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CURTIN UNIVERSITY Faculty of Science and Engineering Department of Imaging and Applied Physics FAR Labs Radiation Experiment Aim · Investigate different types of radioactivity · Investigate the penetration of radiation through various materials. · Investigate the attenuation of radiation with distance · Understand that radiation is an unstable isotope’s way of trying to become stable. Theory Although we can’t see it, radiation is a part of our everyday lives. It has always been there –radiation has been a part of human lives since long before we were humans. Radiation refers to a transmission of energy. There are two types of radiation: electromagnetic radiation and nuclear radiation. Electromagnetic radiation refers to energy transmitted as waves. Low frequency (low energy, non dangerous) oscillations are radio waves, microwaves, infrared (heat) waves, or visible light. Energy transmitted as high frequency waves such as ultraviolet light, x-ray or gamma waves have enough energy to be capable of ionising atoms (removing their electrons and turning them into charged ‘ions’). The ionisation process can cause damage to living tissue. Nuclear radiation refers to energy transmitted by particles emitted from the nuclei of atoms. In this experiment you will be exploring nuclear and ionising radiation. Figure 1: Types of radiation: showing both nuclear particles and waves (rays), which can also be divided into ionising and non-ionising radiation. Simply by living on planet earth, we are exposed to a level of ionising radiation. This is because the earth itself is radioactive! Not only that, but certain foods are actually slightly radioactive. Don’t worry though, you don’t have to change what you eat – if you attempted to eat enough bananas to have a significant radiation dose, well, you’d get pretty massively sick long before you succeeded. Aside from the ground and some foods, some of our regular background radiation dose comes from stars and outer space! Nuclear radiation is caused by an imbalance of force and energy inside an atom. Atoms exist in a range of different sizes, depending on the number of protons, neutrons and electrons they have. All atoms of a particular type, say silver, have the same number of protons (for silver, that’s 47 protons). So any atom with 47 protons is silver. However, not all silver atoms will have the same number of neutrons in the nucleus. The same goes for all elements. Atoms of a given element with different numbers of neutrons are called different isotopes. Key terms: Radiation refers to the emitted particles or energy from an unstable radioisotope. Radioactivity (also referred to as the activity of a radioactive material) is the frequency or rate that the radioisotope emits radiation. Figure 2: The three isotopes of hydrogen. ‘Normal’ hydrogen is H-1, also known as ‘Protium’. Image sourced from: http://www.ducksters.com/science/chemistry/isotopes.php (accessed 17/02/2016) An imbalanced isotope (also known as a radioisotope) may spontaneously transform itself into a different isotope by ejecting mass and or energy in the form of either alpha or beta particles, and/or the emission of energy as gamma rays. This new isotope may or may not be stable – achieving atomic stability can be a multistage process. This process is known as radioactive decay. Since we can’t see radioactive decay occurring – its way too small – we need two things. One – we need to be able to measure it, and two, we need to be able to protect ourselves from large quantities of radiation, which have the potential to harm us. Detecting Radiation In this experiment, you will be using a Geiger counter, which provides a ‘count’ of each radioactive event it measures. Geiger counters are just one method of detecting radiation. Why does this matter? · In everyday life, we are exposed to a low level of non-harmful, background radiation. · Standard smoke detectors contain a small quantity of an alpha emitter. The alpha particles ionise air particles, producing a current within the detector. Smoke neutralises these ions, causing the current to decrease, activating the alarm. · A fairly common medical imaging test is a PET (Positron Emission Tomography) scan. The patient is given a small dose of a radioactive isotope which emits positive beta particles. These interact with electrons to produce gamma rays, allowing the detector to produce a high resolution image. In particular, this is often used to detect cancerous tumours. Radiation in the Workplace Workplaces which need to consider and understand the effects of radiation include: o Geo-science, geo-physics, mining People who deal with naturally occurring radioactive minerals · Airlines, pilots Working at high altitude means a greater exposure to cosmic radiation · Medicine People who work in medical imaging (such as detecting cancer or taking PET scans) or provide medical radiation treatments · NASA Astronauts are exposed to greater amounts