To determine which of the given nuclear reactions is an example of positron emission, we need to know the characteristics of positron emission. Positron emission involves the emission of a positron, which is denoted as [tex]\( { }_{+1}^0 \beta \)[/tex]. Let’s analyze each reaction for the presence of positron emission.
1. [tex]\( { }_{94}^{238} \text{Pu} +{ }_2^4 \text{He} \rightarrow{ }_{96}^{242} \text{Cm} \)[/tex]
- In this reaction, plutonium-238 and helium-4 (alpha particle) combine to form curium-242.
- There is no emission of a positron ([tex]\( { }_{+1}^0 \beta \)[/tex]) in this reaction.
- This is not a positron emission reaction.
2. [tex]\( { }_{11}^{22} \text{Na} \rightarrow{ }_{10}^{22} \text{Ne} +{ }_{+1}^0 \beta \)[/tex]
- In this reaction, sodium-22 decays into neon-22 with the emission of a positron ([tex]\( { }_{+1}^0 \beta \)[/tex]).
- This exactly matches the definition of positron emission.
- This is a positron emission reaction.
3. [tex]\( { }_{28}^{63} \text{Ni} \rightarrow{ }_{29}^{63} \text{Cu} +{ }_{-1}^0 \beta \)[/tex]
- In this reaction, nickel-63 decays into copper-63 with the emission of a beta particle ([tex]\( { }_{-1}^0 \beta \)[/tex]).
- Beta particles are electrons, not positrons.
- This is not a positron emission reaction.
4. [tex]\( { }_{18}^{41} \text{Ar} +{ }_{-1}^0 \beta \rightarrow{ }_{17}^{41} \text{Cl} \)[/tex]
- In this reaction, argon-41 captures an electron ([tex]\( { }_{-1}^0 \beta \)[/tex]) to form chlorine-41.
- This represents electron capture, not positron emission.
- This is not a positron emission reaction.
5. [tex]\( { }_{96}^{242} \text{Cm} \rightarrow{ }_{94}^{238} \text{Pu} +{ }_2^4 \text{He} \)[/tex]
- In this reaction, curium-242 decays to form plutonium-238 and an alpha particle ([tex]\( { }_2^4 \text{He} \)[/tex]).
- There is no emission of a positron ([tex]\( { }_{+1}^0 \beta \)[/tex]) in this reaction.
- This is not a positron emission reaction.
Based on the analysis, the reaction that exemplifies positron emission is:
[tex]\[ { }_{11}^{22} \text{Na} \rightarrow{ }_{10}^{22} \text{Ne} +{ }_{+1}^0 \beta \][/tex]
Thus, the correct reaction is the second one, which is positron emission.
