# Episode 516: Exponential and logarithmic equations

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1 Episode 516: Exponential and logarithmic equations Students may find this mathematical section difficult. It is worth pointing out that they have already covered the basic ideas of radioactive decay in the earlier episodes. Summary Discussion: The exponential decay equation. (15 minutes) Student question: An example using the equation (20 minutes) Discussion: The logarithmic form of the equation. (15 minutes) Worked example: Using the log equation. (20 minutes) Student questions: Practice calculations. (60 minutes) Discussion (optional): Using Lilley s formula. (15 minutes) Discussion: The exponential decay equation Explain that the equation N = N 0 e -λt can be used to generate an exponential decay graph. Work through a numerical example, perhaps related to the dice-throwing analogue (N 0 = 100; λ = 1/6). Make sure that your students know how to use the e x key on their calculators. Emphasise that similar equations apply to activity A (A = A 0 e -λt ) and count rate C (C = C 0 e -λt ). (Not all the radiation emitted in all directions by a source will collected by a detector lined up in one direction from a source) Student question: An example using the equation Set students the task of drawing a graph for a lab source, e.g. Co-60 (λ = y -1, C 0 = 200 counts s -1 ). They should first calculate and tabulate values of C at intervals of 1 year, and then draw a graph. From the graph, deduce half-life. Does this agree with the value from T 1/2 = ln 2/ λ = 0.693/ λ? Discussion: The logarithmic form of the equation Point out that a straight line graph is usually more useful than a curve, particularly when dealing with experimental data. Introduce the equation lnn = lnn 0 - λt. Emphasise that this embodies the same relationship as the exponential equation. Use a sketch graph to how its relationship to the straight line equation y = mx c (intercept = lnn 0, gradient = λ). 1

3 This formula works for non integral values of n; i.e. it also gives the fraction remaining yet to decay after any non-whole number of half-lives (e.g. 2.4, or 3.794). To use this formula, a little skill with a calculator is all that is required. For example: The T 1/2 of 14 C is 5730 yrs. What fraction f of a sample of 14 C remains after years? Answer: f = (1/2) n The number of half lives n = /5730 = Thus the fraction remaining f = (1/2) And using the y x button on a calculator gives f = If students know how to take logs (or can lean the log version of the formula), they can solve other problems: How many years will it take for 99% of 60 Co to decay if its half life is 5.23 yr? The fraction remaining = 1%, so f = Hence 0.01 = (1/2) n Taking logs of both sides gives ln (0.01) = n ln (1/2) giving n = 6.64 half lives, and so the number of years = = 34.7 years. 3

4 TAP 516-1: Decay in theory and practice Fractions and activity 1. In exponential decay the number of atoms remaining after a given interval of time is always the same fraction of the number present at the beginning of the time interval. Therefore, someone might say, however long you wait there will always be some left. A sample of radioactive atoms will last for an infinite time. Is this reasonable? Explain your answer. 2. The decay constant for caesium-137 is s 1. Calculate the number of atoms in a sample of caesium-137 that has an activity of Bq. Experimental exponentials No ordinary place on the Earth s surface is free from background radiation. In making a measurement of this background a student set up a GM tube and counter and recorded the counter reading every 30 s. Time / s Count Using these data, determine the background count per minute. What would you expect the total count to be after a further 30 seconds? 4

5 A radioactive source is set up in front of the same GM tube in the same laboratory. Counter readings of the activity of the source are taken for 1 minute at time intervals of 1 hour. Time / h A / counts min What action should be taken to deal with the background count? 5. Plot a graph to show how the activity of the radioactive substance changes with time and derive from it three different values of the half-life. Calculate the mean of these. 5

6 6. Worried by the obvious fluctuations in the final hours of the count, one student suggests that it might be wise to continue counting for at least as long again. Is this a good idea? Explain your answer. 7. There is a slight increase in the count rate towards the end of the experiment. Is this significant? Explain your answer. 8. Suggest steps you might take to improve the experiment. 6

