WEBVTT 00:00:00.500 --> 00:00:03.976 Welcome to GCSE Edexcel Science revision. 00:00:04.126 --> 00:00:06.917 Unit S P 6: Radioactivity. 00:00:09.567 --> 00:00:15.699 Thomson’s plum-pudding model described negative electrons embedded in spread-out positive charge. 00:00:15.849 --> 00:00:18.174 Later evidence changed this model. 00:00:20.833 --> 00:00:25.224 Rutherford’s team directed positive alpha particles at thin gold foil. 00:00:25.374 --> 00:00:31.805 Most passed through; some were deflected; very few were deflected through large angles or back towards the source. 00:00:34.467 --> 00:00:37.994 Most passing through showed the atom is mostly empty space. 00:00:38.144 --> 00:00:46.559 Large deflections showed positive charge and much of the mass concentrated in a tiny nucleus; the plum-pudding model could not explain them. 00:00:46.709 --> 00:00:50.705 A small concentrated nucleus explains rare large deflections. 00:00:55.367 --> 00:01:01.903 The nuclear model has positive protons and neutral neutrons in the nucleus, with negative electrons outside it. 00:01:02.053 --> 00:01:06.937 Rutherford’s original model did not yet include the later discovery of neutrons. 00:01:09.600 --> 00:01:23.167 Protons have relative charge plus 1 and mass about 1; neutrons charge 0 and mass about 1; electrons charge minus 1 and much smaller mass, about 1 divided by 1840 of a proton. 00:01:25.833 --> 00:01:31.688 Atomic number Z counts protons; mass or nucleon number A counts protons plus neutrons. 00:01:31.838 --> 00:01:34.099 Neutrons equals A minus Z. 00:01:34.249 --> 00:01:39.208 In a neutral atom, electrons equals protons; ions have unequal numbers. 00:01:39.358 --> 00:01:43.644 A neutral sodium-23 atom also has 11 electrons. 00:01:48.300 --> 00:01:52.462 Isotopes have the same proton number but different neutron numbers. 00:01:52.612 --> 00:01:57.118 Carbon-12, carbon-13 and carbon-14 are isotopes of carbon. 00:01:59.800 --> 00:02:03.403 Bohr’s model uses fixed electron energy levels or shells. 00:02:03.553 --> 00:02:11.017 Electrons absorb energy to move to higher levels and emit electromagnetic radiation when returning to lower levels. 00:02:13.700 --> 00:02:20.109 When electrons move between energy levels, different energy changes produce radiation of different frequencies. 00:02:20.259 --> 00:02:26.669 The separate lines in emission and absorption spectra show that the energy levels have fixed, distinct values. 00:02:26.819 --> 00:02:29.507 Different elements produce different line patterns. 00:02:32.167 --> 00:02:36.525 Ionisation is removal or addition of electrons to form ions. 00:02:36.675 --> 00:02:42.721 Radiation that removes an electron leaves a positive ion; it does not change the number of protons. 00:02:45.400 --> 00:02:51.649 Background radiation is radiation present around us even without a nearby experimental source. 00:02:51.799 --> 00:02:58.824 Natural sources include rocks, radon gas, cosmic radiation and naturally radioactive materials in food. 00:03:01.500 --> 00:03:08.603 Artificial sources include medical uses and smaller contributions from nuclear industry and past activities. 00:03:08.753 --> 00:03:11.982 Contributions vary with location and occupation. 00:03:14.633 --> 00:03:19.537 A Geiger, Müller tube and counter detect ionising radiation as pulses. 00:03:19.687 --> 00:03:24.773 Count rate is counts per second or minute; not every decay in the source is detected. 00:03:27.433 --> 00:03:32.883 Source activity is the number of nuclear decays per second, measured in becquerels (becquerels). 00:03:33.033 --> 00:03:38.876 One becquerels means one decay per second; activity and detector count rate are different quantities. 00:03:41.533 --> 00:03:44.678 Measure background without the source for a suitable time. 00:03:44.828 --> 00:03:51.708 Divide counts by time, then subtract this background count rate from the source-plus-background rate measured consistently. 00:03:54.367 --> 00:03:57.159 Radioactive counts fluctuate randomly. 