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DESCRIPTIONThis powerpoint relates to Topic 3 Section 4 of the SACE in South Australia
- 1.RADIOACTIVITY 12 SACE PHYSICS-STAGE 2 SECTION 4 TOPIC 3 PRINCE ALFRED COLLEGE
- Henri Becquerel, a French physicist was working with potassium uranyl sulfate (a very fluorescent compound) in the 1890s.
- He placed samples of the salt on photographic plate wrapped in light tight black paper. After leaving it in the sun for a while, the plate was developed.
- As the plate was fogged, he assumed that X-rays were produced.
- He repeated the same experiment on a cloudy day and since there was no sun, there should be no UV to cause the fluorescence and so no X-rays produced.
- However, the plates were fogged. Becquerel had stumbled upon the fact that something in the salt was unstable (Uranium).
- The reason why the plates were fogged was that the Uranium was emitting particles to become more stable.
- We call this phenomena radioactivity.
- The Curies (Marie and Pierre), were interested in his workand named it Radioactivity.
- They continued the work and managed to discover two new elements polonium (named after Maries homeland) and radium (named due to its intense radioactivity).
- It has been determined that many isotopes of radioactive nuclei are unstable. They become more stable by emitting sub atomic particles or photons.
- Radioactive nuclei decay by the emission of alpha or beta particles or gamma radiation. These methods will be covered in more detail later.
9. NEUTRON/PROTON STABILITY
- By comparing stable nuclei, we can examine their neutron/proton ratio. This is shown on the graph where protons are on the x-axis and neutrons are on the y-axis. The line shows stable isotopes.
10. NEUTRON/PROTON STABILITY
- Anything off the line will spontaneously decay.
- For light elements (up to approx. 20) the N/Z ratio is close to 1.
- Towards the top end, the ratio is more like 1.6/1.
11. NEUTRON/PROTON STABILITY
- This suggests that protons and neutrons bind in pairs.
- However, as the line curves upwards, more neutrons are needed to overcome the repulsive force between protons.
12. NEUTRON/PROTON STABILITY
- Eventually, at 84 protons, no amount of neutrons can dilute the repulsive force and all elements aboveZ= 84 are radioactive.
- ElementsZ= 84 to 92 can be found in the Earths crust but above 92 the nuclei are too unstable to still be present in the crust.
13. NEUTRON/PROTON STABILITY
- Remember, the reason why a nucleus stays together is because of the strong NUCLEAR FORCES found between NUCLEONS (Neutrons and/or protons).
- We discussed this in the last topic (topic 2).
- The ELECTRICAL REPULSION between like charged (positive) protons tries to tear the nucleus apart.
14. NEUTRON/PROTON STABILITY
- At low atomic numbers (under 20), the attractive nuclear forces overcome the repulsive electrical forces within the nucleus.
- The protons and neutrons exist in a 1 to 1 ratio.
15. NEUTRON/PROTON STABILITY
- At higher atomic numbers (between 20 and 84), the nucleus gets larger.
- The repulsive electrical forces act between all protons
- The attractive nuclear forces are only found between adjacent nucleons.
- The nucleus needs more neutrons to create a stronger nuclear force without adding to the repulsive electrical force.
16. NEUTRON/PROTON STABILITY
- This is why elements with high atomic numbers have a greater number of neutrons than protons.
- Eventually the nucleus gets so large (atomic number = 84) that no number of neutrons would create enough of a attractive nuclear force to counteract the high number of protons.
- The nucleus then becomes unstable and will eventually break apart.This is called radioactive decay.
17. THE FOUR TYPES OF RADIOACTIVE DECAY
- There are four types of radioactive decay included in the syllabus.
- They are:
- beta minus
- beta plus
- gamma decay.
18. ALPHA DECAY
- Very heavy nuclei are often unstable as they contain too many protons.
- Typical alpha emitters have an atomic number > lead (82).
- Alpha particles are helium nuclei.
- Alpha particles are emitted, as they are extremely stable. They have high binding energy.
