7 Radioactivity and particles - iGCSE Physics Edexcel

7.1 use the following units: becquerel (Bq), centimetre (cm), hour (h), minute (min) and second (s)
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(+) Protons: The number of protons is equal to the atomic number

(-) Electrons: Electrons equal to number of protons, due to atom being neutral

Neutrons: Atomic number minus mass number = amount of neutrons

Atoms of same element have same proton and electron amount

Isotopes have varying amounts of neutrons, only affecting its mass.
Isotopes unstable due to size or number of protons and neutrons are unbalanced.

These isotope will decay - emitting little chunks (radiation).
In order to reduce size or stabilize

alpha (α) particles
beta (β−) particles
gamma (γ) rays

Radiation particles can knock out electrons of other atoms, ionising them.

Ionisation causes atoms to become ions, and can cause chemical changes in materials, and can damage or kill living cells
Emits 2 protons and 2 neutrons (same as helium nucleus)
Emitted from nuclei that are too large and have too many protons

Neutron becomes a proton and electron, to balance neutron to proton ratio.
Electron created is emitted

Electromagnetic waves
Emitted for nuclei to lose energy

(α) - Few cm (due to being very heavy)
(β−) - Few 10s of cm
(γ) - Infinite

(α) - Stopped by paper (stopped by human skin)
(β−) - Stopped by few mm of aluminum
(γ) - Reduced by few mm of lead

(α) - High
(β−) - Medium
(γ) - Low
Detect using a Geiger Müller Tube connected to a counter.

Try the three different materials absorbers in order, paper then aluminum then lead.

Repeat 3 times and take an average

Count rate will significantly decrease if radiation is stopped.

When not using a source, keep it in a lead lined container.
When in use, try and keep a good distance
Using tweezers (or tongs) and point the source away from you when handling
Radiation detectors work by detecting the presence of these ionised atoms or the chemical changes produces when ionised.

Photographic film
Geiger-Muller (GM) tubes - electron in gas particle is knocked out, is attracted to tube, forms ion pairs and a creates current.
Ionisation chambers
Scintillation counters
Spark counters
Background radiation is the radiation that is always present around us in the environment

Most background radiation is natural
Artificial sources exist, such as medical procedures like XRAY
BG radiation varies with location.

