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. 1
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. 2
Radiation 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. 3
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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. 7
Ionisation and Penetration Radiation 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. 8 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-3.0
Demonstrate the Ionising Ability of Radiation 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. 9
Demonstrate the Penetrating Power of Radiation 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. 10
Demonstrate the Penetrating Power of Radiation 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. 11
Radiation NatureChargeIonising Ability Penetrating Power Range helium nucleus +2greatestleasta few cm of air, a piece of paper electronmedium a few cm of aluminium photons0leastgreatesta few cm of lead, thick concrete 12
Uses of Radioisotopes 13
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. 14 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. by svjo-2 – CC-BY-SA-3.0
Solid State Detector Solid state detectors consist of a reverse biasedp-n junction which is sensitive to ionising radiation. 15 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.
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. 16 Higher Level
Activity 17 Higher Level
Half-Life 18
Half-Life 19 Higher Level
Half-Life e.g. What is the half- life of the isotope depicted in the graph if the t-axis shows N? 20 Higher Level