19. ALPHA DECAY
- When a nucleus undergoes alpha decay, the parent nucleus will suffer a decrease in atomic number ( Z ) of two and a decrease of four in mass number ( A ).
- The daughter nucleus is now a different element.
- Alpha Decay Example
20. ALPHA DECAY
- An example is:
- This is a Nuclear Reaction as new elements have been produced.
- The daughter nucleus will be more stable than the parent nucleus (the daughter nucleus has a lower atomic number).
21. ALPHA DECAY
- Note, the sum of the atomic numbers and the mass numbers are the same on both sides of the equation. Conservation laws still hold.
- The above equation is EXOTHERMIC as there is a loss of mass in the reaction. The energy produced goes to the alpha particle as kinetic energy.
22. ALPHA DECAY
- Alpha particles have a relatively high mass and so are ejected with a moderate speed, typically about2 x 10 7ms -1 .
- Because their charge is high (2+) and speed low, they interact with matter easily, thus they are able to penetrate air only by a few centimetres.
23. ALPHA DECAY
- A thin piece of cardboard is enough to stop a beam of alpha particles.
- As alpha particles do have large amounts of kinetic energy, they can damage human flesh by destroying parts of cells on impact.
24. ALPHA DECAY
- When alpha particles come near atoms, they are strongly ionising.
- Their high charge means they can displace electrons easily leaving behind an ion pair (an ion and free electron).
- This will slow an alpha particle down.
25. ALPHA DECAY
- Alpha particles are emitted with quantised energy, which suggests that the nucleus may have a discrete energy level structure.
26. DISCRETE ENERGY LEVELS
- Lets take a look at the alpha decay of Radium to Radon.
- Radium decays to Radon at different energy levels.
27. DISCRETE ENERGY LEVELS
- This suggests that the nucleons are arranged in the nucleus into energy shells (just like electrons).
28. DISCRETE ENERGY LEVELS
- -particles are ejected at certain discrete velocities (energies). The energy depends on which level the Radium decays to in the Radon.
29. DISCRETE ENERGY LEVELS
- Example: In the diagram,Ra 226 decays giving off an Bparticle that has a specific Kinetic Energy when it decays to Rn 222in the 2 ndexcited state.
30. DISCRETE ENERGY LEVELS
- The Rn 222then might return to the ground state giving off a photon of energy in the MeV range called a GAMMA PHOTON ( )
31. THE EFFECTS OF ELECTRIC AND MAGNETIC FIELDS ON DECAY
- As alpha particles are positively charged, they will be deflected by electric fields and magnetic fields.
- The force they experience can be found fromF=E q .
32. THE EFFECTS OF ELECTRIC AND MAGNETIC FIELDS ON DECAY
- This is in contrast togamma rays or an x-raythat would not be deflected by an electric field because theydo not have a charge.
- The path of the alpha particle is parabolic. As the mass of an alpha particle is relatively large, the acceleration is low compared to other forms of radiation.
33. THE EFFECTS OF ELECTRIC AND MAGNETIC FIELDS ON DECAY
- In a magnetic field, the deflection can be either upwards or downwards (depending on the direction of the field), in a circular path. The force can be found byF=B q v .
Direction of Current 34. THE EFFECTS OF ELECTRIC AND MAGNETIC FIELDS ON DECAY 35. THE EFFECTS OF ELECTRIC AND MAGNETIC FIELDS ON DECAY
- Note- gamma photon radiation has no charge, therefore itis not affectedby electric or magnetic fields, it instead passes straight through them.
- Remember that the reason an atom undergoes alpha decay is because it has too many protons or neutrons.
- It therefore ejects a helium nucleus.
36. BETA DECAY
- Nuclei that have an imbalance of protons or neutrons can be unstable and also undergo radioactive decay.
- The process involves thechangeof a proton into a neutron or more commonly a neutron into a proton with the ejection of an electron from the nucleus.
- This decay is called beta decay, and the electron is referred to as a beta particle.
37. BETA + DECAY
- BETA +DECAY(too many protons)-
- When a nuc