7 Practical advice This question compares the theoretical exponential decay with the messy practical situation. This comparison can be used to bring out some of the characteristics of the models. Answers and worked solutions 1. No. The smooth exponential model provides a good description only when there are a very large number of atoms. However, when the sample is reduced to a few atoms, the model is a poor fit. How long an atom will last on average is known, but it is not possible to say how long one particular atom will last. 2. dn dt = λn N = dn / dt λ = activity λ = Bq s 1 = atoms counts background count = = 30 counts min 4 minutes After further half minute count = / 2 = 134 counts. 4. Subtract the background count from each of the tabulated counts min The half-life is about 80 minutes. 6. The count rate would not change significantly if the counting were continued for a longer period. Very little of the substance remains after 8 hours. 7. No; random fluctuations in the background count rate are probably responsible for this increase. 8. The experiment could be repeated and readings taken at intervals closer than 1 hour during the first few hours. 1 External reference This activity is taken from Advancing Physics chapter 10, 30S 7

8 TAP 516-2: Radioactive decay with exponentials 1. The half-life of one radioactive isotope of sodium is 2.6 years. Show that its decay constant is s Calculate the activity of a sample containing one mole of the sodium. (One mole contains atoms.) A scientist wishes to find the age of a sample of rock. Realising that it contains radioactive potassium, which decays to give a stable form of argon, the scientist started by making the following measurements: decay rate of the potassium in the sample = 0.16 Bq mass of potassium in the sample = g mass of argon in the sample = g 3. The molar mass of the potassium is 40 g. Show that the decay constant λ for potassium is s 1 and its half life is years. 4. Calculate the age of the rock, assuming that originally there was no argon in the sample and the total mass has not changed. Show the steps in your calculation. 5. Identify and explain a difficulty involved in measuring the decay rate of 0.16 Bq given above. 6. Iodine 124, which is used in medical diagnosis, has a half-life of 4.2 days. Estimate the fraction remaining after 10 days. 8

9 7. Explain how you would find the half-life of a substance when it is known to be more than years. Assume that a sample of the substance can be isolated. In an experiment to find the half-life of zinc-63, a sample containing a sample of the radioactive zinc was placed close to a GM tube and the following readings were recorded. The background count rate was 30 min 1. Time / hours Counts / min Plot a graph of count rate against time and use this to find the average time for the count rate to fall to one-half of its previous value. 9. Plot a second graph, ln (count rate) against time, and use it to find the half-life. 10. Discuss which method, 8 or 9, provides a more reliable value. 9

10 Practical advice The questions provide a variety of kinds of practice, at post-16 standard, with radioactive decay, half life and decay constants. Answers and worked solutions 1. ln 2 λ = = t y s y 1 = s 1 2. dn A = dt = λn = ( = s Bq 1 ) ( ) 3. t dn / dt λ = N 1 2 = [( = ln2 = λ = = g)/40 g] ( s s Bq s = years 23 ) g g = 1 8 = so there have been three half-lives years = years 5. The background count rate is likely to be higher than 0.16 Bq. 6. There have been 10 days 1 = 2.38 half-lives. The ratio remaining is days 2 = Place a GM tube near the sample and measure the count rate over a long period of time. Determine a value for dn/dt. Determine N by chemical means, and from that find λ and T 1/2. 10

11 8. Subtract the background count rate from all readings. The half-life is about 40 minutes. first sample half life 200 to 100 in 0.6h 200 second sample half life 160 to 80 in 0.65h third sample half life 120 to 60 in 0.7h average time = 0.65 h 40 min h 0.65 h 0.7 h t / h 9. Corrected count rate / min 1 ln corrected count rate / min

12 = 2.0 h gradient = λ = h t1 = ln 2 = ln h = 0.60 h 2 λ 2.3 = 36 min t / h 10. The second method is more reliable. By drawing a straight line of best fit in question 9, all the data are being averaged rather than just the three pairs taken from the curve in question 8. External reference This activity is taken from Advancing Physics chapter 10, 90S 12

13 TAP 516-3: Radioactive decay used as a clock Because of the predictability of the random behaviour of large numbers of atoms, activity can be used as a clock. Clocking radioactive decay Half-life Activity N 0 N 0 /2 number N of nuclei halves every time t increases by half-life t 1/2 slope = activity = dn dt halves every half-life N 0 /4 N 0 /8 t 1/2 t 1/2 t 1/2 t 1/2 time t In any time t the number N is reduced by a constant factor In one half-life t 1/2 the number N is reduced by a factor 2 In L half-lives the number N is reduced by a factor 2 L (e.g. in 3 half-lives N is reduced by the factor 2 3 = 8) t 1/2 time t Radioactive clock Measure activity. Activity proportional to number N left Find factor F by which activity has been reduced Calculate L so that 2 L = F L = log 2 F age = t 1/2 L 13