00:03:57.309 --> 00:04:05.008 Count for longer and repeat to reduce relative random uncertainty; keep distance, detector position and counting time controlled. 00:04:07.667 --> 00:04:10.365 Photographic film darkens on exposure. 00:04:10.515 --> 00:04:17.395 Film badges and other dosimeters monitor exposure; they do not shield the wearer or replace limiting exposure time. 00:04:20.067 --> 00:04:24.536 An unstable nucleus can decay spontaneously, emitting radiation. 00:04:24.686 --> 00:04:30.041 Decay is random: it is impossible to predict when one particular nucleus will decay. 00:04:32.700 --> 00:04:38.393 An alpha particle is a helium nucleus: two protons and two neutrons, charge plus 2. 00:04:38.543 --> 00:04:44.207 Alpha is strongly ionising, short-range in air and stopped by paper or the outer skin layer. 00:04:46.867 --> 00:04:52.818 Beta minus is a fast electron emitted from the nucleus when a neutron changes into a proton. 00:04:52.968 --> 00:04:56.016 It is not an electron ejected from an outer shell. 00:04:58.667 --> 00:05:05.780 Beta plus is a positron, the positive counterpart of an electron, emitted when a proton changes into a neutron. 00:05:05.930 --> 00:05:11.231 Both beta types are moderately penetrating and ionising compared with alpha and gamma. 00:05:13.900 --> 00:05:20.736 A thin aluminium sheet can substantially stop beta radiation; required thickness depends on beta energy. 00:05:20.886 --> 00:05:23.446 Beta travels further in air than alpha. 00:05:26.100 --> 00:05:32.403 Gamma is high-frequency electromagnetic radiation emitted when a nucleus loses excess energy. 00:05:32.553 --> 00:05:36.372 It has no charge or rest mass and does not alter A or Z. 00:05:39.033 --> 00:05:43.357 Gamma is highly penetrating and weakly ionising compared with alpha. 00:05:43.507 --> 00:05:50.183 Thick lead or concrete reduces intensity substantially, rather than guaranteeing every gamma photon is stopped. 00:05:50.333 --> 00:05:55.645 These are qualitative comparisons; shielding reduces the radiation reaching a detector. 00:06:00.300 --> 00:06:06.795 A neutron can also be emitted from a nucleus: mass number falls by 1 and proton number stays unchanged. 00:06:06.945 --> 00:06:09.931 Different radiation types need suitable shielding. 00:06:12.600 --> 00:06:16.655 In alpha decay, A decreases by 4 and Z decreases by 2. 00:06:16.805 --> 00:06:27.503 For example, radium-226 (Z equals 88) becomes radon-222 (Z equals 86) plus helium-4 (Z equals 2). 00:06:30.167 --> 00:06:34.849 In beta-minus decay, A stays unchanged and Z increases by 1. 00:06:34.999 --> 00:06:41.397 The emitted electron has nucleon number 0 and charge number minus 1, keeping equation totals balanced. 00:06:44.067 --> 00:06:48.482 In beta-plus decay, A stays unchanged and Z decreases by 1. 00:06:48.632 --> 00:06:53.239 The emitted positron has nucleon number 0 and charge number plus 1. 00:06:55.900 --> 00:06:58.564 In gamma emission, A and Z stay the same. 00:06:58.714 --> 00:07:05.443 The nucleus has less energy, but it still has the same numbers of protons and neutrons: it is the same nuclide. 00:07:08.100 --> 00:07:14.626 Check nuclear equations by balancing total nucleon numbers and total charge numbers on both sides. 00:07:14.776 --> 00:07:18.597 The identity of the element is set by Z, not by A alone. 00:07:18.747 --> 00:07:22.371 Balance nucleon numbers and charge numbers in each equation. 00:07:27.033 --> 00:07:33.453 Half-life is the time for half the undecayed nuclei in a large sample to decay, or for the activity to halve. 00:07:33.603 --> 00:07:36.652 It does not mean half the total material vanishes. 00:07:39.333 --> 00:07:42.882 The activity decreases as fewer undecayed nuclei remain. 00:07:43.032 --> 00:07:48.988 After 1, 2 and 3 half-lives, the fraction remaining is one half, one quarter and one eighth. 00:07:51.667 --> 00:07:58.183 After n half-lives, remaining activity equals initial activity divided by two to the power n. 00:07:58.333 --> 00:08:02.845 Time elapsed equals number of half-lives times half-life duration. 