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A nucleus of polonium Pm may decay to the stable isotope of lead \(^{208}_{82}Pb\) through a chain of emissions following the sequence \(\alpha \rightarrow \beta \rightarrow \alpha\) - AQA - A-Level Physics - Question 7 - 2018 - Paper 2
Question 7
A nucleus of polonium Pm may decay to the stable isotope of lead \(^{208}_{82}Pb\) through a chain of emissions following the sequence \(\alpha \rightarrow \beta \ri... show full transcript
Worked Solution & Example Answer:A nucleus of polonium Pm may decay to the stable isotope of lead \(^{208}_{82}Pb\) through a chain of emissions following the sequence \(\alpha \rightarrow \beta \rightarrow \alpha\) - 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 \(^{208}_{82}Pb\).
Answer
- Start at polonium on the grid with coordinates (N, Z) for polonium.
- The first emission is an alpha decay, moving left two units and down two units. (E.g., (N \rightarrow N-2), (Z \rightarrow Z-2)).
- The next is beta decay, which increases Z by 1 while keeping N the same, moving right one unit. (E.g., (Z \rightarrow Z+1)).
- The final emission is another alpha decay, moving left two units and down two units once again.
Thus, you should have a sequence reflecting these changes in the net N and Z values.
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 a stable nucleus is primarily due to the strong nuclear force (SNF) and electromagnetic repulsion.
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The strong nuclear force binds protons and neutrons together within the nucleus, but its effectiveness is limited to very short distances (up to 3 fm).
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It is attractive and works equally well between all nucleons (protons and neutrons).
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In contrast, electromagnetic forces cause repulsion between protons, which are positively charged. This repulsion increases with the number of protons in the nucleus.
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Neutrons help to mitigate this repulsion, but as the number of protons increases, a higher ratio of neutrons to protons is often required to maintain nuclear stability. Thus, a larger number of neutrons is present.
Step 3
Step 4
Explain the origin and action of two sources of this radiation.
Answer
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Gamma Radiation: This occurs when the excited thallium nucleus releases energy as it transitions to a lower energy state, emitting gamma rays in the process.
- The transition releases electromagnetic radiation, leading to a more stable state without mass loss.
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X-Ray Emission: During the electron capture, an inner-shell electron vacancy can occur, prompting electrons from higher energy levels to fall into the vacancy.
- This transition releases energy in the form of X-rays, contributing to the electromagnetic radiation emitted.
Step 5
Explain why the metastable form of the isotope of technetium \(^{99m}_{43}Tc\) is a radioactive source suitable for use in medical diagnosis.
Answer
The metastable isotope (^{99m}_{43}Tc) is particularly valuable in medical diagnostics for several reasons:
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Short Half-Life: It has a half-life of about 6 hours, which is long enough to conduct medical procedures but short enough to minimize radiation exposure to patients.
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Beta and Gamma Emissions: It primarily emits gamma radiation upon decay, which is highly useful for imaging techniques such as SPECT (Single Photon Emission Computed Tomography), allowing for clear imaging of biological processes.
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Chemical Versatility: Technetium has the ability to form various compounds, enabling its use in a wide range of diagnostic imaging across different organs, enhancing its applicability in clinical settings.