What do we know from GCSE?

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Peregrine Butler
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1 Radioactivity Department of Physics & Centre for Plastic Electronics, Imperial College London Faculty of Natural & Mathematical Sciences, King s College London

2 What do we know from GCSE? Sphere of positive charge with negative electrons embedded throughout it. Mostly empty space with a small, dense, positive nucleus and negative electrons orbiting it.

3 Alpha, Beta and Gamma Alpha Beta Gamma Alpha is intensely ionising.. but short range and kept out by skin Ingestion of alpha emitters can do immense damage to the cells. Beta is less ionising but can penetrate the body. Gamma is highly penetrating but causes little ionisation. Long term exposure leads to damage to DNA

Properties of Radiation Radiation Description Penetration Ionisation Effect of E or B field Alpha Beta Gamma Helium nucleus 2p + 2n Q = + 2 e High speed electron Q = -1 e High speed electron Q = -1 e

4 Properties of Radiation Radiation Description Penetration Ionisation Effect of E or B field Alpha Beta Gamma Helium nucleus 2p + 2n Q = + 2 e High speed electron Q = -1 e High speed electron Q = -1 e Background Radiation: fragments of radioactive elements in rocks and soil, atmosphere, cosmic rays, igneous rocks Few cm air Thin paper Few mm of aluminium Several cm lead, couple of m of concrete Intense (10 4 ion pairs per mm) Less intense (10 2 ion pairs per mm) Weak interaction (1 ion pair per mm). Slight deflection as a positive charge Strong deflection in opposite direction to α. No effect. Gamma decay emits in all directions: Distance x away from source, spread over the surface of a sphere radius = r and area = 4πr 2 Intensity = Energy 4πr 2

5 How to prove inverse square law Plot corrected count rate R (measured count rate background count rate) We know R -. / Where d = x + c (c goes to point of emission of radiation) x = kr 34 /+c Plot x against R 34 /

6 What did we know in 1913? There were 3 types of radiation Alpha (α) Radiation = fast moving +ve particles If, as I have reason to believe, I have disintegrated the nucleus of the atom, this is of far greater significance than the war Rutherford s apology to the international anti-submarine committee for being absent from several meetings during WW1

7 Rutherford, Geiger and Marsden Experiment: Radioactive source in a lead box emitting high-speed alpha particles Vacuum to avoid unwanted collisions Thin gold foil (a few atoms thick) A detector of the scattered particles A means to measure the paths of the scattered particles

Rutherford Model of an Atom Result Most α particles passed straight through the foil with little or no deflection Some α-particles were deflected.

8 Rutherford Model of an Atom Result Most α particles passed straight through the foil with little or no deflection Some α-particles were deflected. Some were deflected only a little, others were more than 90 o. Some alpha particles (around 1 in 10,000) were reflected back. Conclusion Most of the atom must be empty space, with no charge. The centre of the atom must have a positive charge, as it is repelling the positively charged α-particle. Most of the atom s mass is concentrated in a small region (the nucleus) at the centre of the atom

9 Geiger-Muller Tubes Sealed metal tube with thin wire down the middle, filled with non-reactive gas at low pressure Wire kept at +450 V with respect to metal case Radiation enters tube through a thin window at the front Radiation ionises the gas atoms Positive gas atoms are attractive to metal case, electrons move towards the wire Charged particles accelerated by voltage and will collide with other atoms, and cause further ionisation >> avalanche effect Large flow of charge causes a pulse of current in central wire >> amplified >> current on a rate meter Dead-time between measurements +450 V 0 V anode low pressure neon gas end window end window anode radioactive particle A typical Geiger tube can detect separate particles as long as they arrive more than 200 μs apart

10 Random Decay and the Exponential Rate The time at which a radioactive nucleus emits radiation cannot be predicted The activity of a radioactive source is the number of ionising particles emitted per second Each emission = change in nucleus of one atom = decay rate Unit of activity is the becquerel (Bq) 1 Bq = 1 decay per second Count rate = # Counts per second The half-life of a sample is the time taken for half of the particles to decay N = λn t Minus sign = N is a reduction in N- number decreases over time Decay constant λ measures the probability a nucleus decays in a second

11 Random Decay and the Exponential Rule N t = dn dt = λn dn N = λdt ln N = λt + A A = constant, ln is the log of base e At t=0, N=N 0 à ln N? = A ln N = λt + ln N? λt = ln N? ln N λt = ln N? ln N

12 Half-Life λt B B à ln B B C = λtà N = N? e 3DE When N = N 0, t = 0, when t = t4, N = - N / F 0 λt - = ln N? F ln N = ln2? 2 = ln 2 λ t - F

