Teaching radioactivity Cloud chamber Practical Physics photos cloud

Teaching radioactivity

Cloud chamber Practical Physics photos - cloud chamber tracks

An overview • recognising & overcoming teaching challenges • ionising radiation: measuring activity & health effects • radiological protection • class experiments • nuclear decay • contexts for teaching radioactivity • support, references • energy from the nucleus, nuclear decommissioning

Teaching challenges Atoms are unimaginably small and cannot be seen (though ‘pictures’ of atoms are created by techniques such as scanning tunnelling microscopy). All the evidence that we have about atoms is indirect; scientists create models to explain the observations. The random nature of radioactive decay is hard to grasp (though it can be heard using a GM tube and counter) and seems even harder to reconcile with the notion of predictable half-life. (Statistics of extremely large numbers of atoms -> predictability. )

Fear of radiation • usually undetectable by human senses • serious consequences – cancers (time-delayed) – contamination long-lasting • unaware of background radiation • media scares - especially after Chernobyl • secrecy - industrial, military & political interests

Teaching order is crucial Education research shows that • Basic misconceptions are widespread. • A conventional approach that puts theoretical ideas first can be a barrier to understanding. Tried and proven effective: • Start with macroscopic phenomena before moving to microscopic descriptions and explanations. • Use a range of examples to illustrate the relative scale sizes of atoms and nuclei. Robin Millar et al (1990) ‘Teaching about radioactivity and ionising radiation: an alternative approach’ Phys. Educ. 25 338 -342

Contexts for radioactivity Medical physics: radioactive materials for diagnosis & treatment Nuclear electric: how nuclear (fission) reactors work; might fusion be a future energy source? Food irradiation: reliable info on the Food Standards Agency website Other uses of radioactive materials: industry, agriculture, estimating age of Earth, archaeology, domestic smoke detectors Disposal of nuclear waste: an unavoidable problem to solve History of nuclear weapons: bomb designs, espionage & international politics Science in the news: e. g. depleted uranium, polonium-210 Note: Controversial issues require a clear & unbiased treatment.

Source v radiation Common descriptions: ‘a cloud of radiation from Chernobyl’ ‘water unfit to drink because it contains radiation’ so … Carefully distinguish • • radioactive material from the radiation it produces. concepts of activity and dose. Radioactive materials produce ionising radiation (e. g. ). Use a source – journey – detector model of radiation.

Contamination v irradiation Radiation absorbed: Many students believe that objects that have been irradiated (e. g. sterilised syringe or dressing, or food) will themselves become radioactive – that they can reemit the radiation some time later. In effect, they seem to think that radiation is somehow ‘conserved’. In everyday language, when we say that a sponge has absorbed water, we assume that it can release the water later. so … Distinguish between contamination & irradiation.

More formal thinking Becoming quantitative: Focus on • the ‘strength’ of radioactive materials (their activity) • the rate of change of this strength (half-life) • radiation damage possibly done to a person (radiation dose). Under a microscope: Consider • • What actually happens when radiation is emitted? Is the material left behind still radioactive? What happens when radiation is absorbed? How is it possible that radiation cause, as well as cure cancer? Atom and nucleus. Nature of . Damaging DNA. Many random decays make a pattern.

Ionisation – the key concept 1896: Becquerel fortuitously discovered radioactivity, while investigating phosphorescence in uranium salts. Invisible rays (ionising radiation) from a fluorescent substance, potassium uranyl sulfate, were detected by a photographic plate. 1903: Becquerel shared a Nobel Prize with Pierre & Marie Curie for discovering radioactivity. The becquerel (Bq) is the SI unit of activity of a radioactive sample. A sample of radioactive material with activity 1 Bq has one nucleus decay per second. 1 Bq = 1 s-1. An adult human has an activity of ~4000 Bq.

Detecting ionising radiation • cloud chamber • spark counter, with related animation • gold leaf electroscope • Geiger-Müller tube • ionization chamber • photographic film • bubble chamber • scintillation counter • semiconductor detectors • multi-wire proportional chamber etc… www. darvill. clara. net/nucrad/detect. htm

UK background radiation

Radiation dose Absorbed dose The amount of energy that cells absorb, measured in grays (Gy). 1 gray = 1 joule absorbed per kg of tissue Equivalent dose A measure of possible harm from radiation, also taking account of the radiation type, measured in sieverts (Sv). UK annual average dose is 2. 6 m. Sv. Maximum allowable dose for employees is 20 m. Sv.

Health effects of radiation Several things can happen when an ionising radiation penetrates a cell: • The cell is unaffected. • The cell is damaged but is able to repair itself. • The cell is killed. • The cell’s DNA is damaged but remains able to reproduce itself, in its modified form. This cell could become cancerous. If a sex cell is hit, ionisation may cause a genetic mutation.

An analogy. . . Here's a way to think about measures of radiation: Imagine that you're out in a rainstorm. • The amount of rain falling is measured in becquerels. • The amount of rain hitting you is measured in grays. • How wet you get is measured in sieverts.

Radiological protection Three principles: justification - Show that the benefits outweigh the harm that the radiation might cause. optimisation - Keep all exposures as low as reasonable achievable (control measures involve increasing distance, using shielding materials, and/or reducing exposure time). dose limitation - Keep the total dose for workers below specified limits. These principles apply to potential accidental exposures as well as predictable normal exposures.

