Ionising radiations

Ionising radiations Radioactive isotopes decay into other elements. When they decay, they give out ionising radiations – so called because they cause ionisation of the medium that they travel through. There are three main types of radiation called alpha (a), beta (b) and gamma (g). They were first identified at the end of the 19th century and were classified according to their penetrating power (alpha is the least penetrative and gamma the most). We now know more about what they are and why they behave as they do.

Picture 2.4 Alpha radiation.	Alpha radiation - a What is it? Alpha radiation is a stream of particles. These are the nuclei of helium – two protons and two neutrons. This means that they have a (relatively) large mass. Alpha particles from a given radioactive decay all have the same energy (of the order of a few MeV). What's its charge? The atomic number of helium is 2 so an alpha particle carries a double positive charge. What's its symbol?  or  How fast do they go? Alpha particles don’t travel very fast (compared with beta particles) because they have such a large mass. They carry energy away from a radioactive decay; this energy is of the order of a few MeV and is given to the kinetic energy of the alpha particle; they have a mass of 6.6 x 10–27 kg so their speed is about 7 x 106 m.s-1 – i.e about a fiftieth of the speed of light. How ionising are they? Their low speed means that they tend to collide with plenty of other atoms and cause a lot of ionisation on their way. An alpha particle can ionise thousands of air particles before it slows down to thermal speeds. How far do they go? All these collisions mean that they lose energy quickly, so they have a short range in air. What do they go through? They are easily stopped by anything solid – even a piece of paper will stop alpha radiation. Effect of magnetic field Alpha particles are deflected slightly in a magnetic field. Their large mass means that they don't get deflected much. They will travel in the arc of a circle in a uniform field (see page 19). Effect of electric field They are deflected slightly in an electric field and will move in a parabolic curve in a uniform field.
Picture 2.5 Beta radiation. Picture 2.6 Bending beta particles in magnetic field. They must take a curved path to reach the detector.	Beta radiation - b What is it? Beta radiation (the sort that you'll come across in school) is a stream of fast moving electrons. Beta plus radiation is a stream of particles called positrons. Beta radiation from a given radioactive decay is given out with a range of energies (see page 16). What's its charge? You will usually come across beta minus (b-) radiation, which has a negative charge. What's its symbol?  or  How fast do they go? These particles have very little mass (about 7, 000 times lighter than an alpha particle) and travel close to the speed of light (300, 000 km.s-1). How ionising are they? They tend to pass through the air and solid matter without many collisions with other atoms. So beta radiation is only weakly ionising. How far do they go? However, it means that it has a long range in air. What do they go through? It will pass through paper, aluminium and steel. However, it is stopped by lead or thick pieces of other metals. Effect of magnetic field Beta particles are easily deflected in a magnetic field because of their small mass. They will travel in a circular path in a uniform field (see page 19). The direction of their deflection tells us whether they are positively or negatively charged. Effect of electric field They are deflected in an electric field and will move in a parabolic curve in a uniform field.
Picture 2.7 Gamma radiation.	Gamma radiation - g What is it? Gamma radiation is at the high frequency end of the electromagnetic spectrum. It has a very short wavelength (much less than the radius of an atom) and will pass through atoms with very little chance of being deflected or absorbed. Gamma radiation is often given out with alpha and beta radiation. What's its charge? It has no charge. What's its symbol? How fast does it go? As it is part of the electromagnetic spectrum, it travels at the speed of light – 300, 000 km.s-1. How ionising is it? It will tend to pass through matter without causing much ionisation. How far does it go? It has an extremely long range in air but gets weaker with distance. Its intensity obeys an inverse square law – getting weaker with the square of distance, i.e. doubling the distance quarters the intensity. Tripling the distance leads to a ninth of the intensity. What does it go through? It will get through thin samples of most materials without any noticeable decrease in intensity. However, its intensity is reduced by lead or very thick pieces of other metals. The thicker the sample, the greater the reduction in intensity. Effect of magnetic field No effect because it has no charge. Effect of electric field No effect because it has no charge.
	Question 6 a) Gamma radiation obeys an inverse square law. Its intensity drops with the square of distance because the radiation spreads out. Explain why alpha and beta do not obey the inverse square law. b) Look at picture 2.6. This experiment can be set up in a school laboratory. It would be more difficult (or impossible) to set up a similar experiment to show alpha particle deflecting. Explain why this is the case.