Use of the Geiger-Müller Counter and the Cloud Chamber to Present Properties of Radioactivity to Youngsters

Transcription

1 Use of the Geiger-Müller Counter and the Cloud Chamber to Present Properties of Radioactivity to Youngsters ABSTRACT Vesna Slapar Borišek Jožef Stefan Institute Jamova , Ljubljana, Slovenia At the Information Centre that was established within the Nuclear Training Centre at the Jožef Stefan Institute visitors are informed about nuclear technology, the basic properties of radioactivity and protection against ionising radiation. The results of the polling of youngsters, conducted over the last 22 years, showed poor understanding of radioactivity. These results are one of the reasons that demonstrations about radioactivity are performed to familiarize visitors with this phenomenon. Demonstrations of the basic properties of the radioactivity are part of each visit, which follow the lecture about nuclear technology. During these demonstrations the Geiger-Müller counter and the cloud chamber are used to show properties of different types of radioactive radiation using training sources. As a part of demonstrations additional experiments are shown to present the natural radioactivity and the comparison of different types of electromagnetic radiation. 1 INTRODUCTION Since the establishment of the Information Centre at the Nuclear Training Centre at the Jožef Stefan Institute in 1993, there have been 3421 visits, resulting in visitors (approximately 7000 visitors per year). Majority of the visitors are youngsters aged between 13 and 16 years old [1]. Every year about 1000 youngsters are polled at the beginning of their visit about their acceptance and knowledge of nuclear energy. Part of this poll is about understanding of some basic facts about nuclear energy and radioactive waste. As can be seen from the poll [1], their knowledge about radioactivity is very poor. For this reason and taking into account that in school curriculums there is almost no time for this subjects it was decided to make demonstrations about basic facts of radioactivity part of every visit. Now every visit consists of a live lecture about nuclear energy, demonstrations about basic properties of radioactivity and protection against ionising radiation and a tour of the permanent exhibition about nuclear technology. Depending on the interest of the visitors a tour to research reactor TRIGA can be performed

2 DEMONSTRATIONS After a live lecture about nuclear power the demonstrations are performed. At the beginning of demonstrations the fundamentals of radioactivity are explained; the difference between nuclear reactions that are happening inside the nuclear fuel and the radioactive decay that is happening in the spent fuel and then the different types of radioactive decay and the differences among them. The demonstrations are divided into two parts. First we present different types of radioactive decay and their properties using the Geiger-Müller counter. Afterwards we talk about naturally occurring radioactivity, performing additional experiments and presenting it with the cloud chamber. 2.1 Geiger-Müller Counter The experimental set-up can be seen on Figure 1. There is a ratemeter and a sensitive End Window GM tube with measuring scale projected using the overhead projector, while the signal of the detector is connected with the speaker, so that the detector response can be monitored over the audio signal. All the experiments are performed using pure school sources below exemption level [2] (Figure 2). Figure 1: Experimental set-up. Figure 2: Training sources. On the left is a source of alpha radiation 210 Po, in the middle is a source of beta radiation 90 Sr, and on the right is a source of gamma radiation 60 Co. As the source of alpha particles 210 Po is used, with initial activity of 3,7 kbq. Due to short half-life of polonium-210 (t1/2 = 138 days), the alpha source has short lifetime, and has to be replaced approximately once a year. Different types of materials, like sheet of paper and aluminium foil are placed between the source and the detector, to show how to stop the alpha

3 106.3 particles. Besides it is shown that just a couple of centimetres of air is enough to stop alpha radiation. And with this conclusion protection from alpha radiation is explained. As a source of beta radiation 90 Sr, with activity of 3,7 kbq, is used. For beta particles a sheet of paper and an aluminium foil is used at the beginning. When youngsters realize that beta particles can penetrate through these materials, greater amount of paper and aluminium are used to explain that the same material can stop alpha and beta particles, but the amount of material is different, like 10 mm of paper instead of one sheet or 5 mm of aluminium. After those two experiments the difference between alpha and beta particles is discussed and explained with some everyday examples why the beta particles have the greater penetration depth in the matter. At the end gamma radiation is presented using the 60 Co, with activity of 37 kbq, as a source of radiation. Here the difference between the particle radiation and the electromagnetic radiation is shown, and explained that gamma radiation can never be stopped, but can be attenuated to the level that is not harmful for humans. To bring the radioactivity closer to youngsters items they know are used. An example of it is an old clock with luminous indicators and dial. Indicators and dial are painted with radium based luminous pain. The reading on the Geiger-Müller counter shows the presence of the radioactivity and the youngsters can quite easily determine that even though radium is alphaactive, some more penetrative particles are emitted from the clock. 2.2 Natural Radiation Due to Radon It is very important that youngsters realize that radioactivity is a natural phenomenon and that ionizing radiation is a part of everyday life. To show the presence of natural radioactivity two simple experiments are used. First is an experiment with a balloon. Empty balloons are first measured with Geiger-Muller counter, to demonstrate that they are not radioactive. The youngsters than inflate the balloons and rub them with wool to electrically charge them. Then they hold the balloons for a couple of minutes. Meanwhile another experiment is performed using a vacuum cleaner and a piece of filter paper. Filter paper is first measured with Geiger- Muller counter to show that it is not radioactive and is then placed on the suction nozzle of the vacuum cleaner which is turned on for one minute. After one minute the filter paper is again measured with Geiger-Muller counter. As the reading on the counter is much higher than before, the reasons for it are discussed. First of all the origin of radioactivity on the filter paper is clarified and where it comes from. It is explained that the radioactivity that was collected on the filter paper with vacuum cleaner due to radon progenies is a part of naturally occurring radioactivity. At this point some words are dedicated to the natural radioactivity in general. It is explained that it is divided to terrestrial radiation coming from the Earth and cosmic radiation coming from the space. Although the focus is on the main contribution to natural radiation, the radon and radon progenies are discussed. After vacuum cleaner experiment the balloons are deflated and measured again. Readings on the Geiger-Muller counter are again higher than they were before the experiment. So this is another way to show the presence of radon and its progenies.

