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A nucleus of polonium P0 may decay to the stable isotope of lead 208Pb through a chain of emissions following the sequence α β- α - AQA - A-Level Physics - Question 7 - 2018 - Paper 2
Question 7
A nucleus of polonium P0 may decay to the stable isotope of lead 208Pb through a chain of emissions following the sequence α β- α. Figure 13 shows the position of ... show full transcript
Worked Solution & Example Answer:A nucleus of polonium P0 may decay to the stable isotope of lead 208Pb through a chain of emissions following the sequence α β- α - AQA - A-Level Physics - Question 7 - 2018 - Paper 2
Step 1
Draw four arrows on Figure 13 to show the sequence of changes to N and Z that occur as the polonium nucleus is transformed into 208Pb.
Answer
- Draw one arrow down 2 positions and left 1 position to indicate the emission of an alpha particle (α).
- Draw one arrow down 1 position to indicate beta decay (β-).
- Draw another arrow down 2 positions and left 1 position to indicate the emission of another alpha particle (α).
- Finally, draw one more arrow indicating the stable position at 208Pb.
Step 2
Explain why there is this imbalance between proton and neutron numbers by referring to the forces that operate within the nucleus.
Answer
The imbalance between proton and neutron numbers in the stable isotope 208Pb can be explained by the competing forces within the nucleus:
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Strong Nuclear Force (SNF): This force acts between nucleons (protons and neutrons) to hold the nucleus together and is attractive at very short ranges (about 1 femtometer). However, it only acts between neighboring nucleons.
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Electromagnetic Repulsion: Protons repel each other due to their positive charge. This repulsive force increases with the number of protons, which can destabilize the nucleus if not counterbalanced.
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Neutrons' Role: Neutrons do not experience electromagnetic forces but contribute to the strong nuclear force. More neutrons help to bind the nucleus together, reducing the repulsion between protons and increasing stability.
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Equilibrium: The stable isotope thus usually has more neutrons than protons, as the excess neutrons provide additional strong force attraction to stabilize the nucleus without adding to the repulsive electromagnetic force.
Step 3
Write the equation to represent this decay, including the isotope of thallium produced.
Answer
The equation for the electron capture of the stable isotope 205Pb can be represented as:
\text{_{82}^{205}Pb} + e^- \rightarrow \text{_{81}^{205}Tl} + \nu_e
where is the electron captured and is the emitted neutrino.
Step 4
Explain the origin and location of two sources of this radiation.
Answer
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Excited State Emission: The thallium nucleus produced after the decay is often left in an excited state. When it transitions to a lower energy state, it emits electromagnetic radiation, primarily in the form of gamma rays.
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Electron Transitions: In addition, when electrons in the thallium atom fall from higher energy levels to lower ones, they can emit X-rays. Both of these processes contribute to the radiation emitted from the thallium atom.
Step 5
Explain why the metastable form of the isotope of technetium 99Tc is a radioactive source suitable for use in medical diagnosis.
Answer
The metastable form of technetium-99 () is widely used in medical diagnostics due to its favorable properties:
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Optimal Half-Life: has a half-life of about 6 hours, which is long enough for medical procedures but short enough to minimize patient exposure to radiation.
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Gamma Emission: It decays by emitting gamma rays, which can be easily detected by medical imaging devices, allowing for non-invasive diagnostic imaging.
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Chemical Versatility: can easily be incorporated into various chemical compounds, making it adaptable for different diagnostic applications, including imaging of the heart, bones, and organs.