13 Uses and Dangers of Radioactive decay Smoke detectors: Small amounts of americium-241 (t4 / Decays via alpha decay = 433 years) Alpha particles collide with air molecules and ionise Ionised air moves to electrodes continuous current Smoke absorbs alpha particles Carbon dating: Carbon-14 radioactive with t4=5730 years / Carbon-14 combines with oxygen >> carbon dioxide Carbon-12 and carbon-14 are taken up by plants When alive, recycle carbon-12 and carbon-14 at same rate Plants die, carbon-14 stops being recycled Measure ratio of carbon-12 to carbon-14 to track age Radioactive waste: Contains both short- and long-lived isotopes which are highly active Isotopes with very long half-lives Nuclear power station can t store underground: very difficult to choose location, very expensive

14 Nuclear Stability Nucleus to be stable, net attractive force (short range nuclear force) must be > net repulsive force (electric) Nearly all nuclear decays = βdecay (electron emitted) Too many neutrons: -J IC -J L C + β 3 Too few neutrons: -O -O N0 L N + β P

15 Excited Nuclear States TTU Technetium JSTc m à metastable (nucleons at an energy level higher than stable technetium) Return to normal with half-life 6 hours, emitting gamma rays of 140 kev = very easy to detect JS TT Tc is radioactive with half-life 216,000 years >> basically stable Inject into bloodstream and track passage through an organ, or radiolabel specific areas

16 Nuclear Radius Louis de Broglie, E = hf applies to photons and electrons E = pc = hf = h c Y λ λ = h p = h mv Electron diffraction was demonstration by Davisson in 1925: bombard nickel target with low energy electrons Rows of atoms in a crystal act as a diffraction grating > reflected beams showed regions with and without electrons nλ = d sin θ Calculate wavelength of electrons

17 Nuclear Radius Radius depends on the total number of nucleons (A) R = R? A 4 _ (R = radius in metres, r 0 radius of nucleon, A = nucleon number) V = a _ πrs = a _ π R?A 4 _ S = a _ πr? S A Where M = mass of nucleons = ma ρ = c d = fu a _ gh C _ f = SU Jgh C _

18 Nuclear Energy E = mc 2 Binding energy per nucleon = energy required to take one nucleon out of the nucleus Tells us how strongly bound nucleons are in different nuclei 1 atomic mass = u = MeV

19 Nuclear Instability Small atoms are stable Big atoms are unstable

20 Fusion vs. Fission > Nuclear FUSION requires atoms to be brought close enough together for long enough time for nuclear force to act > Nuclear FISSION is the result of an unstable nucleus

21 The Nuclear Reactor Uranium-235: fission occurs all the time and produces neutrons The uranium splits up into two lighter nuclei (barium & krypton) and 2 neutrons are released. These two neutrons can collide with other uranium nuclei to cause more fission More fission = more neutrons à large enough mass of fissile (fusionable material) = runaway chain reaction If there is only a small amount of uranium most of the neutrons will leave the material without colliding with other uranium nuclei. For a chain reaction the uranium must be above the critical mass (this minimum mass required for a chain reaction to take place).

22 The Nuclear Reactor Structure of nuclear reactor: Control rods: Reactor core: Coolant

23 The Nuclear Reactor Neutrons are fast moving = to increase the probability of a neutron entering a nucleus they must be slowed down by the moderator. Slow moving neutrons are called thermal neutrons. The moderator = graphite or even water. How to choose material for the moderator; they should be available in large amounts they should have a small/light nuclei they should not become radioactive when bombarded with neutrons if a solid is used it should have a high melting point

24 The Nuclear Reactor The control rods can be moved into and out of the core to slow down or speed up the rate of the nuclear fission. They control rods have a high melting point, and slow fusion by absorbing neutrons. The control rods can be made of boron or cadmium. The coolant in the reactor which can be water or carbon dioxide gas. Removes the heat energy from the reactor before the heat energy is transferred to water in the heat exchanger. It should have a high specific heat capacity, be noncorrosive and stable at high temperatures

25 Safety Shielding is provided by the steel casing of the reactor and several metres of concrete. Radioactive waste materials: Low level waste like protective clothing and obsolete equipment is buried underground. Intermediate waste like parts of the reactor or fuel cladding are put in steel drums and encased in concrete for storage underground. High level waste like very radioactive materials or unwanted fission products are fused into glass blocks (vitrification) and stored underground. Emergency shut down If the rate of the nuclear fission reaction is increasing or if there is some other emergency then the reactor is shut down by dropping the control rods into the core of the reactor.

PHYSICS A2 UNIT 2 SECTION 1: RADIOACTIVITY & NUCLEAR ENERGY

PHYSICS A2 UNIT 2 SECTION 1: RADIOACTIVITY & NUCLEAR ENERGY THE ATOMIC NUCLEUS / NUCLEAR RADIUS & DENSITY / PROPERTIES OF NUCLEAR RADIATION / INTENSITY & BACKGROUND RADIATION / EXPONENTIAL LAW OF DECAY