Sealed radioactive sources Currently available from education suppliers: • • cobalt-60: pure gamma (if low energy betas are filtered out) strontium-90: pure beta americium-241: alpha and some gamma caesium-137: beta, then gamma (from its decay product, metastable Ba-137) Other sources you may have in your school • radium-226: alpha, beta and gamma • plutonium-239: pure alpha

Ionising Radiations Regs (1999) The employer must appoint a qualified Radiation Protection Adviser. Schools and colleges must – account for, store properly, handle safely, & monitor radioactive substances – have standard operating procedures, with a designated Radiation Protection Supervisor – ensure suitable risk assessments in advance of practical work CLEAPSS booklet L 93 Managing Ionising radiations & radioactive substances

School-based training • security & storage arrangements • record keeping • safe handling of each type of source • correct use of associated equipment, monitoring • action if source is dropped or a spill occurs • when to seek help & advice from the RPS No source should ever be left unattended by the teacher in charge.

A useful comparison In the UK annual whole body dose from background radiation ranges from 1 - 10 m. Sv diagnostic medical radiation gives average dose 0. 37 m. Sv A teacher's hand receives dose of 0. 01 m. Sv during a standard school demonstration. (Dose to students is far lower because of their distance. )

Experiments • investigating natural radioactivity • detectors of ionising radiation: cloud chamber, spark counter, electroscope, GM tube • ionising radiations and their properties • simple model of exponential decay (100 s of dice) A-level: measuring the half-life of Pa-234 or Ba-137

Penetration and absorption Alpha & beta lose Ek in ionising encounters with atoms of the absorbing medium - see IOP animation Gamma may interact with an electron or with a nucleus (in several ways), producing 1 or more ‘secondary electrons’ which ionise. type of radiation number of ion-pairs formed per cm of path in air thickness of aluminium to reduce beam intensity by half /cm alpha 105 0. 0005 beta 103 0. 05 gamma depends on energy of the secondary 8. 0 electrons

A nuclear atom Rutherford, Geiger & Marsden, 1909 -11

Inside the nucleus ‘Nucleons’ (protons and neutrons) are held together by the strong force. Z = proton number (atomic number) A = nucleon number (atomic mass) N = neutron number, A - Z isotopes: same element, different mass e. g.

Nuclear disintegrations Alpha decay Beta decay Gamma decay: Nucleus left in an excited state after emission of alpha or beta. No change in A or Z.

Random decay Radioactivity is a chance process. • The chance of decay for each nucleus is constant with time, independent of temperature, pressure, other physical conditions. • The properties of random decay are best displayed if large numbers of events are involved. • The rate of decay is proportional to the number of undecayed nuclei present. • The half-life of a radioisotope is the average time for half the nuclei present to decay (for the activity to fall to ½ its previous value).

Popcorn! We know that the popcorn will go ‘pop’, but we don’t know exactly which kernel will pop at any given time.

Half-life equal ratios in equal times

Teaching how science works • process of scientific enquiry (how we know what we know) • applications, implications, benefits and risks • making decisions (health, social, economic & environmental effects), including ethical issues • uncertainties in science

Support, references Example teaching scheme, from the Practical Physics website IOP DVD Teaching Radioactivity www. talkphysics. org join the Group “Io. P Teacher Network, London” David Sang (ed, 2011) Teaching secondary physics ASE / Hodder Teachers TV - Demonstrating Physics: Radioactivity www. peep. ac. uk controversy & ethics

Stability Isotopes shown here in black are stable. Radioisotopes are unstable. As the proton number increases, an increasing fraction of neutrons is needed to form a stable nucleus. http: //prezi. com/hllfbv 98 zptq/p 2 -nuclear-decay/

Energy from the nucleus (1) four natural radioactive decay series (start with thorium, neptunium, uranium, or actinium) • spontaneous nuclear reactions: mother, daughter, radiation emitted • (with time) all four series end at Pb (lead) ‘Mass defect’: mass of products is less than mass of reactants • energy released as Ek of fragments,

Energy from the nucleus (2) Fission • 1932: neutron discovered by Chadwick • 1934 onwards: experiments done by Fermi et al in Rome - neutron irradiation of elements, starting with lightest & working through the periodic table up to uranium. Expected transuranic elements. Hahn, Strassman, Lise Meitner repeat with U. • with a critical mass of U, neutrons emitted give a ‘chain reaction’ Fusion • small nuclei combine, releasing more energy than fission e. g.

Nuclear & radiological skills Nuclear and radiological technology has key roles in • the health sector • national defence • nuclear power stations • the clean-up of nuclear legacy • a wide spectrum of research, development and manufacturing activities • a shortage of skilled workers, getting worse DTI report, December 2002 Nuclear decommissioning

Nuclear power in the UK All existing UK nuclear power stations to be decommissioned by 2023, except for Sizewell B. Road to 2010 strategy: "Nuclear power is a proven technology which generates low carbon electricity. It is affordable, dependable, safe, and capable of increasing diversity of energy supply. It is therefore an essential part of any global solution to the related and serious challenges of climate change and energy security. “ 10 sites identified by the Government where new nuclear power stations could be built • streamlined planning process, so 10 stations can open by 2018 • providing 40% of the country’s electricity by 2025

The legacy … ‘Hazardous life’: after 20 half-lives, activity falls to a millionth

Nuclear Decommissioning Authority Government has taken on liabilities from BNFL, UKAEA • research, education and training • operation, decommissioning of nuclear installations • clean-up of 18 nuclear sites • operation of facilities for treating, storing, transporting, disposing of hazardous material 2005: estimated £ 1 b a year for 10 - 15 years, total £ 48 b. Feb 2013 (Commons Public Accounts Committee): ‘total lifetime cost of decommissioning [Sellafield] has now reached £ 67. 5 billion and there’s no indication of when that cost will stop rising. ’