Consider a sample of a pure β-active material and select the correct information(s) — Nuclear Chemistry and Radioactivity Chemistry Question

💡 Solution & Explanation

Step 1 - Evaluate Statement (A): Beta particles may have different energies In $\beta^-$ decay, a neutron converts to a proton, emitting both an electron and an antineutrino: $$\ce{n -> p + e^- + \bar{\nu}_e}$$ The total decay energy $Q$ is shared between the electron (beta particle) and the antineutrino. The sharing is continuous and random, producing a **continuous energy spectrum** for the beta particles, from near zero up to a maximum $Q_{\max}$. → **(A) is correct.** Step 2 - Evaluate Statement (B): β particles exist inside the nucleus before ejection This is **incorrect**. Beta particles (electrons) do not pre-exist inside the nucleus. They are **created at the moment of decay** when a neutron transforms into a proton. The nucleus contains nucleons (protons and neutrons), not free electrons. → **(B) is incorrect.** Step 3 - Evaluate Statement (C): Antineutrino has zero mass and zero momentum The antineutrino has essentially zero rest mass but carries kinetic energy. Since it travels at (essentially) the speed of light, it has **non-zero momentum** ($p = E/c$). Zero mass does not imply zero momentum for a massless particle. → **(C) is incorrect.** Step 4 - Evaluate Statement (D): Active nucleus changes to its isobar In $\beta^-$ decay: $\ce{^A_Z X -> ^A_{Z+1} Y + e^- + \bar{\nu}_e}$ The mass number $A$ remains the same while $Z$ changes by 1. Isobars are nuclides with the same $A$ but different $Z$. → **(D) is correct.** The daughter is an isobar of the parent. $$\boxed{\text{Answer: A, D}}$$