14 Practical advice This diagram is reproduced here so that you can talk through it, or adapt it to your own purposes. External reference This activity is taken from Advancing Physics chapter 10, Display material 20O 14

15 TAP 516-4: Two important dating techniques Radiocarbon dating This technique was devised by American physicist W. F. Libby in Carbon-14 ( 14 C) is a naturally occurring radioactive isotope of carbon which is produced when neutrons associated with cosmic rays collide with nitrogen ( 14 N) in the atmosphere. The 14 C is taken up by living organisms (plants, animals); the ratio of 14 C to 12 C in living tissue remains constant during the life of the organism and depends only on the relative proportions of 14 C and 12 C present in the atmosphere. ( 12 C is the `ordinary', stable, abundant isotope of carbon.) When the organism dies, the ratio of 14 C to 12 C decreases because the 14 C decays to produce nitrogen: After about 5700 years (the half-life of 14 C), the ratio of 14 C to 12 C falls to half its initial value; after a further 5700 years the ratio halves again ± and so on. The 14 C / 12 C ratio therefore decreases in a known and predictable way as the sample ages. If the original ratio of 14 C / 12 C is known for an organic sample, say bone, and the present ratio can be measured, then an accurate value for the age of the sample can be calculated. To measure the ratio of 14 C to 12 C, the sample needs to be ground up, thus destroyed. The 14 C in a sample is detected and measured via its radioactive emission, and the concentration of carbon is measured chemically. The method works best on samples containing large quantities of carbon. Thus wood, charcoal, bone and shells of land and sea animals are good archaeological samples. Around 100 g of wood or 30 g of charcoal is required to obtain a date. Bone contains a smaller proportion of carbon so more bone is required than wood (around 1 kg). Around 100 g of shell is required for dating. The precision of the age measurement decreases with age of the sample because the amount of 14 C decreases with time. The half-life of 14 C is 5730 ± 40 years so the method is most reliable for dating samples no more than a few thousand years old. For a sample about years old the uncertainty is about ± 2000 years. The range of radiocarbon dating is at most years. The method rests on the assumption that the sample gains no 14 C after death, and that the atmospheric ratio of 14 C / 12 C has remained constant with time. It is therefore necessary to compare radiocarbon dates against other independent methods of dating such as dendrochronology. For an article on the radiocarbon method, see: Radioactive decay. Dobson, K., Physics Review, Vol. 4, No. 2, pp A good World Wide Web site for details on the method and much besides is: 15

16 Dendrochronology The name of this method comes from the Greek `dendron' (tree) and `chronos' (time). The cross-section of a tree trunk shows a series of concentric circles known as growth rings. Each growth ring represents one year of the life of the tree. The thickness of each ring depends on the climatic conditions in the year that the ring was laid down. Thus the rings show a distinctive pattern of varying thickness. The same pattern can be seen in timber of different ages and so a pattern covering a considerable period of time can be constructed. Once the pattern has been established and tied to known dates, a pattern in a timber of unknown date can be matched against the established pattern and its date determined. The first person to suggest dendrochronology as a dating method was Thomas Jefferson, one of the first American Presidents, who suggested its use in dating Native American burial mounds. The method can be used with wood grown since the last Ice Age, about years ago. One of the longest patterns established is that using the Bristlecone Pine which grows in California. The arid conditions of the area allow samples of the pine to survive in good condition for thousands of years. Apart from its use as an absolute method of dating wood, dendrochronology is also used to calibrate radiocarbon dates. When radiocarbon dates are matched against the tree ring dates, it is found that radiocarbon `years' do not equate directly with calendar years because the amount of 14 C in the atmosphere varies slightly. In general, radiocarbon dating gives a young age for older samples (e.g. a radiocarbon date of 4100 BC may be closer to a real date of 5000 BC). The method does have its limitations. Climatic conditions vary from place to place and so a particular pattern can only be used locally. The wood has to be in a good state of preservation; such wood is difficult to find on archaeological sites. The wood on a site may not indicate the true age of the site. For example, the wood could have been cut down many years before its use on a particular site. And not all types of wood can be used since some types have growth rings of uniform thickness regardless of climatic condition 16

17 Practical advice This activity summarises two important dating techniques used in modern archaeology. External reference This activity is taken from Salters Horners Advanced Physics, AS, DIG additional activity 10 17

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