of cosmic radiation than on earth. Workers who deal with radiation above a certain level (i.e. the occupations listed above) are required to have their dosage monitored. Radiation Protection There are four methods of minimising a radiation dose: · Maximising Distance between yourself and the source of radiation · Minimising Time you are near or handling the sample · Use Shielding to minimise exposure · Containment – keep radioactive materials separate from everyday environment Maximising distance How does distance help? Imagine clapping your hands in the middle of a room, with a friend at the other end. They’d hear the sound, and probably wonder what you were doing. Now imagine clapping your hands right next to their head – they’d hear the clap a lot louder, and probably react rather differently. When you clap your hands, or make any other type of sound – the vibrations of that sound spread out in all directions around you, so the initial energy spreads out in an expanding sphere away from the source. The further away from the source of the sound you are, the larger the area the initial energy is spread over, and the quieter the sound seems. Think of it as the surface of an expanding balloon – the bigger the balloon gets, the more the balloon walls need to stretch and the surface of the balloon becomes thinner or more see-through. Radiation – of all types – behaves the same. As you move further and further from the source, the same initial amount of radiation (particles or gamma Figure 3: If we imagine the spherical sound waves produced by a screaming Calvin we notice the sound get rays) becomes spread over a larger and larger area. fainter with distance, as the energy spreads over a This means that as you move away, less and less of it is larger area. Radiation follows the same principle – as it passing through the area you (or your Geiger counter) gets further from the source, the same initial quantity is spread over a larger area, and thus the intensity is in. decreases as distance increases. This reduction in intensity (or radioactivity) is known as the Inverse Square Law: ??2??1 2 = ??1??2 This shows that as you increase your distance (d), by moving from distance 1 to distance 2, from the source, the amount of radioactivity (A) you will record decreases by your increase in distance squared. Thus, if you want to decrease your exposure by a factor of 4 (have an activity level of ¼ your existing activity), you will need to be…. ??1 2 ??2 = ??2??1 Thus, new activity (1) divided by original activity (4) gives: 1 2 1 = ?? 2 1 = 1 4 4 ??2 ??2 = ??2 =2 …. To decrease your exposure by a factor of 4, you need to be √4 times as far away. (2 times as far). Using Shielding Different materials can be used to ‘shield’ or ‘absorb’ a radioactive source. When choosing shielding, both the material and the thickness are important. Different materials will be more or less effective at stopping different types of radiation. Alpha particles are very large and heavy (compared to a beta particle), as well as carrying two protons (double the amount of charge of a beta particle) – which means they have a lot of energy and are highly ionising. The size and weight of an alpha particle also means they are easy to stop. Even air acts as an impediment to alpha particles. Gamma radiation – being a highly energetic form of light – is highly penetrating (hard to stop), but not as ionising as alpha particles. Beta particles are very small, and have a ‘medium’ level of penetration – they are not as penetrating as gamma radiation, but not so easily stopped as alpha particles. Inverse Square Figure 5: Inverse square law experiment. The radioactive source (green) moves along the track and can be positioned close to the detector (blue) or far away. This image shows the source close to the detector, i.e. a small gap. (Image sourced from FAR labs) Figure 6: Turn table experiment. Absorbers and sources rotate into position underneath the detector. (Image sourced from FAR labs) A note about Histograms Histograms are a type of graph that records how often a type of measurement is recorded. They are very similar to a bar graph, except that bar graphs are used for measuring how often a discrete ‘thing’ occurs, whereas histograms measure how often a value falls into a particular ‘range’. Both of these graphs can be used to tell us how often something occurs. A graph to show how many cats, dogs or llamas there are in a backyard is going to be a bar graph (there is no ‘spectrum’ of dog-llama, or cat-dog), but something like the height or age of various trees can only be divided into arbitrary groups. Figure 7: The height of a group of trees is sorted into ranges (groups of heights), using a histogram. Looking at this group of trees, you can see that most trees are between 5 and 10m tall. So a histogram can be a really easy way to determine which category is most popular or occurs most often. The shielding experiment shows you the number of counts the Geiger counter records in