00:08:05.500 --> 00:08:14.276 If activity starts at 80 becquerels and half-life is 5 years, activities after 5, 10 and 15 years are 40, 20 and 10 becquerels. 00:08:14.426 --> 00:08:19.381 Equal time intervals halve the activity; the curve is exponential, not linear. 00:08:24.033 --> 00:08:30.155 To read half-life from a graph, choose an activity, find when it falls to half, and subtract the times. 00:08:30.305 --> 00:08:35.082 Check several pairs and subtract background from measured count-rate data first. 00:08:37.733 --> 00:08:45.349 Half-life lets us predict how a large radioactive sample will behave overall, even though individual decays are random. 00:08:45.499 --> 00:08:47.962 Smaller samples show more variation. 00:08:48.112 --> 00:08:53.903 As fewer undecayed nuclei remain, fewer decay each second: the decrease is not a straight line. 00:08:56.567 --> 00:09:02.891 Ionising radiation can damage cells and D N A, causing mutations and increasing cancer risk. 00:09:03.041 --> 00:09:07.927 Effects depend on dose and exposure; not every exposure causes a tumour. 00:09:10.600 --> 00:09:17.447 Keep sources at a distance using tongs, minimise exposure time, use appropriate shielding and store them securely. 00:09:17.597 --> 00:09:22.153 Follow supervised handling procedures and never point a source towards someone. 00:09:24.833 --> 00:09:27.702 Irradiation means exposure to radiation. 00:09:27.852 --> 00:09:36.139 Contamination means radioactive material gets onto or into an object or person, so exposure can continue while it remains there. 00:09:36.289 --> 00:09:42.217 Contamination can continue irradiating the person until the material is removed or decays. 00:09:46.900 --> 00:09:52.072 Ordinary irradiation does not mean radioactive material has been deposited. 00:09:52.222 --> 00:09:58.684 Contamination may need removal and containment; merely moving away from the original source is insufficient. 00:10:01.367 --> 00:10:09.962 Alpha is particularly dangerous if an alpha-emitting material enters the body: strong ionisation affects nearby tissue without the skin barrier. 00:10:10.112 --> 00:10:13.652 Penetrating gamma is also an external-exposure hazard. 00:10:16.333 --> 00:10:25.729 Medical staff monitor dose and limit time near sources, use shielding and distance, and balance treatment or diagnostic benefit against radiation risk. 00:10:28.400 --> 00:10:34.522 An ionisation smoke alarm can use an alpha source to ionise air and allow a small current. 00:10:34.672 --> 00:10:41.198 Smoke disrupts this current, triggering an alarm; the source is safely enclosed and should not be handled by users. 00:10:43.867 --> 00:10:49.700 Gamma irradiation can sterilise medical equipment or reduce microorganisms in food. 00:10:49.850 --> 00:11:00.014 Irradiation exposes the item to radiation and does not automatically make it radioactive; contamination means radioactive material is actually transferred. 00:11:02.667 --> 00:11:10.366 A beta source and detector can monitor thin sheet thickness: a thicker sheet absorbs more radiation, reducing detector count. 00:11:10.516 --> 00:11:17.011 Choose radiation with suitable penetration and a source whose output changes slowly enough for the application. 00:11:19.667 --> 00:11:24.467 A radioactive tracer can be introduced into the body and detected externally. 00:11:24.617 --> 00:11:32.295 It needs suitable radiation penetration and a half-life long enough for observation but short enough to limit prolonged exposure. 00:11:34.967 --> 00:11:38.188 PET scanning uses positron-emitting tracers. 00:11:38.338 --> 00:11:49.515 A positron meeting an electron leads to gamma photons that are detected to map activity; short-lived tracers must be produced nearby so useful activity remains when administered. 00:11:52.167 --> 00:11:57.138 External radiotherapy directs radiation at a tumour from outside the body. 00:11:57.288 --> 00:12:06.044 Internal treatment places or delivers a source close to the tumour; targeting and dose planning aim to damage cancer while limiting harm to healthy tissue. 