4 106.4 Figure 3: Experiments used to present the natural radioactivity. On the left children inflating balloons and on the right vacuum cleaner with filter paper. 2.3 Cloud Chamber Experiment Presence of the naturally occurring radioactivity can also be demonstrated using the cloud chamber. Our cloud chamber experiments are performed with a small cloud chamber, which uses a cooler with pump for water cooling of the lower surface and electricity for heating the upper plate. On the bottom plate there is a thin layer of alcohol (isopropyl). Conditions inside the cloud chamber results in forming a layer of supersaturated alcohol vapor near the bottom plate. Because of the small upper surface of our cloud chamber the image is recorded by a camera and shown on a computer screen (Figure 4). Figure 4: Cloud chamber set-up. Traces of charged particles can be seen inside the cloud chamber (Figure 5 and Figure 6). At this point it is explained how the cloud chamber is constructed and how it works. It is explained that when charged particles such as cosmic ray muons, alpha particles, and electrons

5 106.5 interacts with air above the supersaturated layer, the air is ionized. The resulting ions act as condensation nuclei, around which the alcohol vapor condenses and leaves a visible trail of the particle s path. Since everyday experiences make things easier to understand, the comparison of traces inside the cloud chamber and the condensation trails that are formed in the sky behind airplanes is made. Like the trails on the sky are the only way to see the airplanes traveling very high, the trails inside the cloud chamber are the only way to see the paths of the charged particles. On the Figure 5 traces of electrons can be seen. On the left picture there is also the trace of alpha particle. It is interesting to hear the youngsters thoughts on how did the alpha particle entered into the cloud chamber. Here again we emphasize the presence of radiation in the environment and it is explained that, since our demonstrations are taking place in the basement, radioactivity that can be seen and measured is mainly due to radon and its progeny. electron alpha electron Figure 5: Natural radioactivity presented with cloud chamber. To emphasize the difference between the alpha particles and electrons a welding rod is inserted into a cloud chamber. The tungsten welding rod doped with thorium-232 ( 232 Th) is used. Fresh thoriated welding rod is pure alpha emitter, but with time on account of thorium progeny ( 228 Ra, 228 Ac, 212 Pb, 212 Bi and 208 Tl) beta and ( 228 Ac and 208 Tl) gamma radiation is present [3]. As can be seen on Figure 6, a lot of alpha particles are emitted from rod s surface. There are also traces of beta particles and some scattered electrons from background radiation and some emitted from the welding rod. When talking about alpha particles emitted from the welding rod during alpha decay of thorium, it is explained that all the alpha particles emitted have the same energy, but as can be seen on the left picture of Figure 6, the traces can differ quite a lot. It is explained that the active layer of our cloud chamber is quite thin, and because the alpha particles are isotropically emitted from the rod, energy left in active layer is different and that is why the traces are not the same length. alpha electron alpha Figure 6: Cloud chamber with welding rod.

6 CONCLUSION The demonstration of radioactivity phenomena is very interesting for the youngsters, because it is not part of the school curriculums. Understanding of the basic properties of the radioactivity and the principles of radiation protection will diminish the unjustified fear. Beside that it is important to familiarize youngsters with presence of naturally occurring radioactivity. They should realize that radioactivity is a natural phenomenon which accompanies us in everyday life and is not just a byproduct of nuclear power plants or means used in medicine. When the properties of the radioactivity are presented by practical experiments it is easier for them to understand and absorb the information. Using tools from everyday life, such as balloons and vacuum cleaner, brings them the topic even closer and makes it more interesting. All the experiments are performed using End Window GM tube, enabling the assessment of the signal on a measuring scale. At this demonstrations the audio signal presents the component that the youngsters pay a lot of attention to, and because of that are more aware of what is happening during demonstrations. Using the cloud chamber as a part of demonstration enables additional visual representation of radiation, which is an additional tool to ease the understanding. Since the visual presentation of radioactivity enables youngsters to be more aware of natural radioactivity and to better understand its properties, the cloud chamber is a good complement to our demonstrations. REFERENCES [1] Radko Istenič, Igor Jenčič (2014). Public Opinion about Nuclear Energy Poll. Nuclear Energy for New Europe Portorož. [2] Matjaž Koželj, Radko Istenič (2011). Radioactivity for Everyone or Twelve Years of Radioativity Demonstrations in Milan Čopič Nuclear Training Centre. Nuclear Energy For New Europe. Bovec. [3] Matjaž Koželj, Radko Istenič (2013). Radioactivity Experiments For Schools. Nuclear Energy For New Europe. Bled.