SOURCES (most to least)
Radon gas
Rocks and building materials
Medical (X-ray)
Cosmic rays
Other (including nuclear power)
7.11 know that the activity of a radioactive source decreases over a period of time and is measured in becquerelsBq - the number of decays occurring in an isotope every second. As an isotope decays, number of nuclei that remain will decrease. Activity of that isotope will also decrease over time.7.12 know the definition of the term half-life and understand that it is different for different radioactive isotopesHALF LIFE Time taken for the activity of an isotope to drop to half of its initial value. Activity and number of nuclei can never fall to zero Different isotopes have different half-lives Length can vary from a fraction of a second to billions of years 1, 1/2, 1/4, 1/8, 1/16, 1/32, 1/64...7.13 use the concept of the half-life to carry out simple calculations on activity, including graphical methodsGRAPHING Time- X axis Activity- Y axis Smooth curve, although never touching Y=0 CALCULATION An isotope has an initial activity of 120 Bq. 6 days later it's activity is 15 Bg. The number of half-lives that have passed is: 120/2 = 60 60/2 = 30 30/2 = 15 We had to halve 120 three times to get to 15, and so three half-lives have passed. Therefore each half-life must be: 6 days/3 = 2 days.7.14 describe uses of radioactivity in industry and medicineSMOKE DETECTOR Alpha particles ionise air inside the detector, allowing a small current to flow within it. When smoke enters the detector the alpha particles are absorbed and the current no longer flows - triggering the alarm. Alpha particles have both a very short range and are unable to penetrate through the casing of the detector MEASURING MATERIAL THICKNESS As a material moves above a beta source, the particles that are able to penetrate it can be monitored using a detector. If the material gets thicker more particles will be absorbed, meaning that less will get through. If the material gets thinner the opposite happens. This allows the machine to make adjustments to keep the thickness of the material constant. TRACERS Radioactive isotopes that can be added to some fluid so that the flow of that fluid can be monitored. Medicine tracers can be added to the blood to check blood flow around the body and search for blockages Industry tracers may be added into an oil pipeline in order to check for any leaks. RADIOTHERAPY Radiation can kill living cells. Bacteria and cancer cells, are more susceptible to radiation than others. Beams of gamma rays are directed at the cancerous tumour, as they can penetrate the body. Meams are moved around to minimise harm to healthy tissue whilst still being aimed at the tumour. STERILISATION The gamma rays kill bacteria on the instruments and destroy viruses. Gamma rays are able to penetrate the instruments reaching areas that may otherwise not be properly sterilised.7.15 describe the difference between contamination and irradiationIRRADIATION Exposing a material to alpha, beta or gamma radiation. Will not make that material radioactive CONTAMINATION Small amounts of the radioactive isotope leak onto the material Contaminated parts of isotope will give off radiation.7.16 describe the dangers of ionising radiations, including: • that radiation can cause mutations in living organisms • that radiation can damage cells and tissue • the problems arising from the disposal of radioactive waste and how the associated risks can be reduced.HEALTH DANGERS Cause mutations. Cause a cell to become cancerous Kill the cell. NUCLEAR WASTE Highly radioactive - thick shielding is required. Very long half-life - need to be stored for long periods of time. Containers must not corrode or degrade over time, which could allow leaks. Geologically safe places - i.e. away from the danger of earthquakes. Kept away from water sources, to prevent water contamination Clearly marked, so that future societies can understand the nature of what is stored there.7.17 know that nuclear reactions, including fission, fusion and radioactive decay, can be a source of energyThe nucleus of the atom contains a huge amount of energy - roughly one million times greater than the amount of energy involved in chemical reactions . As a result, nuclear reactions (such as nuclear fission) have the potential to produce great amounts of energy. If harnessed in a safe way, nuclear energy could reduce or replace our dependency on fossil fuels, reducing pollution and the emission of greenhouse gases. RADIOACTIVE DECAY Radioactive isotopes of elements such as uranium, thorium and potassium release energy through decay7.18 understand how a nucleus of U-235 can be split (the process of fission) by collision with a neutron, and that this process releases energy as kinetic energy of the fission productsUsually, large unstable nuclei break up gradually by the process of radioactive decay Uranium-235 among few others can break up in one go, a process known as nuclear fission. p:n ratio unbalanced, neutrons loosely bound. Entry of neutron causes high energy due to instability to eject multiple neutrons. In order to undergo nuclear fission, the nucleus usually requires energy, given by hitting the nucleus with a neutron. Neutrons not repelled by positive charge of nucleus7.19 know that the fission of U-235 produces two radioactive daughter nuclei and a small number of neutronsUranium-235 nucleus is struck by a neutron Breaks into two smaller daughter nuclei and 2 or 3 neutrons (in order to balance p:n ratio) Fission products carry away energy released in the form of kinetic energy Smaller daughter nuclei need less energy to be kept bound, excess energy is released.7.20 describe how a chain reaction can be set up if the neutrons produced by one fission strike other U-235 nucleiEach fission requires a neutron to start it, but releases a further 2 or 3 neutrons. These neutrons can go on to create further fissions Known as a chain reaction.7.21 describe the role played by the control rods and moderator in the fission processCONTROL RODS (silver, boron) - Reactor might become too hot and melt (lots of energy) due to amount of chain reactions - Control rods absorb neutrons, decreasing number of fissions - If rate of reactions decreases too much, rods can be withdrawn a little bit - Prevents a runaway chain reaction (explosions) caused when more than one neutron is used to cause fission MODERATORS - Uranium used in reactors contain large amounts of Uranium 238 which are non fissile due to even mass. - Absorb fast neutrons and may stop chain reactions - Neutrons are slowed by a moderator, preventing their absorption by Uradium 238, allowing them to create further fission - Moderators have light nuclei, include carbon and water7.22 understand the role of shielding around a nuclear reactorNuclear fission releases huge amounts of radiation in neutrons and beta particles. Radiation from reactions is hazardous Shielding absorbs radiation, preventing it from leaving the reactor7.23 explain the difference between nuclear fusion and nuclear fissionNUCLEAR FISSION Large nuclei are split into two smaller nuclei, releasing energy in the process. NUCLEAR FUSION Smaller nuclei brought together to form a bigger nucleus.7.24 describe nuclear fusion as the creation of larger nuclei resulting in a loss of mass from smaller nuclei, accompanied by a release of energyWhen fused, mass of the final nucleus is slightly less than the combined mass of the original nuclei. Due to release of energy, in accordance to part of E=mc^2 (mass and energy can convert into each other)7.25 know that fusion is the energy source for starsStars are giant balls consisting mainly of hydrogen gas. At the centre of stars hydrogen nuclei are fused together to form heavier helium nuclei, releasing energy in the process. Helium can then be fused to form even heavier elements in larger stars7.26 explain why nuclear fusion does not happen at low temperatures and pressures, due to electrostatic repulsion of protonsProtons have a positive charge, and repel each other. Protons travelling towards each other at very high speeds overcome repulsion Gas has to be heated to millions of degrees Celsius for that speed High pressure needed to increase collisions that occur, thus increasing the probability and rate of fusion Fusion is not used as a power source on earth due to the need for high temperature and pressure