many m of concrete plus steel to absorb it all. The lowest ionising power of the three, gamma radiation carries no electric charge and has virtually no mass, so not much of a 'punch' when colliding with an atom to remove an electron, weak ionisation

e+ beta plus particle emission high KE positive electron called a positron, mass = 1/1850, charge = +1, expelled when a proton changes to a neutron in the nucleus. beta plus, beta + The positron is the antiparticle of the electron. it is identical to an electron but opposite in charge. It is rapidly destroyed when it meets any electron (see on right) producing two high energy gamma ray photons, so it doesn't get very far! The co-destruction of particle and anti-particle is called annihilation! The effect is used in PET scanning in medicine. Theoretically as above, BUT when electron meets positron, kapow ! e+ + e – ==> 2 equation for annihilation !

n neutron radiation neutron, mass = 1, charge = 0, fundamental particle of the nucleus Highly penetrating (more than alpha & beta & sometimes gamma). However, neutrons are most readily absorbed by light nuclei so hydrogen-rich materials like water, poly(ethene) plastic and concrete are used for neutron radiation shielding. The nuclei formed often emit gamma radiation so an extra thick protective layer of lead is needed around a neutron rich environment ! Can't ionise directly, but they are absorbed by the nuclei of atoms they pass through. This can make the atom unstable - radioactive, hence other nuclear radiations may then be produced, producing an 'indirect ionisation' effect. So neutron radiation is as dangerous as any of the others.