00:12:08.700 --> 00:12:15.085 The risk from a source depends on its activity, radiation type, penetration, exposure route and half-life. 00:12:15.235 --> 00:12:25.153 A very short half-life can mean high initial activity for a fixed initial number of nuclei; a long half-life can create a prolonged contamination or waste problem. 00:12:27.833 --> 00:12:31.849 Nuclear fission splits a heavy nucleus into smaller daughter nuclei. 00:12:31.999 --> 00:12:43.037 A uranium-235 nucleus can absorb a neutron, become unstable and split, releasing energy and two or more neutrons; several different daughter pairs are possible. 00:12:45.700 --> 00:12:49.798 Released neutrons can cause further fissions, forming a chain reaction. 00:12:49.948 --> 00:12:58.246 A controlled reactor maintains a suitable average number of further fissions rather than allowing an uncontrolled rapidly growing reaction. 00:12:58.396 --> 00:13:02.453 Control rods absorb neutrons; they do not act as the moderator. 00:13:07.133 --> 00:13:12.678 A moderator slows neutrons so they are more likely to produce further fission in suitable fuel. 00:13:12.828 --> 00:13:19.877 Control rods absorb neutrons; inserting them further reduces the reaction rate, while withdrawing them can increase it. 00:13:22.533 --> 00:13:24.795 Energy from fission heats a coolant. 00:13:24.945 --> 00:13:34.459 Heat is used to produce steam, which turns a turbine connected to a generator; the nuclear reaction does not itself directly send electricity down the power line. 00:13:37.133 --> 00:13:42.699 Fission products are radioactive and require shielding, cooling and suitable waste management. 00:13:42.849 --> 00:13:48.607 Some wastes stay hazardous for long periods; short half-life does not mean immediate absence of risk. 00:13:51.267 --> 00:13:56.843 Nuclear generation has low direct carbon-dioxide emissions and can provide steady power, 00:13:56.993 --> 00:13:59.148 but evaluate fuel extraction, 00:13:59.298 --> 00:14:00.557 construction, 00:14:00.707 --> 00:14:01.791 cost, 00:14:01.941 --> 00:14:03.370 accident risk, 00:14:03.520 --> 00:14:04.971 public perception, 00:14:05.121 --> 00:14:07.669 safety systems and waste disposal. 00:14:07.819 --> 00:14:11.412 Low direct emissions do not mean zero whole-life emissions. 00:14:14.067 --> 00:14:19.143 Nuclear fusion joins smaller nuclei to make a larger nucleus, releasing energy. 00:14:19.293 --> 00:14:25.873 The products have slightly less total mass than the starting particles; the mass difference corresponds to released energy. 00:14:26.023 --> 00:14:31.005 This is a comparison of processes, not a sequence in which fission becomes fusion. 00:14:35.667 --> 00:14:37.416 Fusion powers stars. 00:14:37.566 --> 00:14:40.361 Positively charged nuclei repel each other, 00:14:40.511 --> 00:14:49.063 so high temperature is needed for sufficient particle energy and suitable pressure or confinement is needed to sustain useful reaction rates. 00:14:51.733 --> 00:14:55.608 Fission splits heavy nuclei; fusion joins light nuclei. 00:14:55.758 --> 00:15:02.187 Radioactive decay, fission and fusion can all release nuclear energy, but they are different processes. 00:15:04.867 --> 00:15:08.203 Generating power by controlled fusion is difficult. 00:15:08.353 --> 00:15:15.723 The very hot ionised gas (plasma) must be kept contained and stable, and the equipment must survive extreme conditions. 00:15:15.873 --> 00:15:19.740 The complete system must produce more useful energy than it uses. 00:15:19.890 --> 00:15:25.648 Producing fusion in a laboratory is not enough by itself to make an economical power station. 00:15:28.300 --> 00:15:36.540 Compare proposed fusion and fission systems using evidence about fuel, waste, safety, technical readiness, costs and useful output. 00:15:36.690 --> 00:15:42.545 Do not describe fusion as an already universal replacement for current electricity generation. 00:15:45.200 --> 00:15:47.440 That completes Radioactivity. 00:15:47.590 --> 00:15:50.935 Revisit the notes and test yourself on the revision website.