Radioactivity Outcomes. Radioactivity Outcomes. Radiation

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1 1 Radioactivity Outcomes Describe the experimental evidence for there being three types of radiation. Discuss the nature and properties of each type. Solve problems about mass and atomic numbers in radioactive decay. Demonstrate ionisation and penetration of each type. Give uses of radioisotopes. Describe the principle of operation of a radiation detector. Demonstrate a radiation detector. 2 Radioactivity Outcomes Define the becquerel (Bq). Interpret nuclear reactions. HL: State the law of radioactive decay. Discuss the concept of half-life. HL: Discuss the decay constant. Solve problems about rates of decay and half-lives. 3 Radioactivity is the decay of unstable nuclei with the emission of one or more types of radiation. There are three types of radiation, evidenced by the effect of an electric field. 1

2 4 Alpha (α) Alpha radiation consists of 2 protons and 2 neutrons. Hence it is often called a helium nucleus. To emit an alpha particle, an atom must therefore lose 2 protons and 2 neutrons. Thus, the atom reduces its atomic number by 2 and its mass number by 4. e.g U Th He e.g Po Pb + α generally, A Z X A 4 Z 2 Y He 5 Beta (β) Beta radiation consists of electrons. A neutron in the nucleus splits up into a proton and an electron. The proton stays in the nucleus and the electron is emitted. Hence, the atom increases its atomic number by 1. e.g Th Pa e e.g Bi Po + β generally, A Z X A Z+1 Y e 6 Gamma (γ) Gamma radiation is high frequency electromagnetic radiation. Particularly after alpha or beta decay, nuclei end up in a high energy excited state, indicated by an asterisk. Falling to the ground state requires emitting a high energy photon. The nucleus does not change composition in gamma decay, so the nuclear reactions are much simpler: e.g Ni Ni + γ 2

3 7 Radioactive Decay e.g. Write the nuclear reaction for potassium-40 undergoing beta decay. e.g. If actinium-225 decays to francium-221, what type of radiation was emitted? e.g. bismuth-214 has a decay chain (i.e. multiple decays in a row) ending at stable lead-206. If lead, bismuth, and polonium are the only elements in the chain, write out each reaction in the decay chain. 8 Ionisation and Penetration can knock electrons out of matter, ionising it. Alpha is the best ioniser, beta is the second best, and gamma is the worst at this. The opposite is true for penetration: gamma requires thick lead or concrete to block it beta will be blocked by a thin sheet of aluminium alpha will be blocked by a sheet of paper, or a few cm of air. by stannered, ehamberg CC-BY-SA Demonstrate the Ionising Ability of 1. Charge an electroscope. 2. Bring a radioactive source near the electroscope. 3. Note that the leaves collapse. 4. The radiation ionises the air around the electroscope and the new charges neutralise the electroscope. 3

4 10 Demonstrate the Penetrating Power of 1. Turn on a GM tube and note the number of counts over two minutes. 2. Aim a source of alpha radiation at the GM tube and record the number of counts over two minutes. 3. Place a sheet of paper between the source and GM tube. Record the number of counts over two minutes. 4. Repeat for sources of beta and gamma radiation, using a thin sheet of aluminium and a thick sheet of lead respectively. 11 Demonstrate the Penetrating Power of 1. Alpha radiation will be blocked by a sheet of paper. 2. Beta radiation will pass through paper, but be blocked by a thin sheet of aluminium. 3. Gamma radiation will pass through paper and aluminium, but be blocked by a thick sheet of lead. 12 Nature Charge Ionising Ability Penetrating Power Range Alpha (α) helium nucleus +2 greatest least a few cm of air, a piece of paper Beta (β) electron -1 medium medium a few cm of aluminium Gamma (γ) photons 0 least greatest a few cm of lead, thick concrete 4

5 by svjo-2 CC-BY-SA Uses of Radioisotopes Medical imaging radioisotopes placed in organs can be used to create an image of the organ. Cancer treatment. Irradiating food to kill bacteria. Carbon dating comparing the presence of C 14 in organisms to the present (1950, before nuclear tests). Tracing movement the movement of isotopes can be tracked in organisms or agriculture. 14 GM Tube A Geiger-Müller tube consists of an inert gas with a high voltage across it. Normally the inert gas does not conduct, but ionising radiation will create ions and electrons. The high voltage accelerates these charges, which bump into neutral molecules, creating more charges. Thus, a single ionisation can produce many charges. Each electron hitting the anode will cause a small current, which is counted. 15 Solid State Detector Solid state detectors consist of a reverse biasedp-n junction which is sensitive to ionising radiation. creates electron-hole pairs in the depletion layer. These charges move due to the voltage across the diode, creating a small pulse of current which can be counted. 5

6 Higher Level Higher Level Activity The activity, A of a radioactive isotope is the number of decays it undergoes per unit time. Activity depends on the type and number of nuclei present. The Becquerel (Bq) is the unit of activity. Activity is 1 Bq if one nucleus decays in one second. The decay of a single nucleus is a random process, so we cannot make predictions or calculations. In a sample, the Law of Radioactive Decay states that the activity, A is proportional to the number of nuclei present, N. Formula: A N 17 Activity The constant of proportionality is the decay constant, λ. Formula: A = λn The decay constant is different for every material. e.g. strontium-90 has a decay constant of s -1. How many atoms are present if it emits beta particles per second? e.g. A sample of radium-226 contains nuclei and is emitting particles per second. Find its decay constant. 18 Half-Life The half-life, T1 Τ2, of a radioactive isotope is the time taken for half of the nuclei in a sample to decay. e.g. After one half-life, half of the sample will remain; after two half-lives, one quarter of the sample will remain, etc. e.g. If the half-life of an isotope is 2 years, what fraction of a sample will have decayed after 10 years? e.g. Technetium-99m is a radioactive isotope used in tracing. If 1 μg is injected into a patient and technetium- 99m has a half-life of 6 hours, how much of the isotope will remain after 1 day? 6

7 Higher Level Higher Level Half-Life Half-life and decay constant are related to each other: Formula: T1 Τ2 = ln λ λ e.g. technetium-99m has a half-life of 6 hours. What is its decay constant? e.g. A sample of a radioactive isotope has atoms. If the half-life of the isotope is s, find its activity. 20 Half-Life e.g. What is the halflife of the isotope depicted in the graph if the t-axis shows N? 7