Ch 13 - NUCLEI12th Physics

Definition: $1/12^{\text{th}}$ of the exact rest mass of an unbonded, neutral Carbon-12 ($^{12}\text{C}$) atom at rest.

Mass Value: $1 \text{ u} = 1.660539 \times 10^{-27} \text{ kg}$.

Energy Equivalent: Using Einstein's $E=mc^2$, the energy equivalent of $1 \text{ u}$ is exactly $931.5 \text{ MeV}$.

Isotopes: Nuclei with the same atomic number ($Z$), but different mass number ($A$). They belong to the same chemical element. (e.g., $^{35}\text{Cl}$, $^{37}\text{Cl}$)

Isobars: Nuclei with the same mass number ($A$), but different atomic number ($Z$). (e.g., $^3\text{H}$, $^3\text{He}$)

Isotones: Nuclei with the exact same number of neutrons ($N = A-Z$), but different atomic number ($Z$). (e.g., $^{198}_{80}\text{Hg}$, $^{197}_{79}\text{Au}$)

Experiment: He observed highly penetrating, electrically neutral radiation when Beryllium nuclei were bombarded with fast-moving Alpha particles ($^4_2\text{He}$).

Reaction: $^9_4\text{Be} + ^4_2\text{He} \rightarrow ^{12}_6\text{C} + ^1_0\text{n}$

Observation: This mysterious radiation could knock out heavy protons from light nuclei targets (like paraffin wax).

Conclusion: By strictly applying the conservation of energy and momentum to the collisions, Chadwick proved the radiation consisted of neutral particles (neutrons) with a mass ($1.00866 \text{ u}$) almost identical to that of a proton.