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WJEC GCSE Additional Science Teacher’s Notes c Use your graph to measure the half-life of iodine-131. PRACTICAL The radioactive decay of protactinium-234 (page 213) Safety: If you are doing this experiment using a protactinium generator, then you must follow the relevant Radioactivity Local Rules for your school and the relevant CLEAPSS guidance. A data-logging GM counter substantially enhances this activity, as the activity can be displayed in real-time as a graph. _ Carbon dating (page 214)______________________ Questions 9. What is carbon dating Carbon dating is a scientific technique for determining the approximate age of an artefact that contains once living matter using the half-life of the naturally occurring carbon-14 isotope. 10. Why do you think that dating dead organic materials over 60 000 years old is almost impossible using carbon dating The half-life of carbon-14 is 5730 years – after 5 half-lives (= 28 650 years), the percentage of carbon-14 left in the sample is very low, after 60 000 years it is so much lower that it cannot be detected. 11. The Turin Shroud is a holy relic, reputedly the shroud used to wrap Jesus in after his crucifixion. The shroud appears to have the image of a man ‘etched’ on one side of the cloth. In 1988, three independent carbon-dating laboratories analysed fibres taken from the shroud and discovered that the 73
WJEC GCSE Additional Science Teacher’s Notes samples studied contained just over 90% of the original amount of carbon-14. Use the carbon dating graph in Figure 19.6 to estimate the age of the Turin Shroud. From the graph, approximately 1/6 of a half-life has elapsed, so 5730/6 = 955 years Sample was produced in 1988 – 955 = 1033AD Discussion point There is much debate about the authenticity of the Turin Shroud. Recent scientific studies have found that while there is no evidence of any scientific forgery, and the origin of the image on the shroud is still unknown, there is also a suggestion that the samples of the cloth examined in 1988 are not representative of the whole shroud. What happens in this case The carbon dating data indicates the shroud is a medieval artefact. Is it possible to prove one way or another that the shroud is real, or a very, very elaborate and clever hoax There are many websites on the net that will help to initiate debate on this topic. This would make an excellent homework task: ‘What evidence is there that the Turin Shroud is a medieval fake’ TASK Using radioactive materials (pages 215–19) 1. Which radioactive element from the table would you use for an RTG Explain your answer. Plutonium-238 is normally used for this application, but from the list on P215, an alpha emitter with a half-life of hundreds of years would be suitable – americium-241. 2. Why do you think that RTGs are only used as the power supply on un-manned devices RTGs are very radioactive and produce too much radiation to be safe on a manned device. 3. Which radioactive elements would be suitable for use as a radioactive tracer Explain your answer. The tracer must be a gamma emitter (so that the radiation can escape the body) with a short half-life (so that the radiation dose received by the patient is minimised) – technicium-99. 4. Why is it important that the radioactive tracer that is used has a very short half-life The tracer must have a short half-life so that the radiation dose received by the patient is minimised. 5. For each of the following forms of radiotherapy suggest and explain which radioisotope(s) you would choose: a external beam radiotherapy A gamma emitter with a half-life measured in hundreds of years would be needed so that the beam of gamma rays remained as constant as possible – Europium-152 or Barium-133. b brachytherapy A beta emitter with a half-life of tens of days would be suitable – iridium-192. c unsealed source radiotherapy A beta emitter with a short half-life – iodine-131. 74
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